Prisoner, Cell Block Hg
By Mike Frost
For more information on the Mercurial behaviour of our innermost planet,
see the article following this story in MIRA 54
It was some time since I had last met my friend Clive, stalwart of the Interplanetary Dangerous Sports Club. When he rang me up and suggested that we met, I was more than happy to agree; evenings spent in Clive's company were always enjoyable, even if I didn't always believe his outrageous stories.
However, this time Clive insisted that we met outdoors, during daylight hours, nowhere near a pub. So I turned up at the local park the following Saturday. Fortunately the weather was fine. Clive arrived late, looking flustered. He checked around to make sure no one was listening to our conversation, and then set off across the park towards the bandstand. I followed him, of course, but I wanted to know why he'd dragged me away from the telly.
Clive looked nervous. "It's Clarissa . . ."
Clarissa, you might remember, was Clive's girlfriend for a short time, during which he had almost dumped her into a black hole; she had almost crashed their ship into a neutron star, and the two of them had got into a shady diamond-smuggling scheme that had led to Clarissa's imprisonment. Aside from that, they were made for each other . . .
"Is she out of prison, then?" I enquired, "I thought she was in for five years."
"Ten years." Clive corrected me.
"Yes, but with parole for good behaviour . . ."
Clive looked at me sternly. "Clarissa hasn't exactly been a model prisoner, Frosty. She has her standards, you see, and prison does not live up to them."
"In what way?"
"The uniform, for example. It clashed horribly with her colour-chart, so she refused to wear it. And the Feng Shui in her cell was completely to pot, so she went on hunger strike until they let her re-align the bunks. And then when she led the prison riot over the toilet paper†. . ."
I was astounded. "Clarissa led the riot!" It had been headline news. Then I remembered the aftermath. "Oh my goodness me. Didn't they send the ringleaders to . . ."
Clive nodded his head sadly. The Home Secretary was in the middle of one of his crackdowns on persistent offenders. Male troublemakers were sent to the maximum security unit on Pluto, which was bad enough, but female prisoners faced a much worse fate . . .
"Mercury . . ." said Clive quietly, "Clarissa is in prison on Mercury." And he started sobbing.
Clive nearly blew his nose on Clarissa's letter, but fortunately I found a pocket-handkerchief just in time. Whilst Clive sniffled, I read the letter. Some bits had been crossed out.
"Darling Clive," it said,
"I am writing to you from the maximum security prison on Mercury. The authorities are ******* this ******, so I shall have to ***** ** ****.
Mercury is a very interesting planet. On the surface, the temperature during the daytime is four hundred degrees, so when I go out on the chain gangs we cannot work for long. Nighttime is a little cooler, about minus two hundred degrees, so we tend not to go out then. There are a lot of rocks to break. The ultra-violet radiation is very strong and so I have a lovely tan.
Inside the prison is very interesting. All the cells are below ground, so we cannot tell if it is day or night above. Water is in very short supply, but I do manage to have a bath every day. A day on Mercury lasts for about twelve Earth weeks. The food is *** very appetizing, but there are lots of cockroaches, which provide extra nutrition.
I have made many friends, who all have sad tales to tell. Noelene, my cellmate, tells me that the police framed her for every single one of her axe murders.
I must close now. Noelene wants to borrow my crayon.
Love and Kisses,
PS. Give Everyone This Message Eat Oranges Until Tuesday. Not On Wednesday."
There were tear-stains all over the letter — I wasn't sure if they were Clarissa's or Clive's.
"Well, what do you think?" demanded Clive.
"Mercury sounds awful," I said, "do you think that's what she means by 'very interesting"? And what was that stuff about oranges at the end?"
Clive sighed. "Initial letters, Frosty; what do they spell?"
"G...E...T....M...E....O...U...T....N...O...W"... "Oh . . ."
Clive looked at me imploringly. "We have to rescue her, Frosty, she's in a terrible mess."
I felt like Tonto, as he and the Lone Ranger are surrounded by whooping hordes of Red Indians. "What do you mean WE, Paleface?" But instead I tried to be more constructive. "Clive, face the facts. Clarissa is in prison on the most inhospitable planet in the solar system, under maximum security guard, for crimes which she admits committing. You and I are law-abiding citizens, with no record of any criminal activity — apart from your habit of jumping off tall buildings and space surfing through air traffic control, of course. I'm not going to risk my good name to save some ungrateful harridan who can't even tolerate prison clothes. Oh Clive, I'm sorry, I didn't mean it like that. She isn't a harridan, she just acts like one . . ."
"No, you're right Frosty, I shouldn't have asked you to help. You just don't understand how much I care for her . . ."
"You were at each other's throats last time you were together . . ."
"Frosty," said Clive, "I am still in love with this woman, harridan or not. We have had our disagreements, but if I can rescue her from this hellhole on Mercury, perhaps she'll tolerate my little foibles a bit more. I guess I'm not going to persuade you to come along, but at least you can give me some advice on how to plan the mission."
"OK" I said. "You're going to have to land close to the terminator . . ."
"Is that a good idea?" said Clive "he was pretty mean in those movies . . ."
I sighed. "The terminator is the line separating the night and day sides of Mercury. Land on the eastern terminator, just after the Sun has set. You see, Mercury is so close to the Sun that your spaceship won't be able to survive being parked in the direct sunlight — the electronics will melt after a few hours. But if you land on the dark side, you risk the opposite — your systems will freeze up. But you are in luck, because Mercury spins very slowly; once every fifty-five days, in fact."
Clive looked at his letter. "Hang on, Clarissa said the days were twelve weeks long. That's not fifty-five days."
I was surprised Clive had noticed. "Mercury spins on its axis in fifty five days, like I said. But mercurial days are longer than that, because you also have to take into account the planet's speed around the Sun; Mercury spins round one and a half rotations each time it orbits the Sun. The effect of the orbital motion is to slow down the Sun's apparent motion across the sky, so what you actually get is eighty-eight days of sunshine and then eighty-eight days of night-time."
