The first detection of a GRB afterglow by UK observers

Gamma ray bursts have intrigued and mystified astronomers since their discovery in the late 1960s. As recently as 1997 even the basic question of their distance was a matter of heated debate amongst researchers. Were they relatively nearby, in a halo around the Milky Way, or at cosmological distances? The highly isotropic distribution of events on the sky tended to favour the cosmological interpretation but this would have meant that GRBs were extremely energetic. Other evidence suggested that the events were nearby and rather less extreme.

In 1997 February, for the first time, the afterglow of a GRB was seen at optical wavelengths. Shortly afterwards the first redshift was measured at z=0.83 proving the cosmological hypothesis correct and implying that GRBs are by far the most luminous phenomena known in the universe. Since then great efforts, and considerable amounts of telescope time, have been devoted to studying GRB afterglows, or 'optical transients'. From the few dozen afterglows which have been investigated in detail, the main, although still tentative, conclusions are that GRBs seem to be associated with the deaths of massive stars, and that the explosions themselves involve the ejection of beams of plasma at highly relativistic velocities. The highest recorded redshift is for GRB000131 at z=4.5. The most luminous burst seen, in both gamma rays and optical, was GRB990123 which peaked at R=8.9, despite being at a redshift of z=1.6. (Gamma ray bursts are identified by the six-digit date - in astronomical format - on which they are first detected.)

Progress is difficult because GRBs are unpredictable and they fade rapidly. They are currently found by satellites at a rate of one every month or two and some characteristics, such as a roughly power-law decline of intensity, are seen in most bursts. Other aspects, such as the rate of decline and the optical magnitude at early times, seem to vary considerably from burst to burst. As a rule of thumb, even 'bright' bursts have usually faded to mag 20 or so after a day.

It is very important to obtain early photometric observations of GRB afterglows. The first few hours after the burst are interesting because they can provide information about conditions relatively close to the GRB. Observing the transient when it is still relatively bright also allows us to use GRBs as a probe of the intervening material between us and the event. At their fastest, the satellite-determined positions (which may be good to several arcminutes) are distributed to researchers via the Internet within a matter of seconds. However, most Earthbound professional telescopes take some time to respond, and of course geographical location and factors such as the weather mean that only a limited number of telescopes will have a chance of making early observations of any given burst. Clearly there is an exciting opportunity for amateur observers with their worldwide distribution to make an important contribution to this field. Many amateurs not only have equipment - large, computer-controlled telescopes with CCD cameras - which is capable of obtaining photometry to magnitude 20, but they have also already shown themselves capable of undertaking programmes to monitor such targets as supernovae, novae and other variable objects.

Amateur successes

The American Association of Variable Star Observers (AAVSO) operates a worldwide GRB alert service for amateur astronomers, and they have been sponsored by NASA to liaise with amateurs elsewhere, including those in the UK. In this country, GRB follow-up has been promoted by one of us (NRT) and it has been coordinated by The Astronomer group.

A number of amateur astronomers have obtained images of GRB afterglows. Notably, observers at the Nyröla Observatory in Finland have attempted more than a dozen in the last two years and successfully imaged several. UK observers started to look for GRB afterglows late in 2000 but a combination of badly placed objects and misfortune with the weather has resulted in frustration until recently.

UK success at last - GRB021004

  On Friday, 2002 October 4, NASA's HETE-II satellite detected a GRB and sent out approximate coordinates within a few seconds. In this instance observers using the 48-inch (120cm) telescope on Mount Palomar located the afterglow in less than 10 minutes - the second earliest optical detection of any GRB. However, even at that time, the magnitude was R=15.5 and fading fast.

By late afternoon in the UK a TA e-circular was issued requesting observations of the burst. The location of the burst was in Pisces so it was well placed for observation as it got dark. Even better, the sky was clear over much of the country. The Sun set at 17.40 UT and before it was dark observers started to train their telescopes on the field. At 18.44 UT Nick James made the first detection of the afterglow (left) at around 18th magnitude. Mark Armstrong (Kent) was having some problems with his telescope but he rapidly obtained the second detection at 18.48. As the evening progressed more telescopes were brought into use and by midnight images had been submitted by Tom Boles (Suffolk), Martin Mobberley (Suffolk), David Strange (Dorset), Peter Birtwhistle (Berkshire), Eddie Guscott (Essex) and by observers at the University of Hertfordshire Observatory.

Since the afterglow was well placed it was possible to follow it through the night and one of us (NDJ) produced the lightcurve shown here. This shows that the GRB was declining by about 0.8 magnitudes for every doubling of the time since the burst. Tom Boles imaged the afterglow again (below) on 2002 October 6, 56 hours after the burst, when the magnitude had fallen to around 19.5. This was brighter than would have been expected given the decline on the first night.

On following days professional spectroscopy showed the GRB to be at a redshift of z=2.3. This is the third largest redshift ever measured for a GRB and it is equivalent to a look-back time of 80% of the age of the universe, about 11 billion years. The photons from this event left the GRB well before our Solar System existed! A pretty amazing catch for amateurs using relatively small telescopes from back gardens in the UK.

The future

For the next year, alerts from HETE-II and INTEGRAL will provide further opportunities for amateurs to observe, and possibly even discover, these fascinating afterglows. Near the end of 2003 the Swift satellite, a US/UK/Italian collaboration, is expected to provide accurate locations for several GRBs per month, with the positions being beamed to Earth, and then around the world in a matter of seconds. An onboard optical telescope will provide some early-time photometry, in blue and ultraviolet bands, but there will still be a requirement for monitoring at redder wavelengths. The sheer number of bursts will be difficult for professional astronomers to follow up, but will provide potentially rich pickings for amateurs.

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