Dr. Scott Barthelmy, Research Scientist, Laboratory for High Energy Astrophysics, Goddard Space Flight Center, Greenbelt, MD
- Monday, 06 July 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.
NTB: What can we learn from studying gamma-ray bursts?
Barthelmy: Well, the first thing is that, as I mentioned earlier, they’re very energetic. They’re the most energetic objects in the universe since the Big Bang. They have huge amounts of energy and they’re also very compact, so you have a lot of energy in a small volume of space, and that produces energy densities that are many, many orders of magnitude greater than what can be achieved in laboratories here on Earth. So, it’s an indirect method of getting at these high-energy densities. Granted, it’s indirect, but it’s still very useful to study these things because we can start to sort of restrict and constrain the models of the fundamental laws and physics of the universe. That’s the one reason.
There’s another reason. These happen at cosmological distances, which mean a quarter of the way to three-quarters of the way across the universe. We can study the star formation rate and galaxy formation rate all the way back into the early days of the universe, 6 to 13 billion years ago, when the universe was very young.
In a somewhat related issue, but also somewhat of a tangent, they’re useful to study to try and understand what these objects are, how often they happen, and more details about what the source objects are. We want to know because if one happens in our galaxy, we want to know about it. These things are very energetic, as I said, and if one happens in our Milky Way galaxy, on our side where the Earth’s solar system is, say within a quadrant of where we are in the galaxy, it would have pretty severe and devastating effects on the atmosphere and climate change. There has been some speculation that some of the mass extinctions that were observed in Earth’s geologic history were not caused by impacts with comets and meteors, but possibly by a nearby gamma-ray burst.
NTB: You are currently the principal investigator for NASA’s Gamma-Ray Bursts Coordinates Network (GCN). What is the Gamma-Ray Bursts Coordinates Network, and what is it used for?
Barthelmy: It’s a collection of computers and programs that collect all the information on gamma-ray bursts from all the instruments and missions that are in orbit right now. It collects it all in real time from the telemetry that is coming down from the missions, from the satellites, and it processes it, finds the gamma-ray bursts’ positions, and then sends that out in real time, with no humans in the loop, to approximately 600 people around the world who are doing gamma-ray burst follow-up activities and research. Some of them have robotic telescopes where the telescope just slews over to the latest burst location that may be only 10 or 20 seconds old. Those ground-based telescopes start taking data…some of them are radio, most of them are optical, and some of them are infrared. It also sends information to humans via email on their cell phones and pagers and directly to their desktop computers, or wherever they choose to receive it so they can be aware of what’s going on, that a burst has happened, and they can make decisions about activating some target of opportunity that they have on some telescope, either ground-based or space-based. They can then make real time, or near real time – and we’re talking minutes and tens-of-minutes – decisions about what to do with their resources in following up on the latest gamma-ray burst.
NTB: You recently proposed applying some of the technology used in the Burst Alert Telescope to homeland security. Please explain that idea to us.
Barthelmy: The Burst Alert Telescope is what’s called a coded aperture instrument, which means there are two parts to the instrument. It’s not like a traditional focusing-optics telescope. You can’t focus gamma-rays; they’re too energetic. The photons are too energetic to focus them like a traditional telescope with mirrors or lenses or a camera system, so you have to use this coded aperture technique. We have about a square meter of detectors, and located about a meter in front of that is this sort of random checkerboard pattern of open cells and lead squares. The lead squares are very dense and they stop the gamma-rays, so when a burst goes off somewhere in the universe, if it’s in the field-of-view of BAT, the gamma-rays eventually, after billions of years, make their way to Earth. Some of them hit this front aperture of the BAT instrument and the gamma-rays that hit the lead tiles stop. The ones that hit the open cells in this random checkerboard pattern go through and hit the detector, so there’s a shadow pattern cast by this random checkerboard, in gamma-ray light, on the detector array by the coded aperture. What the onboard computers and software do, when a burst goes off, is detect the shift of this shadow pattern on the detector array. The shift, and whether it’s so far to the left and so far up or whatever, tells you in instrument coordinates what direction the gamma-rays came from. You can then turn the instrument coordinates into celestial coordinates, because the spacecraft has a star tracker and knows where it’s pointing at every instant in time, so we get the celestial coordinates of the gamma-ray burst, and then we send those to the spacecraft and it decides whether to slew or not. So that’s the coded aperture technique; that’s how the basic technology of the Burst Alert Telescope works.
NTB: And you can use this, you believe, to detect nuclear weapons?
Barthelmy: Right. The coded aperture technique can be used to locate any point source of gamma-rays. Gamma-ray bursts are one point source, and yes, they’re halfway across the universe. But there are other point sources, like radioactive material, that are being shipped in railcars, or ship containers, or the cargo holds of airplanes. There are a lot of radioactive sources being shipped all the time, perfectly legitimate ones because there are lots of industrial uses and medical uses for radioactive sources, so the people responsible for homeland security have this terrible chore of trying to identify the legitimate shipments of radioactive sources from the not-so-legitimate sources, such as dirty bombs and nuclear weapons.
If you take this coded aperture, basically the BAT telescope, and you tip it over on its side, and now it’s now looking out across the Baltimore Harbor, and there are maybe 500 or 1,000 sea containers in this large field-of-view, BAT can image all of the radioactive sources. Because gamma rays are very energetic, they penetrate through the sidewalls of the shipping containers, and BAT’s coded aperture technique can make an image of those gamma-ray sources. It can locate them in X-Y-Z space to plus-or-minus 4-inches in about three seconds if the source is bright enough. By bright enough I mean about 1-curie source (3.7 x 1010 photons per second).