Clive looked confused. "If you say so . . ."
"Anyway," I concluded, "this is the clever bit. Park your spaceship on the terminator, just after sunset, so that the ground temperature will still be warm, but your spaceship will be out of direct sunlight. Even so you can't leave your spaceship parked for too long, or the ground will cool and the batteries will start to drain. So figure out when the Sun sets at the prison, and time your landing for shortly afterwards. And land close to the terminator."
I knew Clive had got the message, because he made himself as big as possible, and announced, in his best Austrian accent, "Hasta la vista, baby! . . . I'll be back . . ." But as he walked away I wondered if I'd ever see him out of prison again.
I didn't see Clive at all for several more weeks, so when I did next encounter him, in the shopping arcade, I was dying to know how his rescue attempt had gone. Clive was sat, on his own, at a table by one of the fast food stalls. No Clarissa, I noticed; not a good sign.
"Oh, hi Frosty." Clive said, absent-mindedly.
I asked him how the mission had gone.
"Oh, OK. I landed my spaceship close to the terminator, like you suggested, on a flat plane just to the east of a ridge. Because of the radar screens I had to land a long way from the prison. I suited up and climbed out of my spaceship. The sight was absolutely breathtaking. To my west lay the horizon and the terminator. The Sun was only just below the horizon; I couldn't see the solar disc, of course, but the solar corona jutted above the horizon. I can see why you go to all those eclipses, Frosty; it was a gorgeous sight — a pearly white glow streaming twenty or more degrees up from the horizon. Where it joined the horizon you could just see the tops of solar flares licking upwards, moving slowly as I watched.
Overhead, the sky was velvet black, but it was difficult to see many stars because the corona to the west was so bright. When I turned to the east, I could see occasional patches of ground, slightly higher than the surroundings, still illuminated by the Sun. These were crater rims, which were high enough to catch the last rays of the Sun. Otherwise the landscape was lit solely by the soft light of the corona, which gave everything the most beautiful deathly white pallor.
It was a wonderful view, Frosty, but I couldn't stand around all day and admire it. So I set off across the surface of Mercury towards the prison. It was hard going, I can tell you; there were hundreds of craters between me and the prison, and I had to detour round them to avoid exposing myself directly to sunlight. There was dust, like the Moon, but also rocks, fused together and slippery. I found I could move fastest when I bounced along like the astronauts do on the Moon.
Eventually, I reached the walls of the prison. I had to cut my way through the fencing, but it was easier than I had thought; maybe the great changes in temperature from day to night had induced metal fatigue. Likewise, getting into the prison itself was easy — there were unguarded airlocks. I guess they really hadn't expected rescue attempts. I made my way down towards where Clarissa's cell was located.
Fortunately, she was in her cell with Noelene, who was busy skinning a rat. 'Strewth!' yelled Noelene as I forced the lock and walked in, 'it's a man!'
'Oh, Clive!' Clarissa screamed, and threw herself on me.
'Jeez', Noelene remarked, 'Wish I had a mate like yours.'
'We can't hang around,' I told Clarissa, once she stopped kissing me, 'We've got to be out of here before anyone notices I'm here.'
'You got room in that spaceship of yours for one more Sheila?' Noelene asked, hopefully.
I shook my head. 'Sorry Noelene, only room for two.' I was a bit worried she might decide to take Clarissa's place in the ship — I wasn't sure there was a lot I could do if she decided she was coming along.
But instead she just shrugged. 'Well, you two lovebirds get out of here before anyone notices. Just let me loose inside the prison and I'll create a diversion.'
Quickly and quietly we made our way to the prison exit. We paused to say good-bye at the airlock door. 'Thanks for everything, Noelene' Clarissa said, tearfully, 'I don't know how I could have coped without your help.'
Noelene was fighting back a tear herself, I noticed. 'No worries mate,' she managed to sniff, 'everybody needs good neighbours . . .'
A few moments later we were suited up and heading across the surface of Mercury. We couldn't talk to each other, in case the guards overheard our radio conversation, but every so often I caught a glimpse of Clarissa's face through the visor and she smiled at me.
It took nearly twice as long to return to the spaceship, as it had to make the outward journey. I was tired and Clarissa wasn't used to the terrain. Not that I was counting, but there seemed to be even more sunlight-bathed crater rims to avoid, than on the outward journey. About half way back to the spaceship, the sirens sounded over the radio. The jailbreak had been discovered! As we neared the spaceship we could hear attempts to organize a search party, but something seemed to be disrupting the guards. At length, there were sounds of scuffles and fighting, and a familiar voice came over the radio.
'Good on yer, cobbers! [Take THAT, you whining drongo!] Good luck to the both of youse! [Get OFF me you miserable pommie . . ]' Then abruptly the radio went dead.
At last the spaceship came into sight. We climbed into the airlock, de-suited, and prepared to enter the ship. Almost straight away I knew something was wrong. The door into the spaceship wouldn't open automatically, we had to winch it open by hand. Inside the spaceship the light was dim and failing.
'What's wrong, darling?' Clarissa demanded. I went straight to the control panels.
'The batteries are low, dear . . . I hope we have enough power for take-off.'
'Clive, darling, you did switch off everything before you left the ship, didn't you?'
'Well, I thought I did, dear . . . oh dear, I think I must have left the computer on. And the air-conditioning. And . . .'
'Clive, darling,' demanded Clarissa, tartly, 'did you put any thought at all into conserving the ship's power?'
'Well I thought I . . .'
'Didn't you realise there would be no solar energy on the dark side of Mercury to re-charge the batteries?'
'Yes, but . . .'
'And how long till dawn?'
'Uh . . . twelve weeks.'
