A Small, Rapid Optical-IR Response Gamma-Ray Burst Space Observatory Concept: The NGRG

After Swift, there is no sure plan to furnish a replacement for the rapidly disseminated, high-precision GRB positions it provides, nor a new type of observatory to probe new GRB parameter space. We propose a new GRB mission concept, the Next Generation Rapid Optical–NIR (near infrared) Response GRB Observatory (NGRG) concept, and demonstrate, through analysis of Swift BAT data, studies of new GRB samples, and extinction predictions, that a relatively modest size observatory will produce valuable new measurements and good GRB detection rates. As with Swift, GRBs are initially located with a coded-mask X-ray camera. However, the NGRG has two distinguishing features: first, a beam-steering system to begin optical observations within ∼1 s after location; second, in addition to the optical camera, a separate near-IR (NIR) camera viewing the same field, greatly increasing sensitivity to extinguished bursts. These features yield the unique capability of exploring the rise phase of GRB optical-NIR emission. Thus far, among GRBs with optical afterglow detections, a peak is measured in only ∼26–40% of the light curves. The rise time for prompt, or pre-afterglow, optical emission is rarely measured, as is the transition to afterglow emission. Prompt or pre-afterglow NIR emission is even less frequently measured. Rapid-response measurements give new tools for exploration of many science topics, including optical emission mechanisms (synchrotron vs. SSC, photospheric emission) and jet characteristics (reverse vs. forward shock emission, baryon-dominated vs. magnetic dominated). The rapid-response capability also allows measurement of dynamic evolution of extinction due to vaporization of progenitor system dust. This dynamic dust measurement is the only tool we know of to separate the effects of star-system-scale dust and galactic-structure-scale dust; it is remarkable that this probe of small-scale phenomena can be used at the high redshifts where GRBs are observed. In this paper, we discuss techniques and the feasibility of these measurements, and give detection rate estimates using only measured Swift performance (without extrapolations). The NGRG will explore two new frontiers: optical and NIR GRB emission measured earlier than ever before, via rapid-response, and potentially fainter, more extinguished GRBs than ever before, via sensitive, early NIR measurements. In an era with little funding for new extragalactic science space missions, costs are important. Our modest NGRG concept will produce new GRB science, while providing crucial access to rapid GRB alerts for the community. An X-ray instrument barely 1/5 the detecting area of Swift BAT, 1024 cm^2, will yield a significant fraction of BAT’s GRB detection rate: more than 65 X-ray detections per year. With a 30 cm optical-IR telescope and modern cameras, more than 19 NIR and 14 optical band detections would be produced each year for community follow-up. In addition, active control of the beam-steering system, via feedback from a fast-read optical camera, would remove the need for arcsec pointing stabilization of the spacecraft platform, for a substantial cost saving and a wider range of potential space platforms