So, this technology – the BAT instrument’s coded aperture – can be used for homeland security to locate all of the radioactive sources, position them, tell the operators where they are, and even what kinds of radioactive sources they are, which is also an important aspect of homeland security. That helps you identify and sort out the legitimate shipments from the not legitimate shipments.
NTB: Has the Department of Homeland Security (DHS) expressed any interest in pursuing this technology?
Barthelmy: Yes. DHS itself has, and the Department of Energy and the Department of Defense have also expressed an interest. Proposals have been written and some of them have been accepted. Some of them have not been accepted. There’s also been interest from the corporate sector. I put together a large, multimillion dollar proposal recently to do radioactive source location. Unfortunately that one wasn’t accepted, but we have others that were accepted and we are working on them right now.
NTB: One of the projects you’re currently working on is a new generation of CZT (cadmium-zinc-telluride) detectors and electronics for possible use on NASA’s proposed Energetic X-Ray Imaging Survey Telescope, known as EXIST. Tell us about your work in that area.
Barthelmy: CZT detectors, like we have in the BAT instrument… This is essentially a second-generation BAT instrument. I’m working with Dr. Josh Grindley up at Harvard CFA (Center for Astrophysics) and several other people, and we’re designing and developing the technology to make a larger BAT – about 10-times bigger – and the reason for that is we want to be able to see farther in the universe, fainter GRBs (gamma-ray bursts), more GRBs. And in addition to locating gamma-ray bursts, we want to locate active galactic nuclei, so-called “AGN.” These are also powerful emitters of gamma-rays.
By having the large fields-of-view that a coded aperture system allows you to have, you can collect lots of gamma-ray bursts and lots of data on theses AGN simultaneously, and by having a much larger detector area – almost 10-times more cadmium-zinc-telluride – we can see fainter and farther.
In addition to going beyond BAT, they are also better because the CZT material is thicker. The CZT material is 5mm thick, whereas in BAT it’s only 2mm thick, and that presents some problems that require new electronics that are capable of handling the thicker detectors. What the thicker detectors allow you to do is go to higher energy gamma-rays. All materials have a certain gamma-ray opacity, and as you go to higher and higher atomic numbers, they become more opaque. But you can also go to thicker and thicker material, and that also makes it more opaque. Therefore, it increases your efficiency in capturing and recording these gamma-rays. So, the EXIST mission will extend the energy range well beyond BAT. BAT stops at roughly 150 keV; the EXIST mission will go to several hundred keV – 300 or 400 keV.
NTB: What is EXIST’s current status within NASA? Do you think it will be one of the missions eventually selected by NASA for full funding?
Barthelmy: There was a competition about a year ago involving about 35 or 37 missions, and out of that competition EXIST was one of seventeen missions selected to receive more money – study money – for an Advanced Mission Concept Study. EXIST got some of that money to do a yearlong study, to fill in some of the details, identify some of the high-risk points, and address them either through direct technology or through different design mitigation choices. That’s what we’re right in the middle of now. We’ve got about four more months of work to do and then we’ll turn in a second-round proposal. If we’re smart, and diligent, and lucky, we may get selected for a flight mission out of that second round choice process.
NTB: When will that decision be made?
Barthelmy: Probably in late 2009.
NTB: You’ve been with NASA since 1985. Of all the projects you’ve worked on in that time, which ones have given you the most personal satisfaction?
Barthelmy: I would say two of them. The Swift mission, which we’ve talked about a lot already, particularly the BAT instrument. That was the most end-to-end process for me. I started in the CZT detector development effort with several other people here at Goddard about 12 or 13 years ago, and in the early days it was very difficult to make the CZT detectors work. But we finally figured out how to get the right crystals and how to apply the right kinds of electrodes to them, and design and attach the right kind of electronics to measure these faint pulses that the gamma-rays leave in the CZT material. Then we made that into giant detector arrays, which then could be incorporated into an instrument the size of BAT. We did the coded aperture system here as well, and we wrote proposals, got them selected, got a mission – the Swift mission – and we built that with two other instruments, and we took it down to the Cape and flew it, and now we’ve been operating it for four years. That’s what I mean by “end to end.” It went from ideas and scratches on paper for a detector technology 13 years ago all the way to fire coming out of the back of the rocket and achieving a good orbit, and everything turned on and worked, and we’ve been detecting gamma-ray bursts. That’s one mission.
I would say the other one is the Gamma-Ray Bursts Coordinates Network, because that’s also an end-to-end thing. That has a little more personal satisfaction to me because it’s just me. Swift and BAT have more than a hundred people involved. The Gamma-Ray Coordinates Network, GCN, is something that I created in the early days when the GRO instrument was having trouble with its onboard tape recorders, so they switched to transmitting their telemetry in real time. I figured out there was a way to capture this real time telemetry, process it in real time, look for gamma-ray bursts, calculate positions, and then set up a system to distribute those burst positions in real time to people around the world who could make use of it. It has since grown from just that one mission – GRO – well… Missions have come and gone – we’ve had about 15 or 16 missions now, and there are 5 or 6 currently operating, producing gamma-ray burst positions in real time. So that’s the second project that I think has a lot of personal satisfaction for me, because it’s also end-to-end and it’s just a team of one. It’s my baby, so to speak.
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