X-ray and γ-ray astronomy
astronomy
Isabelle A Grenier
Philippe Laurent
he Universe, as seen in X- and y-rays, is a very exotic place, T largely filled with extremely hot gas, with temperatures of 1()6 to 108 K, and dotted with cosmic accelerators of all sizes launching particles to relativistic speeds. One can see bright eruptions from young stars, admire cosmic laser shows with beams of radiation circling in the sky from spinning neutron stars, watch matter fall inside a black hole or «miss" the hole and shoot out at nearly the speed of light, follow matter blasted away by giant stellar eXplosions or by titanic explosions when neutron stars and black holes collide and coalesce. On a quieter but even more energetic scale, one can witness the colossal merging of clusters of galaxies.l In these exceptional conditions, far beyond the dreams of the 19th & mid-20th century physicists, both hot gas and relativistic particles are intimately related and their joint observation in X- and y-rays bears new diagnostics to investigate these extraordinary media. Most X- and y-rays are absorbed in the Earth's atmosphere, and so must be detected from space-borne telescopes. Only the highest-energy y-rays (those above 50 GeV) can be observed from the ground by means of the particle showers they initiate in the upper atmosphere. New X-ray instruments, such as Chandra andXMM-Newton2, are revealing hot plasmas with unprecedented angular (sub-arcsecond) & spectral precision. Detailed maps ofthe density, temperature, «chemical", and velocity distributions of the hot gas are derived from precise spectroscopic line measurements from many atoms in various ionisation states. The y-ray telescopes still struggle to catch sparse y-photons one by one and strive for sensitivity and angular resolution in the realm of arcminutes to degrees. Yet, the late Compton Observatory in space and the ground-based instruments (e.g. Whipple, CAT, Cangaroo, Celeste)3 have revealed many powerful cosmic accelerators and have used the penetrating power of y-rays to deeply probe the conditions inside accelerator cores, otherwise hidden from view at other wavelengths.
-
There is now convincing evidence that black holes exist in two
varieties: stellar-mass ones, produced by the implosion of the
core of a massive star at the end ofits life, and super-massive ones
weighing 1()6 to 109 solar masses, lurking at the centres of galax
ies. Whether black holes of intermediate mass exist (with 1()l to
11)5 solar masses), and why, are still hotly debated. The gathered
evidence indicates that black holes of all masses show strikingly
similar behaviour. They attract matter from their environment
(stellar companion or host galaxy) into a thin disc of gas that spi.
rals inwards; heated byturbulent friction, it brightly shines in UV
and X rays. The hungrier the black hole, the softer the radiation!
During violent flare episodes, black holes expel highly collimated
jets of plasma accelerated to relativistic speeds. Their synchro
tron radiation is seen from the radio to X rays. Inside the jets,
e+-e- pairs are further accelerated to TeV energies and shine pro
fusely in y-rays. Given these basic ingredients, a source can,
however, show up in many disguises from different points ofview
because of screening by intervening matter and relativistic
effects. Sources possess different states and can change from one
to another within hours or days, supposedly because they "eat"
more or less. Thus it took forty years to disentangle the underly
ing continuity in the wide variety of sources seen at all
wavelengths and this daunting task is far from complete today!
The huge energy release, of lOZ9 W to Hyl W in stellar systems
and 1()36 W to Hrl W in super-massive ones, originates from the
intense gravitational potential of the black hole. Interestingly,
neutron stars accreting matter from a companion star develop
similar features. With increasing gravitational energy, jet plasma
is propelled to o.sc by neutron stars, 0.9c by stellar black holes,
0.995c by super-massive black holes, and over 0.99999c by y-ray
bursts (c being the speed oflight). Jets from stellar systems extend
over light-years while the galactic ones span millions of light
years. Which mechanisms can generate the acceleration required
to maintain jet collimation over such distances? What triggers the
ejections? Theorists are at a loss, but answers will hopefully come
in the near future with greatly improved observations.
When a star explodes into a supernova, matter is ejected at
thousands ofkm S-I and the released energy of 1()44 J is enough to
power the Sun for 8 billion years! The blast wave rams into the
surrounding interstellar gas and heats it up to loa K while a
reverse shockwave travels back to the centre and heats the ejecta
up to 107K. Thus the whole remnant becomes a bubble ofhot gas
that brightly shines in X rays. It provides an attractive laboratory
for atomic physics and thermodynamics, illustrating how ionisa
tion slo (...truncated)