'And how long will it take the guards to find us?!'
'A few hours, maybe, but we've . . .'
'So we are going to be stranded here, darling, until the guards track us down, all because you are a complete incompetent!!'
'I was only trying to . . .'
'YOU ARE USELESS, CLIVE, COMPLETELY USELESS!! You can't even be trusted to switch off the lights. Typical male . . .'
'Yes dear, but . . .'
'DON'T INTERRUPT!! You haven't changed a bit, Clive, have you?'
'I still love you, Clarissa, if that's what you . .'
'DON'T CHANGE THE SUBJECT!!'
'Isn't the horizon getting brighter over there??'
'I SAID DON'T CHANGE THE . . . What did you say!?'
"Well this is a nice surprise, isn't it?" Clarissa had emerged from the shops, and strode over to grasp Clive's hand. "You're the chap who helped Clive plan my escape mission. Well may I say thank you . . ." She leant over and pecked me on the cheek. "If only you had given him a little more advice on how to power down spaceship systems."
"But we didn't need to, dear . . ."
"Yes of course, darling. Clever Clive, eh? He unset the Sun for me . . ."
Clive looked pleased with himself. "Yep, the Sun just popped back above the horizon. Not very high; not for very long, but just enough to recharge the batteries. And off we went!"
Something rang a bell in my memory. "Oh, eccentricity!"
Clarissa was defensive, "He may be eccentric, but I still love him!"
"Not Clive," I said, "Mercury! Mercury has a very eccentric orbit — not at all circular like Earth's. And that really upsets the length of the day — because, when Mercury is at the closest point in its orbit, the Sun is moving at its fastest through the sky. You remember me telling you that Mercury spins very slowly? Well, usually, the Sun's motion through the sky due to Mercury's spin is greater than the motion due its orbit. But not at perihelion. At the closest point in its orbit, the Sun appears first to stop in the sky, and then to change direction. The upshot is, the Sun can actually rise again after sunset, if you land on the terminator at just the right time of year. You must have got lucky!"
"Luck had nothing to do with it," said Clive, smugly, "like you said, it was pure egocentricity." I decided not to correct him.
"Anyway," said Clarissa, "we got away. And now we're going to get married."
Clive dropped his drink. "What!"
"Oh didn't I tell you, darling? Daddy's got it all arranged. He was a bit dubious when I first described you to him, but I did point out that you sprang me from jail. And you don't have to worry about the ring, darling, I still have one or two diamonds that those beasts in customs weren't able to find on me."
There was a look of pure horror on Clive's face. Clarissa didn't notice it; she was already immersed in the minutiae of wedding preparations. She turned instead to me.
"Anyway, we'd like you to be the best man. I want someone I can trust to look after Clive on the big day. I can't trust the poor darling to get himself to the church in one piece . . ."
Clive was standing open-mouthed. I could see him thinking. 'For all her years in prison, Clarissa hasn't changed one bit.'
And I felt sorry for him. Clive had rescued Clarissa from one prison, only to be in grave danger of being trapped in another prison himself. . .
To Be Continued
One of the things that really bugs me is when people come up to me
and say "I hear you're interested in astrology". There, I've
said it. Excuse me while I wash my mouth out with soap and water. "No", I'll say, "it's Astronomy I'm into, not mumbo jumbo nonsense." For that I get queer looks off some and I have to start to explain the
difference between science and superstition and fiction. Most of
you in this Society, must at one time or another, have had to explain this
to someone. I bet I am not the only one to be constantly surprised
by how little some folk know of the sky and how it works, many believe
all the rubbish they read in the morning papers written by a so called
One of the best ways to upset some, is to point out that the star sign
they were born under (whatever that means) is wrong for some because the
Sun is not in the constellation that they thought it was from reading about
their stars in the dally paper.
So here below is a list of the correct dates for this year then the
Sun moves into one of the 13, yes 13, Zodiacal constellations. All
these dates have been worked out from a star atlas and a computer Orrery
program. The Position Hour Angle is the Right Ascension (ie. the
longitude measurement in the sky) which runs from a point in Pisces where
the ecliptic, which is the path of the Sun across the sky, crosses the
0° declination co-ordinates and runs around the sky for 24 hours until
it returns to the same spot. So at the start of this year the Sun
was in Sagittarius which it had entered on December 19th 1999 at 17h 45m
RA, and passed into Capricornus at 20h 10m RA on January 21st. The
RA position is the boundary line of that constellation and the next.
So as the Sun, or rather as the Earth orbits the Sun, so different
star groups pass behind the Sun at about 1° per day. It is the
lines on the constellation maps and star charts which tell you where the
Sun is currently at a specific date. To check this out I have given
two sets of dates, one for this year and the other for 1900. This
is because the boundaries of the constellations where changed around 1930
by the IAU into todays straight lines from the older system of curves.
To check I used an old book on astronomy with the maps drawn for the 1900
epoch, and was surprised at how closely the dates matched.
Some folk may find that their star sign
belongs to a different group to what they thought it was, especially around
November time when the Sun takes just a week to pass through Scorpus and
on into the forgotten constellation of Ophiuchus where it spends about
20 days. Have you ever heard anyone say their birthday is in Ophiuchus?
A quick glance at a star atlas will show you how small is the section of
Scorpus in that part of the sky crossed by the ecliptic. The reason
2000's start and finish dates in December 19/18 are different is because
of the leap year helping to keep the sky in step with the Earth's orbit.
I hope this provides some ammunition
for those who like to get there bit in when asked what's their star sign,
as if it matters at all. The only worry we have about stars is how
far away will the next supernova be, anything less than 1,000 light years
and we may be in trouble. But that's another story.
