The University of Wyoming GRB Afterglow Follow-Up Program

S. L. Savage1,2, M. Pierce1, R. Canterna1, A. S. Kutyrev2, J. P. Norris2

1 University of Wyoming, 2 NASA GSFC

Abstract As the Swift era approaches, the University of Wyoming in Laramie has been preparing its two research observatories for an extensive GRB afterglow follow-up program. The 2.3-m telescope at Wyoming Infrared Observatory (WIRO) is located on Jelm Mt. (2944-m elevation) in a semi-arid atmosphere, 40 km west of Laramie. On dry, cold winter nights, WIRO's sensitivity rivals that of 4-m class telescopes at more temperate sites. Three instruments are currently in use at the observatory: WIRO-Prime, WIRO-Spec, and the Goddard IR camera. WIRO-Prime is a 20482 prime-focus CCD camera with a 20 arcmin diameter FOV (f/2.1). A 5-minute exposure reaches point sources as faint as 24th magnitude in V in 1-arcsec seeing. WIRO-Spec is an integral field, holographic spectrometer which utilizes Volume-Phase-Holographic gratings with a 20482 CCD detector. A 15 X 20 array of 1 arcsec optical fibers will allow simultaneous spectroscopy over an equivalent region on the sky for rapid follow-up spectroscopy of GRB afterglows. The high system efficiency (~ 40%) should enable us to reach S/N ~ 10 for a 20th mag point source in a 10-min exposure with a typical resolution of ~ 5 Angstrom. The Goddard IR Camera is a 2562 InSb camera (FOV ~ 108 arc sec) mounted at Cassegrain and operated at ~ 15K. Available filters for GRB observations include R, I, J, H, and K’. Red Buttes Observatory (RBO) features a 0.6-m f/8 Cassegrain telescope, 19 km south of Laramie. RBO's 10242 CCD camera has a limiting magnitude of ~ 20 in a 5-minute exposure and an 18 arc minute FOV, sufficiently large for BAT localizations. At WIRO we plan to pursue afterglows with whichever instrument is in use at the time of an alert and hope to obtain a response time of ~ 120 s, comparable to Swift’s slew timescale. Together, these facilities should significantly enhance the current capabilities for rapid follow-up response to GRB events and thereby provide valuable optical and near-IR photometry and spectroscopy of the afterglows.

Motivation

To complement the GRB response from Swift, the combined efforts of the two research observatories at the University of Wyoming, WIRO and RBO, will enable rapid reaction to burst alerts, offer a large FOV for targeting bursts, and provide wavelength coverage ranging from optical through near-infrared. RBO’s automated response, with a slewing rate comparable to Swift’s, coupled with a large 18-arcmin FOV assures targeting a burst in its early afterglow stages and possibly detecting the elusive afterglow of short bursts – a feat yet to be accomplished. The large collecting power of the 2.3-m WIRO telescope and infrared optimization ensure deep imaging in wavelengths not accessible by Swift. WIRO and RBO will also be able to overlap with Swift’s coverage in the optical.

Red Buttes Observatory

RBO is located 19 km south of Laramie on a dark site which, combined with the dry, thin atmosphere (~ 2200-m elevation), enables relatively deep imaging with its Apogee AP8p 10242

CCD camera mounted to a 0.6-m Cassegrain DFM reflector. Ten-minute exposures can yield typical limiting magnitudes of ~ 19.5 (19) for V and R (B). The large 18-arcmin FOV is sufficient for rapidly acquiring and imaging Swift’s BAT error regions (~ 4 arcmin radius) and relaying the locations to WIRO for deeper imaging in the IR and spectroscopy.

Demands on RBO by the U. Wyoming Physics and Astronomy Department have greatly increased for various scientific endeavors in addition to GRB afterglow follow-up study. To realize rapid response and to modernize the facility, RBO has been extensively renovated and upgraded. Among the upgrades are refurbishment of the telescope platform and facility, acquisition of faster computers, establishment of a microwave link, and installation of a GPS clock, flat-field lights/screen, a weather station, and an all-sky camera for local weather monitoring. All of the improvements were commensurate with the goal of realizing a completely automated GRB afterglow response. Current follow-up operations require human intervention; however, total automation is planned for the near future.

