Paul Butterworth

A giant periodic flare from the soft γ-ray repeater SGR1900+14

Soft γ-ray repeaters are transient sources of high-energy photons; they emit sporadic and short (about 0.1 s) bursts of ‘soft’ γ-rays during periods of activity, which are often broken by long stretches of quiescence. These objects are associated with neutron stars in young supernova remnants. The event of 5 March 1979 was the most intense burst to date, and the only one that showed a clear periodicity in the signal. Here we report the detection, on 27 August 1998, of an even more intense burst from a different soft γ-ray repeater. This event was characterized by ‘hard’ γ-rays at its peak, followed by a tail 300 s long with a soft spectrum and a clear periodicity of 5.16 s. The burst was probably initiated by a massive disruption of the crust of the neutron star, followed by an outflow of energetic particles rotating with the period of the star. A comparison of the events of 27 August 1998 and 5 March 1979 supports the idea that magnetic energy plays an important role in the genesis of such events. Although these giant flares are rare, they are not unique events and may occur at any time in a neutron stars activity cycle.

Reactivation and Precise Interplanetary Network Localization of the Soft Gamma Repeater SGR 1900+14

In 1998 May, the soft gamma repeater SGR 1900+14 emerged from several years of quiescence and emitted a series of intense bursts, one with a time history unlike any previously observed from this source. Triangulation using Ulysses, BATSE, and KONUS data gives a 1.6 arcmin2 error box near the Galactic supernova remnant G42.8+0.6. This error box contains a quiescent soft X-ray source that is probably a neutron star associated with the soft repeater.

The GRB coordinates network (GCN): A status report

The GRB Coordinates Network (GCN) continues to deliver locations of GRBs to instruments and observers in real-time (a few seconds)—while the burst is still bursting—so that they can make multi-band simultaneous follow-up observations. This was routine during the GRO-BATSE years and has resumed with HETE. This goal was realized with the optical detection of the burst counterpart for GRB990123 by the ROTSE instrument [1]. A brief review of the function and capabilities of the GCN system is given. Complementing the real-time location Notices, the GCN Circulars allow the follow-up observers to share the results of their observations rapidly with the community. A status report on recent improvements to the GCN system and a list of future improvements is given.

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.

Unusual Burst Emission from the New Soft Gamma Repeater SGR 1627–41

In June-July 1998 the Konus-Wind burst spectrometer observed a series of bursts from the new soft gamma repeater SGR 1627-41. Time histories and energy spectra of the bursts have been studied, revealing fluences and peak fluxes in the ranges of 3 ·10−7 7.5 ·10−6 erg cm−2 and 10−5 10−4 erg cm−2 s−1 respectively. One event, 18 June 6153.5s UT stands out dramatically from this series. Its fluence is ∼ 7 · 10−4 erg cm−2 and peak flux ∼ 2 · 10−2 erg cm−2 s−1. These values from a source at a distance of 5.8 kpc yield an energy output of ∼ 3 · 10 erg and maximum luminosity of ∼ 8 · 10 erg s−1, similar to the values for the famous March 5, 1979 and August 27, 1998 events. In terms of energy, this event is another giant outburst seen in a third SGR! However, this very energetic burst differs significantly from the other giant outbursts. It exhibits no separate initial pulse with a fast rise time, no extended tail, and no pulsations. It is rather similar to ordinary repeated bursts but is a few hundred times stronger in intensity. According to the magnetar model by Thompson and Duncan (1995) such a burst may be initiated by a strong starquake when a crust fracture propagates over the whole surface of a neutron star.In 1998 June-July, the Konus-Wind burst spectrometer observed a series of bursts from the new soft gamma repeater SGR 1627-41. Time histories and energy spectra of the bursts have been studied, revealing fluences and peak fluxes in the ranges 3 × 10-7 to 7.5 × 10-6 ergs cm-2 and 10-5 to 10-4 ergs cm-2 s-1, respectively. One event, 18 June 6153.5 s UT, stands out dramatically from this series. Its fluence is ~7 × 10-4 ergs cm-2, and its peak flux is ~2 × 10-2 ergs cm-2 s-1. These values from a source at a distance of 5.8 kpc yield an energy output of ~3 × 1042 ergs and a maximum luminosity of ~8 × 1043 ergs s-1 for isotropic emission, similar to the values for the famous 1979 March 5 and 1998 August 27 events. In terms of energy, this event is another giant outburst seen in a third soft gamma repeater! However, this very energetic burst differs significantly from the other giant outbursts. It exhibits no separate initial pulse with a fast rise time, no extended tail, and no pulsations. It is rather similar to ordinary repeated bursts, but is a few hundred times stronger in intensity. According to the magnetar model by Thompson & Duncan, such a burst may be initiated by a strong starquake when a crust fracture propagates over the whole surface of a neutron star.

