Robert L. Kehoe

Observation of contemporaneous optical radiation from a gamma-ray burst

The origin of γ-ray bursts (GRBs) has been enigmatic since their discovery. The situation improved dramatically in 1997, when the rapid availability of precise coordinates, for the bursts allowed the detection of faint optical and radio afterglows — optical spectra thus obtained have demonstrated conclusively that the bursts occur at cosmological distances. But, despite efforts by several groups, optical detection has not hitherto been achieved during the brief duration of a burst. Here we report the detection of bright optical emission from GRB990123 while the burst was still in progress. Our observations begin 22 seconds after the onset of the burst and show an increase in brightness by a factor of 14 during the first 25 seconds; the brightness then declines by a factor of 100, at which point (700 seconds after the burst onset) it falls below our detection threshold. The redshift of this burst, z ≈ 1.6 (refs 8, 9), implies a peak optical luminosity of 5× 1049u2009ergu2009s−1. Optical emission from γ-ray bursts has been generally thought to take place at the shock fronts generated by interaction of the primary energy source with the surrounding medium, where the γ-rays might also be produced. The lack of a significant change in the γ-ray light curve when the optical emission develops suggests that the γ-rays are not produced at the shock front, but closer to the site of the original explosion.

Northern Sky Variability Survey: Public Data Release*

The Northern Sky Variability Survey (NSVS) is a temporal record of the sky over the optical magnitude range from 8 to 15.5. It was conducted in the course of the first-generation Robotic Optical Transient Search Experiment (ROTSE-I) using a robotic system of four comounted unfiltered telephoto lenses equipped with CCD cameras. The survey was conducted from Los Alamos, New Mexico, and primarily covers the entire northern sky. Some data in southern fields between declinations 0° and -38° are also available, although with fewer epochs and noticeably lesser quality. The NSVS contains light curves for approximately 14 million objects. With a 1 yr baseline and typically 100–500 measurements per object, the NSVS is the most extensive record of stellar variability across the bright sky available today. In a median field, bright unsaturated stars attain a point-to-point photometric scatter of ~0.02 mag and position errors within 2. At Galactic latitudes |b| < 20°, the data quality is limited by severe blending due to the ~14 pixel size. We present basic characteristics of the data set and describe data collection, analysis, and distribution. All NSVS photometric measurements are available for on-line public access from the Sky Database for Objects in Time-Domain (SkyDOT) at Los Alamos National Laboratory. Copies of the full survey photometry may also be requested on tape.

Rotse all sky surveys for variable stars I: test fields

The Robotic Optical Transient Search Experiment I (ROTSE-I) experiment has generated CCD photometry for the entire northern sky in two epochs nightly since 1998 March. These sky patrol data are a powerful resource for studies of astrophysical transients. As a demonstration project, we present first results of a search for periodic variable stars derived from ROTSE-I observations. Variable identification, period determination, and type classification are conducted via automatic algorithms. In a set of nine ROTSE-I sky patrol fields covering roughly 2000 deg2, we identify 1781 periodic variable stars with mean magnitudes between mv = 10.0 and mv = 15.5. About 90% of these objects are newly identified as variable. Examples of many familiar types are presented. All classifications for this study have been manually confirmed. The selection criteria for this analysis have been conservatively defined and are known to be biased against some variable classes. This preliminary study includes only 5.6% of the total ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will include more than 32,000 periodic variable stars.

The ROTSE‐III Robotic Telescope System

The observation of a prompt optical flash from GRB 990123 convincingly demonstrated the value of autonomous robotic telescope systems. Pursuing a program of rapid follow-up observations of gamma- ray bursts, the Robotic Optical Transient Search Experiment (ROTSE) has developed a next-generation instrument, ROTSE-III, that will continue the search for fast optical transients. The entire system was designed as an economical robotic facility to be installed at remote sites throughout the world. There are seven major system components: optics, optical tube assembly, CCD camera, telescope mount, enclosure, environmental sensing and protection, and data acquisition. Each is described in turn in the hope that the techniques developed here will be useful in similar contexts elsewhere.

Prompt Optical Observations of Gamma-Ray Bursts

The Robotic Optical Transient Search Experiment (ROTSE) seeks to measure simultaneous and early afterglow optical emission from gamma-ray bursts (GRBs). A search for optical counterparts to six GRBs with localization errors of 1 deg2 or better produced no detections. The earliest limiting sensitivity is mROTSE>13.1 at 10.85 s (5 s exposure) after the gamma-ray rise, and the best limit is mROTSE>16.0 at 62 minutes (897 s exposure). These are the most stringent limits obtained for the GRB optical counterpart brightness in the first hour after the burst. Consideration of the gamma-ray fluence and peak flux for these bursts and for GRB 990123 indicates that there is not a strong positive correlation between optical flux and gamma-ray emission.

