Gamma-Ray Bursts Dazzle Astronomers

Gamma-Ray Bursts Dazzle Astronomers

by Simon Mitton

WHAT'S HOT IN PHYSICS...

Rank	Paper	Citations This Period Jan-Feb 99	Rank Last Period Nov-Dec 98
1	T. Banks, et al., "M theory as a matrix model: a conjecture," Phys. Rev. D, 55(8):5112-28, 15 April 1997. [Rutgers U., Piscataway, NJ; U. Texas, Austin; Stanford U., CA] *WV250	32	1
2	S. Perlmutter, et al., "Discovery of a supernova explosion at half the age of the Universe," Nature, 391(6662):51-4, 1 January 1998. [16 institutions worldwide] *YP888	27	†
3	H.L. Lai, et al., "Improved parton distributions from global analysis of recent deep inelastic scattering and inclusive jet data," Phys. Rev. D, 55(3):1280-96, 1 February 1997. [Michigan St. U., Lansing; Argonne Natl. Lab., IL; Southern Methodist U., Dallas, TX; Florida St. U., Tallahassee; U. Oregon, Eugene] *WF297	24	2
4	C. Alcock, et al., "The MACHO Project Large Magellanic Cloud microlensing results from the first of two years and the nature of the galactic dark halo," Astrophys. J., 486(2):697-726, 10 September 1997. [12 institutions worldwide] *XX173	24	†
5	S.R. Kulkarni, et al., "Identification of a host galaxy at redshift z = 3.42 for the g-ray burst of 14 December 1997," Nature, 393(6680):35-9, 7 May 1998. [9 institutions worldwide] *ZM028	24	†
6	D.J. Schlegel, D.P. Finkbeiner, M. Davis, "Maps of dust infrared emission for use in estimation of reddening and cosmic microwave background radiation foregrounds," Astrophys. J., 500(2):525-53, 20 June 1998. [U. Durham, U.K.; U. Calif., Berkeley] *ZX419	23	†
7	J.D. Lowenthal, et al., "Keck spectroscopy of redshift z ~ 3 galaxies in the Hubble deep field," Astrophys. J., 481(2):673-88, 1 June 1997. [U. Calif., Santa Cruz] *XB884	21	†
8	M.R. Metzger,et al., "Spectral constraints on the redshift of the optical counterpart to the g-ray burst of 8 May 1997," Nature, 387(6636):878-80, 26 June 1997. [Caltech, Pasadena; Natl. Radio Astron. Observ., Socorro, NM; Inst. Space Astrophys., Frascati, Italy; U. Ferrara, Bologna, Italy] *XG416	19	†
9	S.S. Gubser, I.R. Klebanov, A.M. Polyakov, "Gauge theory correlators from non-critical string theory," Phys. Lett. B, 428(1,2):105-14, 28 May 1998. [Princeton U., NJ] *ZY402	18	†
10	E. Costa, et al., "Discovery of an X-ray afterglow associated with the g-ray burst of 28 February 1997," Nature, 387(6635):783-5, 19 June 1997. [12 institutions worldwide] *XF144	17	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

stronomy turned in a storming performance this period, capturing seven of the Physics Top Ten, in a near rout which has all but seen off string theory, now represented only at #1 and #9. Three papers, #5, 8, and 10 showcase the solution to one of the great mysteries of high-energy astrophysics, the nature of gamma-ray bursts.

First detected in the 1960s by the U.S. military Vela satellites, gamma-ray bursts (GRBs) are mysterious flashes of high-energy radiation that appear from random directions in space and typically last a few seconds. They are unpredictable and can come from any part of the sky. Until 1991 astrophysicists assumed they must come from collapsed neutron stars in the Milky Way. But by 1992 NASA's Burst and Transient Source Experiment mapped the distribution of GRBs for the first time. This showed that the sources were either uniformly spread through the halo of the Milky Way, or from the remote universe. Determining the location of these bursts became a fascinating detective story, which is solved in #5, 8 , and 10.

The solution began with the 1996 launch of the Italian-Dutch BeppoSAX satellite. Its battery of experiments had one aim: detect and locate GRBs. On February 28, 1997 the All-Sky Monitor registered a hit in the 40-700 keV range. By good fortune one of the Wide Field Cameras (WFC) covering ~5% of the sky also recorded an X-ray flash. In #10, the Italian-Dutch team describe a race against time to match the position of the X-ray flash against a visible object. BeppoSAX had narrowed the field of search to an error box 3 arc min wide. Within eight hours, the Narrow Field Instruments on BeppoSAX were registering an X-ray afterglow. This followed a power-law decay function (consistent with an expanding envelope of high-energy debris following an initial explosion), which was undetectable three days later.

The position information from BeppoSAX subsequently enabled an international observing team led by Jan van Paradijs, University of Amsterdam, to lock down on a faint galaxy. Within 21 hours of the GRB they were swinging the 4.2m William Herschel Telescope of La Palma Observatory onto the source. They detected an optical transient in a galaxy billions of light years away.

With a single coincidence in one galaxy, the case for extragalactic GRBs was not an overwhelming one. But the BeppoSAX WFC grabbed the X-ray signature of another GRB on May 8, 1997. Howard Bond, Space Telescope Science Institute, Baltimore, found a faint optical variable source at the same location. Armed with this information Mark Metzger, Caltech, and collaborators used the Keck Observatory to obtain a spectrum. As described in #8, the distant galaxy had intergalactic absorption lines at z=0.835, strengthening the case for a remote origin for GRBs, but still not the "clincher."

Hot Paper #5 records that for the GRB of December 14, 1997, the Caltech team led by Shrinivas Kulkarni were on the case within 13 hours. At the Keck Observatory they followed the decay of an optical transient, which had the power-law of a simple fireball. Bright moonlight thwarted attempts to get a spectrum of the transient itself. Subsequent observations gave z=3.42 for the underlying host galaxy. This converts to a distance of 12 billion light years. If the burst was isotropic, the total energy released was about 16% the rest mass energy of the Sun, or 40 times as much energy as the Sun will radiate in its entire lifetime.

On January 23, 1999, the Caltech rapid-response team took a wake-up call at 5.00 am and were observing within three hours. A robotic telescope at the University of Michigan had imaged the optical transient within 22 seconds of GRB. This event is the current record-holder: an optical burst 9 billion light years away but so bright it could have been seen with binoculars. For a minute or so it was a thousand times more luminous than a quasar!

Although GRBs are dethroned from the galactic halo, there may be MACHOs out there according to Charles Alcock. In #4 he gives a report on two years of photometric monitoring of stars in the Large Magellanic Cloud. This is demanding surveillance work, with 8.5 million stars being checked for a characteristic up-tick in their brightness which would indicate a MAssive Compact Halo Object (MACHO) drifting across the line of sight. The time scale for the brightening ranges from 34 to 145 days. The MACHO collaboration describe eight candidate microlensing events, which is about six times higher than the rate expected from known stars. The implied halo mass in MACHOs is 2 x 10¹¹solar masses, in the form of objects of about half a solar mass, most probably white dwarfs.

Dr. Simon Mitton is the Senior Fellow of St Edmund's College,
Cambridge University Press, Cambridge, UK.