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.

Discovery of the Optical Transient of GRB 990308

The optical transient of the faint gamma-ray burst GRB 990308 was detected by the QUEST camera on the Venezuelan 1 m Schmidt telescope starting 3.28 hr after the burst. Our photometry gives V = 18.32 ± 0.07, R = 18.14 ± 0.06, B = 18.65 ± 0.23, and R = 18.22 ± 0.05 for times ranging from 3.28 to 3.47 hr after the burst. The colors correspond to a spectral slope of close to fν ∝ ν1/3. Within the standard synchrotron fireball model, this requires that the external medium be less dense than 104 cm-3, the electrons contain more than 20% of the shock energy, and the magnetic field energy be less than 24% of the energy in the electrons for normal interstellar or circumstellar densities. We also report upper limits of V > 12.0 at 132 s (with LOTIS), V > 13.4 from 132 to 1029 s (with LOTIS), V > 15.3 at 28.2 minutes (with Super-LOTIS), and a 8.5 GHz flux of less than 114 μJy at 110 days (with the Very Large Array). Wisconsin-Indiana-Yale-NOAO 3.5 m and Keck 10 m telescopes reveal this location to be empty of any host galaxy to R > 25.7 and K > 23.3. The lack of a host galaxy likely implies that it is either substantially subluminous or more distant than a redshift of ~1.2.

Real-Time Optical Flux Limits from Gamma-Ray Bursts Measured by the Gamma-Ray Optical Counterpart Search Experiment

The Gamma-Ray Optical Counterpart Search Experiment presents new experimental upper limits on the optical flux from gamma-ray bursts (GRBs). Our experiment consisted of a fully automated very wide-field opto-electronic detection system that imaged locations of GRBs within a few seconds of receiving trigger signals provided by BATSEs real-time burst coordinate distribution network. The experiment acquired 3800 observing hours, recording 22 gamma-ray burst triggers within ~30 s of the start of the burst event. Some of these bursts were imaged while gamma-ray radiation was being detected by BATSE. We identified no optical counterparts associated with gamma-ray bursts among these events at the mV ~ 7.0-8.3 sensitivity level. We find the ratio of the upper limit to the V-band optical flux, Fν, to the gamma-ray fluence, Φγ, from these data to be 1 × 10-18 < Fν/Φγ < 2 × 10-16.

New Constraints on Simultaneous Optical Emission from Gamma-Ray Bursts Measured by the Livermore Optical Transient Imaging System Experiment

LOTIS is a gamma-ray burst optical counterpart search experiment located near Lawrence Livermore National Laboratory in California. Since operations began in 1996 October, LOTIS has responded to five triggers as of 1997 July 30, which occurred during good weather conditions. GRB 970223 (BATSE trigger 6100) was an exceptionally strong burst, lasting ~30 s with a peak at ~8 s. LOTIS began imaging the error box ~11 s after the burst began and achieved simultaneous optical coverage of 100% of the region enclosed by the BATSE 3 σ error circle and the interplanetary network annulus. No optical transients were observed brighter than the mV~11 completeness limit of the resulting images, providing a new upper limit on the ratio of simultaneous optical to gamma-ray fluence of RL<1.1×10−4 and on the ratio of simultaneous optical (at 700 nm) to gamma-ray (at 100 keV) flux density of RF<305 for a B-type spectrum and RF<475 for an M-type spectrum.

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.