Hour Angle RA Date Sun moves
2000 in 1900
17h 45m December 19
20h 10m January 21 January 18
22h 00m February 17 February 14
23h 30m March
12 March 11
1h 49m April
19 April 17
3h 22m May
13 May 14
6h 1m June
21 June 21
8h 1m July
20 July 19
9h 22m August
10 August 10
11h 37m September 16 September 16
14h 23m October
31 October 29
15m 55m November
23 November 22
16h 22m November
17h 45m December
QUASARS: the strange ones
by Paritosh Maulik
These very bright objects, often associated with radio sources, were
first detected to be far far away. All the facts are still
not known. It is believed that perhaps massive black holes are the
power sources behind these objects. Similar, but smaller,
objects have also been found within our galaxy as well. These
are powered by smaller black holes. This article is based on
an article by Patrick Osmer of Ohio State University.
There was something odd about the spectrum of 3C273, the 273rd
object in the 3rd Cambridge Catalogue of the radio sources. Maarten
Schmidt produced the most convincing interpretation of the spectrum.
1) It was a star like object
of 13th magnitude, but . . .
2) The spectrum showed well
defined lines and it was continuum, i.e. in between peaks, the intensity
did not drop to near zero; the spectrum of the stars and galaxies is not
3) The spectrum was red shifted
to longer wavelength by about 15.8% of the regular or rest wavelength.
A red shift of 15.8% corresponds to the object being 3x109 light years
away, yet it appears to be of 13th magnitude; this means that the object
is very bright. To put it another way, if our Sun was moved to 140
light years away, it would look like a 13th magnitude object.
These highly luminous, compact objects, about 1013 times solar mass
in a volume the size of solar system, with high red shifts and often with
radio sources were named as Quasi-Stellar Radio Sources. But since
then, similar objects without radio sources have been detected and now
this group of objects are known as Quasi- Stellar Objects. Astronomically
speaking these objects are somewhat rare and it is generally believed that
the vast distance (i.e. large redshift) of these objects is due to the
expansion of the universe. These formed when the universe was young
and since then have moved away with the expansion of the universe.
Quasars are found at the centre of the galaxies. There are some
galaxies close to our galaxy with power output of about 109 suns. These
are called active galactic nuclei. The power output of the quasars
are about 1012 suns and outshines the luminosity of the active galactic
nuclei. Earth based images of quasars show fuzziness around the quasars
due to looking through the Earth's atmosphere. Now the Hubble Space
Telescope has given us a clearer view through and confirms that the quasars
do reside in the galaxies and hence the fuzziness.
1) In the optical images,
the quasars have a star like nucleus at the centre and reside at the centre
of a galaxy.
2) Radio observations suggest
that these objects are very compact. Therefore if these objects are
really so old (from the redshift), then these are perhaps the distant objects
of such compact nature.
3) These objects have high
energy output in the energy range of infrared to radio and the power is
similar in all the ranges.
4) Red shift in the range
of 0.1 to 4.9
5) Variable brightness in
different bands and the time period can be as short as days to weeks.
6) Highly luminous, can be
up to 1014 suns
7) Radio emission studies
indicate possible high energy sources up to 1060ergs in high energy electrons.
Possible Power Sources
The energy sources of the stars and the galaxies are powered by nuclear
fusion reactions; these are near equilibrium processes and is reasonably
However the properties of the quasars do not quite follow the fusion
based reactions. The general consensus now is that the powerhouse
is gravity driven. If a system collapses such that under gravity
1% of its mass is converted into energy, then the system collapses unstoppable
the system becomes a powerful and efficient energy source. This is
a black hole scenario.
Martin Rees of Cambridge University has suggested that a massive black
hole of 108 solar masses at the centre of the galaxy may be the possible
explanation of the quasar power source. At a distance of about one light
year the velocity of the gases could be as high as 10000km sec. Nearer
to the black hole, the temperature can be greater than 109 k and the physics
takes over from Newtonian motion to Einstein's relativistic motion.
Fig 1 shows a schematic model proposed by Martin Rees. The diameter
of the hole is about 16 light minutes, approx. the size of Earths orbit.
The source of the radio emission is approximately 1x106 light years from
the centre laying at the outer edge of the system. There remains
a lot unknowns for these objects.
Quasars: Are These Old Fossils?
The most distant quasar is 23x103 light years away — this corresponds
to an age of 12x109 years and the universe is about 13x109 years old;
this is assuming the expansion of the universe and the inflation theory,
i.e. the early rapid expansion of the universe.
Large amount of gases present in the intergalactic space absorbs part
of the spectrum from the source behind the gas cloud (see box 2). The neutral
hydrogen present in intergalactic space, absorbs one of the characteristic
lines of the hydrogen spectrum, rest wavelength at 121.6nm. This
appears as a absorption line in the spectrum of the quasars, although redshifted
to higher wavelength. This may be interpreted as - the intergalactic
space is perhaps not that full; but it is not totally empty either.
This has an interesting effect on the spectrum of the quasars.
i) these dim the spectrum
ii) produce some absorption
lines due to the background gases.
One of the most distinctive features of the spectrum of the quasars
is continues radiation and some broad features above the back ground level.
The density of the gas cloud which causes the absorption lines, may be
100 atoms per cm3, and this occurs when a part of another galaxy lies between
us and the distant quasar. Therefore by analysing the spectrum we
can get some idea about the nature of the dust cloud. Both the dust
and the gas may exist in the same cloud. Now, if the dust cloud is
at present close to the quasar, it would absorb most of the radiation from
the quasar. The redshifts of the absorption lines from the gas cloud
are smaller than the redshifts of the quasars. Hence it is likely
that the gas is a distance from the quasars. We also know that most
of the quasars are far away objects formed earlier then the intervening
gas cloud, that must have formed at a later stage; this implies that the
earlier universe was relatively dust free.
Evolution of Quasars
There is a theory in astronomy called luminosity function — it
is used to calculate the density or population of objects based on luminosity.
