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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.
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.
Motivation
Wyoming Infrared Observatory
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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.
Red Buttes Observatory
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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.
Results & Expectations
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-&alpha 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 top 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.
WIRO INSTRUMENTATION
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&bull 20482 13.5-um pixel CCD
&bull 0.55 arcsec / pixel &bull Prime-focus mount &bull 20 arcmin diameter FOV (f/2.1) &bull 5 minute exposure &rarr 24th &bull V mag with 1 arcsec seeing &bull ~ 400 – 1000 nm |
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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.
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&bull 2562 InSb detector
&bull Operates at 15K &bull Cassegrain mount &bull ~ 108 arcsec FOV &bull R, I, J, H, K’ |
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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&gamma). | |
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
Acknowledgements
This research has been supported by NSF grant AST 00-97356, NASA EPSCoR grant NCC5-578,
and NASA grant NAG5-11191.
