The Next Generation Rapid-Optical-NIR Response GRB Observatory (NGRG)
Dr. Bruce Grossan of the University of California Space Sciences
Lab presents the concept of the NGRG.
- NGRG Paper accepted 2014 Aug 222014 Grossan, Kumar, Perley, Smoot. You can find a nice copy with figures embedded here. (More formally: http://arxiv.org/abs/1402.4856)
What is the NGRG?
Swift is well past its design lifetime, and a replacement is required to continue to furnish rapid GRB locations for the community. The NGRG concept utilizes a beam-steering mirror (instead of moving the entire spacecraft, as Swift does) to point the telescope beam extremely rapidly, ~ 1 s including settling time. Swift/UVOT takes more than 60 s to respond to a GRB trigger.
|Swift: > 60 s response time. |
|NGRG: ~ 1 s Response Time
Swift Response Time is Finite, and Misses the Rise Phase of Most GRBs.
The NGRG beam-steering mirror is designed to be observing in ~ 1 s.
In addition to providing Swift-like alerts for the community, there are two new science focuses:
- Rapid-response science: looking at the rise phase of the Opt-NIR light curve, rarely measured
- NIR measurements: NIR is very sensitive in space, and more sensitive than optical for extinguished GRBs
What's In The Paper?
We wanted to show the value of a Rapid-Reponse to GRB trigger in Optical-NIR Observatory. We point out the science one can do and
we took the trouble to carefully and conservatively show the feasibility of such an observatory, even if of modest size and cost, via the caluclation of event rates in a very conservative way. Along the way,
- We define a Swift X-ray Super Sample of the "Early UVOT Brightness Distribution Sample", just below
- We define the Early UVOT Brightness Distribution Sample, about the most uniform, sizable sample of early GRB optical observations.
- We look at how you might observe dynamic dust destruction with optical and NIR filter channels.
Please use the samples above for further study and visit our data page.
What are the Observatory Goals and Particular Strengths?
The NGRG is intended to be a very modest observatory for an era of limited fund. Given the known properties of GRBs, we can make a modest mission with a fast OIR (optical-near-IR) beam steering system that produces lots of detection and new science. Surprisingly, with a relatively small X-ray instruments (1024 sq. cm) and OIR telescope (30 cm) we can produce more than one OIR GRB detection each month. This is a very conservative estimate based on actual swift data, not a theoretical projection. Taking advantage of our beam-steering system, we don't even need a precision-stabilized spacecraft (see below).
The Swift observatory is a boon to science, but its response time is finite. Most bursts are responded to in ~ 100s, with a hard minimum of about 60 s response (see responsehistogram above). This is not fast ehough to catch the rise of most bursts. We are aiming to systematically survey the first ~100s of OIR emission to undersand the prompt (rise) phase, the transition to afterglow emission, and all that comes with it.
What is Rapid-Repsonse Science?
Most theoretical discussions of GRB emission mechanisms start with a bifurcation: either OIR emission correlates or does not correlate with the "gamma ray burst" (Note 1). So, there is quite an emphasis on "prompt" or simultaneous X/gamma and OIR emission. Considering how important this is, it's unfortunate we don't have a systematic survey of prompt emission. At this time, it's really only when we get lucky, e.g. when gamma emission lasts longer than ~100 s, typically when most of the emission is over. ... Until I add more here, I recommend you read the paper, but ...by looking at the ratio of X/gamma to OIR emission you can learn a lot about emission mechanisms, baryon vs. magnetic jets, an independent measure of the bulk Lorentz factor, and several other topics.
What is this about NIR and extinguished bursts?
The majority of bursts are extinguished (look at Perley+09). Therefore, to have a good systematic understanding of GRB, you have to be able to probe some distance into the parameter space of extinguished bursts. This is more easily done with NIR emission, which helps to look at redder wavelenghts, where the extinction is less. I am very excieted about the NGRG's ability to dynamically observe dust destruction. By measuring an optical to NIR color of the emission with rapid response, you see the effects of dust extinction change as the dust is evaporated; this is only possible with two colors, preferably including NIR, and only possible with rapid-response (the entire process is believed to be over in ~ 60 s).
Come on, this is some blue-sky absurdly optimistic prediction. You won't ever get those detection rates.
Not so. After we've seen overly optimistic projections that don't stand up to scrutiny, we've based our entire analysis on actual Swift measurements. Most of our analysis for rates was made by using only Swift measurements, selected by our instrument's capabilities. For the NIR rates, we scaled from swift's ACTUAL performance, using GRB spectra and measured QE curves.
We had to examine hundreds of BAT light curves and run them through our triggering algoritm to get our X-ray detection and location rates. We defined and studied a systematic sample of Swift early optical observations to determine the very conservative Early Sample of GRB brightness at ~ 140 s after trigger. This forms the basis of our extremely conservative numbers on early observations.
Why aren't there more details here? Where are the pretty pictures?
I promise more on this website regularly. But there is lots in the paper and you can always email me and ask me about it, I'm happy to respond.
Note 1: There is some indication OIR is better correlated with >100keV emission than 15-100keV BAT emission, i.e. one needs to be careful about the "gamma-ray burst" in which band.
Dr. Bruce Grossan
Lawrence Berkeley Lab 50R-5005
1 Cyclotron Road
Berkeley, CA 94720
Office Location for visits: 50-5005D
bruce [at] singu[dot]lbl[dot]gov [anti-spam format address]
Fax -please esnd scans by email.
Mail comments to bruce [at] singu[dot]lbl[dot]gov [anti-spam format address]
Last Update: 2014 Sep 9