By analysing the luminosity and correcting for the redshift we can determine
the luminosity function. In a given volume of space it is likely
that there will be more faint quasars than bright ones.
As early as 1968, Maarten Schmidt realised that the majority of the
quasars occur at a redshift of 2, i.e. when the universe was about 20%
of the present value. Over the years these observations have been
refined and now it appears that the quasars formation really took off when
the size of the universe was around 1.5x1010 light years or, about 1.4x1023km.
All this detailed work has opened up further questions.
1) Is the number of quasars
higher at the time of formation at redshift = 2 or were the quasars brighter
around that time period, and
2) Does the density of the
quasars drop off after redshift of 2?
There are reasons to know all these, because it can throw some light
on the nature and the life of quasars.
i) Density Evolution
model: The formation of quasars were frequent but short lived and
the brightest galaxies took part when the universe was young.
ii) Luminosity Evolution
model: The formation of quasars were rare events, occurring perhaps
in only 2% of all galaxies.
The next question is, did the quasar formation really drop off after
a redshift of 2? To answer this question we have to find these obscure
objects and count them.
To Look for the Chosen Few
Let us start with some statistics of a typical telescope that can be
used to look for quasars. The Anglo-Australian UK Schmidt telescope has
a plate size of 356mm x 356mm, this will cover an area of 6.4° x 6.4°degrees
of sky. A typical image will have about 200,000 features, including
galaxies, stars and quasars. On a single plate there may be 1000
quasars, but only a few would have a redshift greater than 4. Telltale
signs of quasars are:
i) Different spectral distribution.
ii) X-ray and radio emission.
iii) On the photographs taken on different nights
the quasars do not show any motion, whereas the images of stars do.
iv) The stars are not generally variable in
their output but the quasars are.
The most distinguishing features of the quasars are their spectral
energy distribution. The techniques used to hunt for the quasars
are low resolution spectroscopy, multicoloured photography, and large format
CCD detectors. The photographic plates have low detection efficiency
compared to the CCD, but the trade off is a large picture, used to image
large areas of the sky, about 106 picture elements. A computer analyses
the pictures and filters out the defects on the plate.
For colour imaging filters in the range of 350 - 800nm are used.
The spectrum of stars are mainly from their thermal energy; these contain
absorption lines; but the quasars on the other hand do not behave like
a normal thermal body. These show emission lines. Some of the quasars
also show ultraviolet lines — this suggests the quasars are hot.
Some of the quasars also show absorption lines — these are due to hot
gases flowing out of the quasars at 10% of the speed of light.
Armed with all these techniques, 100 quasars greater than redshift
of 2.2 but only three quasars with redshift greater than 4 were detected.
If the photographic colour pattern of the quasars are compared with
that of the starts, the quasars clearly stand out. This is caused
by the redshift and absorption of the spectral lines. In the optical
images there is a large component of ultraviolet radiation. The spectrum
of a faraway quasar at a distance of redshift 4.43 Fig 3 shows
the emission lines of oxygen, silicon and carbon. Lya is a characteristic
hydrogen emission line called the Lyman alpha line. In this spectrum
it has been redshifted to around 670nm, i.e. in the ultraviolet range.
In the laboratory or rest condition we should expect this line to be around
121nm wavelength. The dip in the region of 790nm is due to absorption
from some gases containing carbon.
One group of workers have reported quasars of redshift higher than
4 and perhaps one at a redshift of 4.9 this perhaps is the
furthest object detected. At such a high redshift the visible spectral
lines would be getting away from detection limit of the terrestrial telescopes
and if the redshift is 6.5, the quasar may not be optically visible at
Features of the spectrum from a high redshift, 4.9 quasar is very similar
to that of a relatively nearby quasar; it shows emission lines from hydrogen,
carbon, nitrogen, oxygen and silicon. Redshift of 4.9 refers to 109 years
after the big bang, and if this is the true picture, then in that short
period many of the elements have formed in the universe, and the overall
composition is not that dissimilar from the present day universe.
When we look at the spectrum of the quasars in the short wavelength,
the intensity is very close to the background level. This is because
the intergalactic hydrogen has absorbed part of the spectrum. Examination
of this part of the spectrum tells us about the nature of the hydrogen
cloud, how far way the quasar is and also the associated hydrogen must
have also formed at the very early days of the universe.
When did the Quasars form
Two independent groups of workers have found that the maximum number
of quasars are found around the redshift of 3.2 and then there is a sudden
drop. An easier way to look at this data is to plot a look back time
vs. space density; space density is something like number of quasars in
a given volume of space. Fig 4
How Did the Quasars Form
We now know that interaction or collision of two galaxies can trigger
the formation of new stars. Two galaxies can merge to form a new
galaxy. When two galaxies merge, the matter in the galaxies can loose a
large amount of angular momentum. Matter drops to the centre of the
merged system. There may already be a black hole or a new black hole can
form. We have already seen that a black hole is the power source
of the quasar. At one time it was believed that since the active
galactic nuclei is more common in the spiral galaxies, quasars live in
these galaxies, but the Hubble images show that quasars are also plentiful
in the elliptical galaxies as well.
The early universe was somewhat tightly packed; objects were about
four times nearer, compared to the present day. It gave the galaxies
to chance to collide with each other. Some recent images of merging
spiral galaxies clearly show star formation, but is yet to show a quasar,
maybe that the time to form a quasar is in the future.
Some of the recent data suggests that perhaps there is a correlation
between the mass of the back hole and the number of the older stars in
a galaxy. In the case of a spiral galaxy, the older stars tend to
be in the bulge, where as, in the elliptical galaxies, all the stars are
old. The mass of a supermassive black hole (of at least 105 suns)
is about 200 times less than the weight of all the older suns in the galaxy
surrounding the black hole. All this suggests that there is a connection
between the black holes and the quasar formation.