A Search for Early Optical Emission from Short- and Long-Duration Gamma-Ray Bursts

Gamma-ray bursts of short duration may harbor vital clues to the range of phenomena producing bursts. However, recent progress from the observation of optical counterparts has not benefited the study of short bursts. We have searched for early optical emission from six gamma-ray bursts using the telephoto array on the Robotic Optical Transient Search Experiment I. Three of these events were of short duration, including GRB 980527, which is among the brightest short bursts yet observed. The data consist of unfiltered CCD optical images taken in response to Burst and Transient Source Experiment triggers delivered via the GRB Coordinates Network. For the first time, we have analyzed the entire 16° × 16° field covered for five of these bursts. In addition, we discuss a search for the optical counterpart to GRB 000201, a well-localized long burst. Single-image sensitivities range from 13th to 14th magnitude around 10 s after the initial burst detection and from 14 to 15.8 mag 1 hr later. No new optical counterparts were discovered in this analysis suggesting short-burst optical and gamma-ray fluxes are uncorrelated.

SDSS J124602.54 + 011318.8: A Highly Luminous Optical Transient at z = 0.385*

We report the discovery of a highly luminous optical transient (OT), SDSS J124602.54 + 011318.8, associated with a galaxy at a redshift of 0.385. In this paper, we consider the possibility that the OT may be a gamma-ray burst (GRB) afterglow. Three sets of images and two sets of spectra were obtained as part of the normal operations of the Sloan Digital Sky Survey (SDSS). In the first two image sets, observed two nights apart, the object appears as a point source at r* ≈ 17. The third image set, observed about 410 days later, shows an extended source which is more than 2.5 mags fainter. The spectra were observed about 400 and 670 days after the first two image sets, and both show an apparently normal galaxy at a redshift of 0.385. Associating the OT with the galaxy, the absolute magnitude was M = -24.8, which is over 4 mag brighter than the most luminous supernova ever measured. The spectral energy distributions of the galaxy-subtracted OT derived from the first two image sets are well-fitted by single power laws with indices of βν = -0.92 and -1.29, respectively, similar to most GRB afterglows. Based upon the luminosity of the OT, nondetections in contemporaneous ROTSE I images and the change in spectral slope, the OT, if an afterglow, was likely discovered early during a plateau or slowly fading phase. The discovery of a GRB afterglow at this stage of the SDSS is consistent with expectations, but only if the optical emission is much less strongly beamed than the gamma rays. We emphasize that other explanations for the OT cannot be ruled out; a recent follow-up study by Gal-Yam et al. (2002) provides strong evidence that this source is in fact an unusual active galactic nucleus (AGN).

An Untriggered Search for Optical Bursts

We present an untriggered search for optical bursts with the Robotic Optical Transient Search Experiment (ROTSE-I) telephoto array. Observations were taken that monitor an effective 256 deg2 field continuously over 125 hr to mROTSE = 15.7. The uniquely large field, moderate limiting magnitude, and fast cadence of ~10 minutes permit transient searches in a new region of sensitivity. Our search reveals no candidate events. To quantify this result, we simulate potential optical bursts with peak magnitude mp at t = 10 s, which fade as f = , where αt < 0. Simple estimates based on observational evidence indicate that a search of this sensitivity begins to probe the possible region occupied by gamma-ray burst (GRB) orphan afterglows. Our observing protocol and image sensitivity result in a broad region of high detection efficiency for light curves to the bright and slowly varying side of a boundary running from [αt,mp] = [-2.0, 6.0] to [-0.3, 13.2]. Within this region, the integrated rate of brief optical bursts is less than 1.1 × 10-8 s-1 deg-2. At ~22 times the observed GRB rate from BATSE, this suggests a limit on θopt/θγ 5, where θopt and θγ are the optical and gamma-ray collimation angles, respectively. Several effects might explain the absence of optical bursts, and a search of the kind described here but more sensitive by about 4 mag should offer a more definitive probe.

Observations of the Optical Counterpart to XTE J1118+480 during Outburst by the Robotic Optical Transient Search Experiment I Telescope

The X-ray nova XTE J1118+480 exhibited two outbursts in the early part of 2000. As detected by the Rossi X-Ray Timing Explorer (RXTE), the first outburst began in early January and the second began in early March. Routine imaging of the northern sky by the Robotic Optical Transient Search Experiment (ROTSE) shows the optical counterpart to XTE J1118+480 during both outbursts. These data include over 60 epochs from January to June 2000. A search of the ROTSE data archives reveal no previous optical outbursts of this source in selected data between 1998 April and 2000 January. While the X-ray-to-optical flux ratio of XTE J1118+480 was low during both outbursts, we suggest that they were full X-ray novae and not minioutbursts based on comparison with similar sources. The ROTSE measurements taken during the 2000 March outburst also indicate a rapid rise in the optical flux that preceded the X-ray emission measured by the RXTE by approximately 10 days. Using these results, we estimate a preoutburst accretion disk inner truncation radius of ~1.2 × 104 Schwarzschild radii.

Rapid optical follow-up observations of SGR events with ROTSE-I

In order to observe nearly simultaneous emission from Gamma-ray Bursts (GRBs), the Robotic Optical Transient Search Experiment (ROTSE) receives triggers via the GRB Coordinates Network (GCN). Since beginning operations in March, 1998, ROTSE has also taken useful data for 10 SGR events: 8 from SGR 1900+14 and 2 from SGR 1806-20. We have searched for new or variable sources in the error regions of these SGRs and no optical counterparts were observed. Limits are in the range m_ROTSE ~ 12.5 - 15.5 during the period 20 seconds to 1 hour after the observed SGR events.