GRB 070125: The AAVSO High Energy Network in Action!

Gamma ray bursts remain one of the most enigmatic classes of variables in the universe. We have much evidence that these high energy events herald the birth of a new black hole, either from the collapse of a supermassive star in a hypernova, or from the merger of two smaller black holes or neutron stars. But even though we've made enormous progress in understanding these objects in the last ten years, the detailed physics behind them remains a mystery. Much of this mystery is caused by the short duration of the phenomenon itself. The burst of gamma rays for which these objects are named never last more than a few hundred seconds, and some come and go within just one second! You have to be quick to even catch one of these things in the act, let alone study them. However, if a gamma-ray burst produces an afterglow at other wavelengths, they often persist for hours or days later, letting astronomers study the after-effects of these violent events long after the gamma rays have faded.

The process of localizing gamma ray bursts has been greatly improved, particularly with the launch of modern satellites dedicated to their detection. Often, the coordinates of gamma ray bursts can be localized to within a few arcminutes on the sky, and these coordinates can then be transmitted to ground-based observers within moments of the burst, giving us a fighting chance at catching these secretive objects. Unfortunately, bursts with visible counterparts at other wavelengths are less common than those without; often the gamma-ray portion of the outburst is the only thing that gets detected, and the afterglows at other wavelengths are never detected. So when "a good one" comes along, they tend to generate a lot of excitement, and many observations too!

An artist's conception of the Swift satellite in action. The AAVSO High Energy Network uses gamma ray burst localizations from Swift and other satellites to search for optical afterglows of these elusive variable stars.

Such is the case with GRB 070125, a recent, bright, long-duration gamma ray burst. This burst was initially localized by a number of interplanetary spacecraft with gamma ray monitors aboard, known as the Interplanetary Network. The satellites that detected this burst included Mars Odyssey, Suzaku, RHESSI, INTEGRAL, and Swift. The gamma ray behavior of this burst is typical of the long-soft class of bursts, although the length of the burst (at least seventy seconds) and its brightness were exceptional. Also of interest, the burst was highly structured in time, with two significant peaks of emission lasting from T=0 to T=13 seconds, and T=30 to T=50 seconds. But what was most exciting was the presence of bright X-ray and optical afterglows. Swift detected an X-ray afterglow more than 12 hours after the initial burst, and Palomar's Robotic 60-inch (P60) telescope detected this burst in the optical the day after it happened. A subsequent spectrum from the Gemini North telescope in Hawaii places GRB 070125 at a redshift greater than z ~ 1.54, billions of light years away.

AAVSO HEN observers Stefano Sposetti, Douglas Durig, and Bruce Gary began imaging the afterglow on the 26th and 27th, and clearly detected the afterglows as well. We released three GCN Circulars on this burst (GCN 6048, 6051, and 6067) showing the evolution of the burst light curve up to three days after the burst; observations were made to fainter than 21st magnitude! The light curve below shows the data from the AAVSO HEN observations plotted against Rc-band observations from other GRB observing programs. Data from all of the GRB networks including AAVSO HEN will be used by astrophysicists trying to understand the origins of gamma ray bursts, and how they fit into the wider picture of star formation, extragalactic astronomy, and cosmology.

The AAVSO is thrilled to see the great results of the follow up observations of this burst, and they prove that amateur observers and small telescopes have a major contribution to make to this important field of research. If you are interested in participating in the AAVSO High Energy Network, or are simply interested in learning more about gamma ray bursts and other high-energy phenomena, please visit the webpage of the AAVSO High Energy Network.

Clear skies, & good observing!

Matthew Templeton,
AAVSO

Optical light curve of GRB 070125, including CV and CR observations from the AAVSO High Energy Network, and Rc observations from the P60, TNT, Mitsume, and Loiano Observatory follow-up groups.