In a simple form the theory is as follows: During the first few
billion years after the big bang, the primordial hydrogen gas cloud collapsed
under its gravity, and fragmented to form stars. But the process
of star formation is not perfect and a lot of hydrogen remains, which continues
to collapse. This cloud is so large that it's very hot and has not
been able to form stars yet. These are the origins of supermassive
black holes. But this theory is yet to be verified. James Dunlop
of Royal Observatory, Edinburgh has suggest that collapse
of hydrogen led to the formation of massive back holes, and once the hydrogen
ran out the, the black hole formation stopped and so also did
the quasar formation.
Quasars Nearer to Home
Small or Micro-quasars have also been reported within our galaxy.
At the centre of these, there are small black holes, maybe just above 3
times solar mass and an accretion disc of 1000km wide, fed by a near by
companion star. Under the prevailing condition, the friction may
be high enough to cause high temperature and therefore X-rays emissions
can be detected from the disc.
At a distance of a few light years, where the radio emission meets
the interstellar medium, jets from the quasars are seen. These jets
appear to travel faster than light. No, Einstein's theory is not
under scrutiny, in reality these only appear to do so. There are
two jets, one heading towards us and the other moving away. If we
calculate the sums correctly, taking into account the line of travel
of the jet and the angle to the observer and relativistic motion, the sums
work out all right. The other half of the jet moves away too quickly to
Originally it was thought that the jet is created by the strong magnetic
fields and the material comes from the accretion disc. But now it
is believed that the energy of propulsion of the jet is from the spinning
black hole and the guidance of the jet is from the magnetic field.
The short regular dip of the X-ray emission is explained as the inner part
of the accretion disc falls into the black hole; this followed by a flare
up in the radio and infrared emission. A similar mechanism also perhaps
to occur in the far away quasars. Some of these micro-quasars may
lie in the Galactic centre a mare stone‘s throw in astronomical terms
and therefore may be somewhat easier to study and can eventually lead to
the understanding of these strange objects.
Opposing the Motion:
Quasars are High Redshifted Objects
1) The Hubble's law applies for the nearby galaxies, and needs modification
if it is to be applied to far away quasars; we may have more meaningful
results, if perhaps quasars of similar spectral types are to be grouped
together when comparing their luminosity.
2) Say by observing with a telescope, we measure the size of the quasar, and
also calculate the distance. Now we know that the quasars also show
variations in radio and / or optical output and we can measure the time
period of this variation. We can cross check these parameters against
each other according to the redshift model. The emission from the
quasars is due to the motion of electrons in the magnetic field (synchrotron
radiation). From the nature of the radiation we can calculate the
size of the object. Now compare the theoretical size from the radiation
to the observational measurements. If these do not match, then we
have a problem. One can argue that perhaps we have to abandon the synchrotron
3) If we measure the redshift of a galaxy and a quasar close to each other
and both of these appear to have similar red shift, we can say that since
the distance of the galaxy is due to the cosmological expansion, we can
conclude the same for the quasar as well. But there has been reported
at least one case of two quasars close to each other on the sky, but their
separation does not correlate to their individual distances measured from
the Earth, we need to verify with the cosmological expansion model.
4) The quasars are objects with enormous mass, so they have a high gravitational
field, and the redshift is in fact gravitational redshift. Models based
on the mass distribution within the quasars (i.e. if the mass is concentrated
nearer to the centre or nearer to the outer surface) and its effect on
the redshift have been suggested, but yet to be verified by observation.
Some recent observations suggest that the space density of the quasars
do not drop at a redshift of 4. It may be possible that the light
from these far away objects are absorbed by a dust cloud; if this is the
case, the images of these quasars would be different from the nearby ones
and therefore we have to look for these with a new light. Now we
have images from the space telescopes to verify theoretical models.
It is hoped soon we shall have a better understanding of these strange
by Paritosh Maulik
NASA successfully launched a mission on 24 July, 1999 with a X-ray
telescope on board, called Chnadra, named after astronomer
S. Chnadrashekahar, to carry out a X-ray survey.
There was a long delay between the planing and deployment into orbit.
This mission is aimed to carry out the most detailed X-ray survey done
to date. Soon the information from this mission will be complimented
by an European and a Japan - US collaboration X-ray survey.
Now is a good time to jog our memories on X-ray astronomy.
A Brief History
Before the 1960's it was realised that the Sun is a source of
X-rays. Since the atmosphere of the Earth absorbs most of the
X-rays and gamma rays from reaching the ground, soon it became apparent
that detection has to be carried out above the atmosphere. Rocket-born
instruments from the US in the early 60's detected a strong
X-ray source, Scorpious X-1 in the constellation of Scorpious.
Then the flood gate opened in the 1970s; X-ray sources were found
in our galaxy from neutron binary stars. Some of these, like their
radio wave emitting counterparts, pulsars, showed a periodic variation
of X-rays. These stars being binary, we were able to calculate mass
and spin rate. Soon it became apparent that the mass is too high
to be a neutron star. This eventually led to the possible location
of black holes. Other galaxies also appear to contain X-ray sources.
Also found was a general back ground of X-ray emission. In other
words a lot of information in the X-ray range was waiting for the astronomers.
Generation of X-ray in Laboratory
If the electrons of the inner core of an atom are exited by being giving
extra energy, the electrons of the atom can go temporary to a higher energy
level but eventually drops to the ground state by a release of energy.
If the exited state is high enough, the released energy is in the form
of X-rays. Wilhelm Röntgen in Germany was the first to produce
X-rays in the laboratory. In a X-ray tube a high voltage is maintained
between the cathode and a hollow anode. The cathode gives off a beam
of electrons, these pass through the hole in the anode and strikes
another metallic target and this gives off a X-ray. The X-ray
spectrum from the target depends on the energy of the electron striking
X-rays can fog or expose photographic plates. For optimum contrast
we use special grades of photographic plates. This is the basic radio-graphy
in hospitals. With a medical X-ray, we can image a fracture in bone,
we can also image a crack in a welded joint or porosity in a casting.
