Shedding Light on Dark Gamma Ray Bursters

Dark gamma ray burst GRB020819. Image credit: Keck. Click to enlarge.
Virtually everything we know about the Universe comes to us through the agency of light. Unlike matter, light is uniquely suited to travel the vast distances across space to our instruments. Most astronomical phenomena however are persistent and repeatable – we can rely on them to “hang around” for long-term observation or “come back around” on a regular basis. But this isn’t so for gamma ray bursts (GRB’s) – those mysterious cosmological events that supercharge photons (and sub-atomic particles) with absurdly high energy levels.

The first detected celestial GRB occurred during nuclear arms treaty monitoring in 1967. That event required years of analysis before its extraterrestrial origin was confirmed. After this discovery, primitive triangulation methods were put in place using detectors located on various space probes within the Interplanetary Network (IPN). Such methods required a great deal of number crunching and made instant follow-up using Earth-based instruments impossible. Despite the delays involved, hundreds of gamma ray sources were catalogued. Today – even using the Internet – it would still require several days to respond using an IPN-type detection approach.

All this began to change in 1991 when NASA put the Compton Gamma Ray Observatory (CGRO) into space using space shuttle Atlantis as part of its “Great Observatories” program. Within four months of scanning the sky, CGRO made it clear to astronomers that the Universe underwent sporadic and widely distributed gamma ray paroxysms on an almost daily basis – paroxysms caused by cataclysmic events that hurl vast quantities of gamma and other high-energy radiation across the abyss of space-time.

But CGRO had one main limitation – although it could detect gamma rays and alert astronomers quickly, it wasn’t particularly accurate as to where such events happened in space. Because of this large “error circle”, astronomers were unable to locate the visible light “afterglow” of such events. Despite this limitation, CGRO went on to detect hundreds of continuous, periodic, and episodic gamma ray sources – including supernovae, pulsars, black holes, quasars, and even the Earth itself! Meanwhile CGRO also discovered something unsuspected – certain pulsars acted as narrow band transmitters of gamma rays without accompanying visible light – and therein lay astronomer?s first sense of “dark” GRBs.

Today we know that “dark pulsars” are not the only “dark” sources of gamma rays in the Universe. Astronomers have determined that some small portion of episodic (one-time-only) GRBs are also low in visible light, and they – like anyone tickled by the unusual and inexplicable – want to know why. In fact GRB’s are so unique that aficionados may often be heard saying “When you’ve seen one GRB, you’ve seen one GRB”.

The first satellite to simplify optical detection of GRB afterglows was BeppoSAX. Developed by the Italian Space Agency in the mid 1990’s, BeppoSAX launched April 30, 1996 from Cape Canaveral and continued to detect and pinpoint X-ray emission sources until 2002. BeppoSax’s error circle was small enough to enable optical astronomers to rapidly track down many GRB afterglows for detailed study in visible light using earth-based instruments.

BeppoSAX re-entered the Earth’s atmosphere April 29, 2003, but by this time NASA’s replacement (HETE-2 the High Energy Transient Explorer-2) was already several years on station in low-earth orbit. Instrument’s on HETE-2 (its first incarnation HETE failed to separate from the third stage of its Pegasus rocket in 1996) expanded the range of X-ray detection and provided even tighter error circles – just the thing astronomers needed to improve their response time in locating GRB afterglows.

Two years and a few months later (Monday, August 19, 2002) HETE-2 set off the bells and whistles as a strong gamma ray source was detected somewhere near the head of the constellation Pisces the Fishes. That event (designated GRB 020819) caused a series of astronomical observatories to begin capturing radio-frequency, near-infrared, and visible light photons in an effort to determine just where the event occurred and help make sense of the phenomenon driving it.

According to the paper “The Radio Afterglow and Host Galaxy of the Dark GRB 020819” published May 2, 2005 by an international team of investigators (including Pall Jakobsson of the Niels Bohr Institute, Copenhagen Denmark who proofed this article), within 4 hrs of detection the 1 meter Siding Spring Observatory (SSO) telescope in Australia was turned to a region of space less than 1/7th the apparent diameter of the Moon. 13 hours later, a second, slightly larger instrument – the 1.5 meter P60 unit at Mt. Palomar – also joined the chase. Neither instrument – despite capturing light as faint as magnitude 22 – caught anything unusual for that region of space. However a large and extremely photogenic 19.5 magnitude face-on barred spiral galaxy fell nicely within the grasp of their instruments.

Fifteen days later, the 10 meter Keck ESI instrument on Mauna Kea, Hawaii imaged the same region in blue and red light down to magnitude 26.9. At this optical depth, a distinct 24th magnitude “blob” (suspected to be an HII star-formation region) could be seen 3 arc seconds north of the spiral galaxy. A final attempt to detect anything further was made January 1, 2003 – again using the Keck 10 meter. No change was seen in optical light emanating from the region of GRB 020819. All this confirmed that no visible afterglow accompanied the gamma ray outburst detected by HETE-2 some 134 days earlier. The investigating team had their “dark gamma ray burster”. Later would come the task of figuring out just what the heck it was – or at least was not…

Periodically throughout the cycle of optical and near-infrared inspection, the region of the burst was monitored in radio-wave frequencies. Using the VLA (Very Large Array – consisting of 27 Y-configured 25 meter dishes located fifty miles west of Socorro, New Mexico) the team succeeded in capturing a dwindling trail of 8.48 Ghz radiation and identified its locale.

First radio waves from GRB 020819 were collected 1.75 days after the HETE-2 alert. By day 157, rf energy levels flattened to the point where the source could no longer be seen with confidence. However by this time, its location had been pinpointed to the “blob” three arc-seconds north of the core of the previously uncharted spiral galaxy. Unfortunately – due to its faintness – the distance to the blob itself could not be determined spectrographically – however the galaxy was found to lie some 6.2 BLY away and enjoys “high-confidence” in terms of having a relationship with the source.

As a result of such investigations astronomers are now learning more and more about a class of cataclysmic events that results in massive fluxes of high and low energy photons while almost completely skipping intermediate frequencies – such as ultraviolet, visible, and near-infrared of light. Is there anything that could account for this?

Based on learning from GRB 020819, the team explored three fireball-shock models of how dark GRBs might occur. Of the three (an even expansion of high energy gases into a homogenous medium, even expansion into a stratified medium, and a collimated jet penetrating either type medium), the best fit against GRB 020819 behaviors was that of an even expansion of high energy gases into a homogenous medium of other gases (a model first proposed by the astrophysicist R. Sari et al in 1998). The virtue of this isotropic-expansion model being (in the words of the investigating team) that “only a modest amount of extinction must be invoked” to account for the absence of visible light.

In addition to narrowing the range of possible scenarios associated with dark GRBs, the team concluded that “GRB 020819, a relatively nearby burst, is only one of two of the 14 GRB’s localized to within (2 arc minutes using) HETE-2 that does not have a reported OA. This lends support to the recent proposition that the dark burst fraction is far lower than previously suggested, perhaps as small as 10%.”

Written by Jeff Barbour