WIRO, situated at 2944 m, 40 km southwest of Laramie in semi-arid conditions, is an optimal site for optical and infrared observations. The 2.3-m telescope is one of the largest with extensive availability to afterglow follow-up research. WIRO’s intermediate size allows fast acquisition strategies in comparison with larger telescopes. WIRO is currently undergoing renovations which include new and upgraded instruments as well as facility improvements. Recent additions are WIRO’s three primary instruments: WIRO-Prime, WIRO-Spec, and the WIRO-Goddard IR camera. Beyond GRB targets of opportunity, WIRO is primarily dedicated to on-going observing programs by the faculty, new graduate students, and visiting astronomers with scientific emphasis on quasars, cataclysmic variables, globular clusters, etc. For speed of acquisition the instrument in use at the time of a burst alert will be utilized to promptly pursue afterglows with photometry or spectroscopy.

WIRO INSTRUMENTATION

WIRO-Prime

• 20482 13.5-um pixel CCD

• 0.55 arcsec / pixel

• Prime-focus mount

• 20 arcmin diameter FOV (f/2.1)

• 5 minute exposure → 24th V mag with 1 arcsec seeing

• ~ 400 – 1000 nm

WIRO-Prime is pictured mounted at prime focus. The above galaxy is a scaled, color image of M101 taken with WIRO-Prime using B (10-min), V (5-min), and R (5-min). Note: The older CCD used to obtain this image had a 10242 24-um pixel CCD and 18 arcmin FOV. The new CCD has better spatial sampling with a similar FOV (see above).

WIRO-Spec

• 20482 CCD detector

• Volume-Phase-Holographic (VPH) gratings

• 293 fiber optical cables

• 1 fiber ~ 1 arcsec

• 15 × 20 fiber array

• Fibers connect Cassegrain mount to stationary spectrometer

• ~ 400 – 1000 nm

WIRO-Spec is pictured from above inside its stationary freezer. The fibers are connected to a Cassegrain platform. The above solar spectra are taken via the moon’s reflection. The curvature of the lines is an optical artifact and are straightened using software and comparison spectra. The spectra collected from individual fibers run vertically and a spectral feature (i.e., a given spectral line) run horizontally. Each fiber looks at a different area of the ~ 15” × 20” FOV. Due to its high efficiency (~ 40%), a 10-min exposure typically yields a S/N ~ 10 with a resolution of ~ 5 Angstrom for a 20th magnitude point source.

WIRO-Goddard IR Camera

• 2562 InSb detector

• Operates at 15K

• Cassegrain mount

• ~ 108 arcsec FOV

• R, I, J, H, K’

To left are pictures of the WIRO-Goddard IR camera. It must be vacuum-pumped and cooled to 15K with liquid nitrogen and liquid helium for operation. The false color image of Jupiter at right was composed using the IR camera under heavy cloud with three filters (J, K’, and Br).

References: 1Bromm, V. & Loeb, A. 2002 ApJ, 575, 111 2Lamb, D.Q. & Reichart, D.E. 2000 ApJ, 536, 1

3Panaitescu, A., Kumar, P., Narayan, R. 2001 ApJ, 561, L171

Because of the transient nature of bursts, GRB follow-up research must be performed on a target-of-opportunity basis. Both observatories, within an hour of Laramie, are owned by the university making rapid and numerous prompt observations as well as long-term follow-up monitoring more feasible. Short bursts have yet to be associated with optical counterparts due to their predicted steep light curves and intrinsically low initial brightness.3 Acquiring light curves for short bursts with the rapid reaction capabilities of RBO would be a significant contribution. In addition to imaging and photometry, spectroscopy is available for determining redshifts of high z burst sources – a necessary ingredient for modeling. GRBs are predicted to occur at considerable rates beyond z ~ 5, thus making them primary candidates as probes for cosmology in the study of the Lyman-α forest, the epoch of reionization, the evolution of metallicity, and large-scale, high-redshift structure.1,2

Prior to the involvement of WIRO in the GRB program, RBO collected data on several GRBs in collaboration with the Follow-Up Network (FUN) GRB group in primary affiliation with Dan Reichart of the University of North Carolina, Chapel Hill. Eight GCN notices have been archived from the University of Wyoming with detection or magnitude contributions. WIRO will continue the lightcurve analysis to deeper magnitudes than RBO, add infrared capability (J, H, and K bands), and spectroscopy for determination of high redshift burst afterglows.

Above left are images taken from RBO of the brightest and one of the closest bursts – GRB030329. Below, its lightcurve is shown as measured by RBO. The results of RBO monitoring have contributed eight GCN notices to date.

Wyoming Infrared Observatory

RESULTS & EXPECTATIONS

Acknowledgements: This research has been supported by NSF grant AST 00-97356, NASA EPSCoR grant NCC5-578, and NASA grant NAG5-11191.

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