For industrial applications X-rays of higher power, capable of penetrating
metals are need.
As X-rays is an electromagnetic radiation, we can also detect X-ray
by electronic devices. These electronic detectors form the image
in Computer Aided Tomography or CAT scan.
Detection of X-rays for Astronomers
The X-ray sources out there are generally weak. So we need a
large array of collimated detectors, the result being poor resolution,
but if the source is strong, we can have a collimator in front of a large
detector; life is a compromise. So like in optical astronomy, if
we have strong detector we can concentrate on a small section of the sky
and can form the image of the X-ray sources or areas of diffuse
One of the major problems with the X-ray is that, unlike the optical
beam, the refractive index of a X-ray radiation is near 1, so it
cannot be refracted by passing through a lens; however it can be reflected
by impinging the beam on a crystalline surface similar to total internal
reflection. The X-ray telescope therefore is made from a number of
small reflecting surfaces arranged to form a parabolic surface. An
incoming beam undergoes several stepwise reflections on the surface and
eventually focuses on the detector.
Since the sources are weak compared to the background, ?integrating
detectors‘ such as film are of limited use. The detection is done
by photon-by photon basis, i.e. the information is slowly build up.
Astronomers consider X-ray source to be bright, if the flux of one
photon of energy range, 1 to 10keV per square centimetre per second
is received on the Earth.
Proportional Counters: It is essentially a reduced pressure, gas cell with an arrangement of electrodes. X-ray beam with energy less than 50keV when it strikes gas atoms, it releases electrons. These electrons are attracted to the positively charged anode. These electrons, as they collide with gas, cause further ionisation of the gas and this results in a cascade of electrons to the anode. The charge picked up by the anode is the measure of the strength of the signal. These counters are very popular.
In order to boost the signal strength, back-ground correction is very important with the proportional counters. There are various methods available and the method used depends on the altitude of the satellite. If the satellite is in high-Earth orbit, the background noise is due to the solar cycle and interplanetary cosmic rays, which is mainly isotropic. On the other hand in near-Earth orbits, the effects are from cosmic rays, radioactivity and electrons from other sources. The background noise is generally low at near-Earth orbits.
Micro-Channel Plates: A type of high gain electron multipliers,
called Micro-Channel Plates or MCPs, although have been widely used
in particle physics, is a relatively newcomer in astronomy.
It is an array of about 10x106 tubes/channels, each of about 10µm
diameter formed in lead glass. To give an idea of the delicate nature
of this instrument, the length to diameter ratio of the channel is in the
range of 75:1 to 750:1. These channels are made either by drawing,
etching, or other chemical methods. X-rays interact with the channel
plate glass and the electrode to produce a signal. This is recorded.
Provided the energy of the X-ray beam is below 5keV, only one channel is
activated, leaving the neighbouring channel unaffected.
The main advantages of these detectors are high resolution, both energy
and spatial distortion free images. High gain could be achieved.
These are relatively immune to magnetic field.
Semiconductor Detectors: As the name suggests these are truly
solid state devices and offer high energy resolution. These are made
by doping semiconducting material like silicon or germanium with impurities.
These create localised imbalance of charge in the crystal and when a X-ray
beam falls on the detecting crystal, a voltage is set up, which is processed.
These crystals generally operate at very low temperatures. CCDs are
also a type of semiconductor; these detectors can offer good energy
Scintillation Counters: When a X-ray in the energy range greater
than about 20keV, falls on certain crystals, like sodium iodite (NaI) or
caesium iodite (CsI), the crystals go through a process of excitation — de-excitation and emit visible light. This optical signal is processed
by a CCD device. These crystals can be produced as relatively large
single crystals and are used for large area detectors. Controlled
amount of doping can increase the efficiency of the scintillating
At a thickness of around 5mm for both of these materials, the
detection efficiency in the range of 20 — 100keV is essentially unity.
Plastics, on the other hand, are very poor in detecting X-rays, but often
play an interesting role. The scintillating crystals generally show
decay of optical signal of the order of a microsecond, while plastics,
show decay of a nanosecond. Scintillation counters are made with
a combination of halide crystals and plastics, and the relative decay is
measured to determine the background subtraction.
The energy resolution of the scintillation detectors are not very good,
but these are very useful as large area detectors.
Similar detectors are used in CT scanners.
Phosphor Detectors: Like the scintillation counters, when a X-ray
beam falls on a phosphor screen, the screen emits visible light.
The main difference between the scintillation counters and the phosphor
screen is that the latter works in the soft or low energy X-ray range,
while the scintillation counters work in the hard or high energy X-ray
range. The phosphor screens are capable of high spatial resolution.
These screens are made by depositing powdery material of about a micron
particle size, to a thickness of about a few microns. One commonly
use material is P43 (Gd2 O2 S(Tb)). The detection efficiency
is quite good.
Negative Electron Affinity Detectors: These are semiconductor
based devices based on crystals such as gallium arsenide (GaAs), and the
surface is treated with caesium and oxygen. If a X-ray beam falls
on the crystal, the surface of the crystal can emit electrons.
These electrons are focused by electrodes on to a phosphor screen.
This concept was developed in the late 1970s and 80s and was hailed
as an ideal detector with high spatial resolution, high efficiency and
moderate energy resolution, but in order to maintain the surface
free of contamination, this device has to be operated under a very
high vacuum. This makes the device unsuitable for a large area detector
for practical purposes, but future developments may make this device to
be more user friendly.
Transition Edge Sensor (TES): It is based on superconducting properties
of metals, when cooled to very low temperatures close to absolute zero.
When a photon strikes a thin film of metal, say tungsten film about 20µm,
cooled to about 80x10-3 K, it heats up and looses its superconducting property.
