Dr. Scott Barthelmy, Research Scientist, Laboratory for High Energy Astrophysics, Goddard Space Flight Center, Greenbelt, MD

Monday, July 06 2009

Dr. Scott Barthelmy is the principal investigator for the Burst Alert Telescope (BAT), a sophisticated instrument that detects and precisely locates elusive gamma-ray bursts in the universe. Developed as part of NASA’s Swift mission, the instrument technology is now being considered for a variety of homeland security applications because of its ability to pinpoint and identify nuclear materials – both legal and illegal – in transit or storage. Dr. Barthelmy also created the Gamma-Ray Bursts Coordinates Network (GCN) to distribute data collected on gamma-ray bursts to researchers throughout the world in real time.

NASA Tech Briefs: You’ve been credited with creating the Burst Alert Telescope. What is the Burst Alert Telescope and how does it work?

Dr. Scott Barthelmy: Well, first I’d like to say that, while I was the lead scientist on the Burst Alert Telescope on the Swift mission, there were many other people involved. Approximately 70 or 80 people were involved in the project and they all played important roles, contributing greatly to the design and success of the BAT instrument. I just want to get that on the record.

The Burst Alert Telescope is one of three instruments on the Swift mission. It was launched four years ago (Nov. 20, 2004) to study gamma-ray bursts in the universe. It’s a wide-field-of-view instrument, about a hundred degrees field-of-view. It needs to look at a large region of the sky to catch these gamma-ray bursts, and when one goes off it quickly calculates the position inside the BAT instrument and sends it to the spacecraft, which then autonomously decides – with no humans in the loop – if it’s safe to slew to this new location and point the other two instruments that are on the Swift observatory. There’s an x-ray telescope and an optical ultraviolet telescope. These are narrow field-of-view instruments – about a third of a degree.

So when BAT locates something in its 100-degree field of view, the spacecraft has to slew 10, 20, 50 degrees to point these two narrow field-of-view at the burst location. It does that within 20 to 70 seconds after the start of the burst so that the two narrow field-of-view instruments can be on target and observe the tail-end of the gamma-ray burst itself and the afterglow emission that lasts for minutes, hours, days, sometimes weeks after the original gamma-ray burst.

NTB: What, exactly, are gamma-ray bursts, and what causes them?

Barthelmy: Gamma-ray bursts are very brief and intense flashes of gamma-rays, which are like x-rays, only more energetic. They’re farther up the electromagnetic spectrum. When I say brief, they may be a fraction of a second to a couple-of-hundred seconds long. The average is about 20 seconds long. When I say they’re intense, when a gamma-ray burst is actually bursting, it’s putting out more energy per second than all of the other stars in all of the galaxies in the universe combined. That makes them very interesting objects.

Gamma-ray bursts only happen once for whatever the source object is. There are two theories as to what the source objects are for gamma-ray bursts. One of them is the collapse of a massive star, very much like a supernova only more massive and, therefore, there’s more energy involved. We’re talking about 20 to 50 solar masses! Sometimes they’re called “hypernova,” which is sort of an extension of the supernova model. The other source object is mergers of neutron stars and/or black holes. You have neutron stars that are orbiting each other and they lose energy due to gravitational radiation. They spiral inward close enough so that they actually touch and merge into a single object. That can be either two neutron stars or a neutron star and a black hole. When that happens, you also get these brief bursts of gamma-rays.