By measuring the changes in the electrical properties one can calculate
the energy of the in coming photon. This devise was originally developed
for detecting elementary particles and now it has been adapted to detect
both in the optical and X-ray range. It is reported to be included
in the next generation X-ray satellite, Constellation-X.
Let us now look at a practical X-ray telescope system, AXAF: Chnadra.
The satellite was carried to low Earth orbit, about 250 km (155 miles),
and then sent to the working orbit, about one third of the distance to
the Moon. The orbit is highly elliptical: altitude, about 140,000km
(86,000 miles) furthest and about 10,000km (6,200 miles) closest to earth.
Time to orbit round the Earth is every 64 hours. This orbit will
allow the satellite to stay outside the radiation belt surrounding the
Earth for a long time. Although this radiation may not be harmful
to life on Earth, it may be powerful enough to interfere with the sensitive
instruments on board. Therefore no measurements will be taken
while the facility is within the radiation belt; however it will carry
out uninterrupted observations for 55 hours during each orbit.
The Advanced X-ray Astrophysics Facility has three major elements
the spacecraft system,
the telescope system and
the science instruments
Let us concentrate on telescope system briefly.
The telescope system: The mirrors are a highly polished set of
four nested grazing incidence cylindrical mirrors, forming a paraboloidal
and hyperboloical mirror assembly. The total number of mirrors is
8. The mirrors are made from low expansion ceramic materials and
then surface is coated with 330A° thick film of metal iridium.
The surface finish is 7A° (X-ray wavelength 0.03 — 3A°).
The mirrors are housed in a constant temperature housing. The effective
area is 400sq. cm at 1keV and resolution is 0.5arc sec. The focal
length is about 10m. At the focal plane there are two sets of instruments
1) High resolution camera: It is a Micro Channel Plate
detector of 10 sq cm. There are 69x106 lead oxide glass tubes 1·2mm
long and 10µm thick. When a X-ray beam strikes these tubes,
electrons are produced and are picked up by a wire grid to image the object.
2) 10 Charged-Couple arrays: This gives simultaneous imaging and spectroscopy. The spectroscopy can be used to identify the elements present in the X-ray source.
Additionally there are two sets of gold diffraction plates in the path
of the beam. These deflect the beam depending on the energy of the
beam and thus are used to determine the energy of the source. One
grating deflects the high to medium energy beam (0.4 — 10keV) toward
the imaging spectrometer and the other deflects the low energy beam (0.09 — 3keV) to the high resolution camera.
Some examples of stellar features where X-ray sources may be involved
are in binary star systems accreting of material one from one star to another.
This causes the material to heat up by friction and the temperature may
be high enough to release X-rays. Pulsars too can give off X-rays
in short busts and the presence of magnetic fields may cause the
X-ray beam to be highly directional.
The brightness of some stars goes through a periodic variation. This period is very regular, for example, for U Gem the optical intensity reaches to a maximum of about 100 times in around 120 days and then falls back to the original level in about 10 days. These stars are called Cataclysmic Variables or CVs.
Many of the CVs are weak X-ray sources. The material leaving the secondary can not fall straight away into the primary. This is due to the different forces acting on the accreting material which forms an accretion disc around the primary. The friction within this layer heats up the material and it slowly spirals into the primary. Once the disc hits the surface of the primary the temperature can go up as high as 106° — high enough to generate X-rays. The surface of the primary is not well defined and its nature is now currently being studied. Again magnetic fields can alter the nature of the radiation.
CVs with nuclear fusion rather than accretion is also possible. This is the case with a classical nova outburst. X-ray astronomy has revealed another group of objects: Super Soft Sources. The energy output from these sources are somewhat low at about 0.5keV. Theoretical calculations suggest that continuous nuclear reaction on the surface of the SSS may eventually lead to a supernovae explosion.
Radio observations have shown that some galaxies can emit powerful radio energy as high as 1038W. The nucleus of these galaxies also emit strong radio energy and appears to be at the centre of the observable galaxy. Jets stream out from the centre in opposite directions into the interstellar medium. Additionally a pair of lobes can de detected that stretch outside the visible limit of the galaxy. At the centre of these galaxies there are huge sources of electrons and protons with high energy. In the presence of strong magnetic fields, these particles can accelerate to nearly the speed of light. This is termed as synchrotron radiation. The is now the acceptable mechanism for the generation of the radio and X-ray emissions.
Approximately once a day there occurs sudden bursts of very powerful gamma sources. Soon after the outburst, the sources disappear, this phenomenon appears to occur at random. Very little is know about this. The sources may be either in the Milky Way or outside. The energy release from these bursts is around 1053 to 1054erg *. A similar level of energy release also take place during supernovae explosions, but of in a supernovae explosion, kinetic energy of the dust and gas cloud and visible light account for about 1050erg of the energy release and only a small fraction is associated with the X-ray energy release. In the case of GRBs all most all of the energy release is the range of X-ray / gamma-ray range.
These are the not only X-ray sources around, active nuclear galaxies, quasars, with massive black holes also may be the source of X-rays. Once we know the nature of the spectrum, we can find out about the nature of the sources. In some cases theories exist and the observations verify the theories; sometimes the other way round. As the infrared, ultraviolet and radio astronomy has complimented the optical observations, now the time of the of X-ray astronomy has already began.
Since the article was written, images have been received from the Chandra
mission and these are currently being used for proving the system.
* 1 erg = 10-7 Jule = 624 MeV; is a measure of energy.
1 watt = Jule / second is measure of power
Power = energy per unit time)
For further information http://chandra.harvard.edu/press_release.html
Some X-ray missions
Italian 1970 Uhuru
US mission July
1999 Advanced X-ray
Astronomy Facility (AXFA)
European January 2000 X-ray Multi- Mirror
Japan-US February 2000 Astro-E