Burst Alert Telescope Instrument Research Articles

On multiple classes of gamma-ray bursts, as deduced from autocorrelation functions or bivariate duration/hardness ratio distributions

Autocorrelation functions (ACFs) of 119 gamma-ray bursts (GRBs) monitored by the Burst Alert Telescope (BAT) on Swift are calculated. Contrasting with previous results for smaller numbers of bursts from other missions, the widths of the ACFs are not bimodally distributed. Although the distribution appears slightly asymmetrical, underlying mixtures of distributions can also probably be ruled out. Factors contributing to differences between the results presented here, and those in the literature, may include the differences in energy passbands used, and the superior sensitivity of the BAT instrument (which affects e.g. the redshift distribution of the detected GRB). The second part of the paper is concerned with the fitting of mixtures of bivariate Gaussians to the joint duration/hardness ratio data of 325 GRBs. A careful analysis confirms that a three-component mixture is the statistically most acceptable, but it is shown that the implied marginal distribution of the hardness ratios does not fit the data very well. It is also stressed that mixture components cannot automatically be assumed to represent different classes of GRBs. The point is illustrated by showing two substantially different, but statistically almost equivalent, mixture models for the distribution of 571 BAT-determined GRB durations.

Discovery of a short orbital period in the Supergiant Fast X-ray Transient IGR J16479–4514

Abstract We report here the discovery of a 3.32 d orbital period in the Supergiant Fast X-ray Transient (SFXT) source IGR J16479–4514. Using the long-term light curve of this source obtained with Swift Burst Alert Telescope (BAT) in the energy range of 15–50 keV, we have clearly detected an orbital modulation including a full eclipse of duration ∼0.6 d. In the hard X-ray band of the BAT instrument, the eclipse ingress and egress are rapid. We have also used the long-term light curve obtained with the Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) in the energy range of 1.5–12 keV. Taken independently, the detection of orbital modulation in the RXTE–ASM light curve is not significant. However, considering a clear detection of orbital modulation in the BAT light curve, we have used the ASM light curve for a more precise determination of the orbital period. IGR J16479–4514 has the shortest orbital period among the three SFXTs with measured/known orbital period. We discuss the implication of a short orbital period with the various mechanisms proposed to explain the transient nature of this class of sources.

Swift BAT Instrument Thermal Control System Recovery after Spacecraft Safehold in August 2007

The Swift mission Burst Alert Telescope (BAT) Detector Array thermal control system includes two propylene loop heat pipes (LHPs), eight ammonia constant conductance heat pipes (CCHPs), a radiator that has AZ-Tek's AZW-LA-II low alpha white paint, and precision heater controllers that have adjustable set points in flight. The Power Converter Box (PCB) and Image Processor Electronics (IPE) boxes (a primary and a redundant) of the BAT have Z93P white paint radiators. Swift was successfully launched into orbit on November 20, 2004. The spacecraft (S/C) was placed into a safehold mode on August 10, 2007 after an anomaly on inertial reference unit (IRU) #3. It was returned to inertial pointing on August 16 and instrument power up followed. This paper presents a thermal assessment of the BAT instrument thermal control system (TCS) shut down and recovery as a result of the S/C safehold mode. The recovery required starting up the LHPs manually.

Using GRO J1655-40 to test Swift/BAT as a monitor for bright hard X-ray sources

While waiting for new gamma-ray burst detections, the Burst Alert Telescope (BAT) on board Swift covers each day ~50% of the sky in the hard X-ray band (``Survey data''). The large field of view (FOV), high sensitivity and good angular resolution make BAT a potentially powerful all-sky hard X-ray monitor, provided that mask--related systematics can be properly accounted for. We have developed and tested a complete procedure entirely based on public Swift/BAT software tools to analyse BAT Survey data, aimed at assessing the flux and spectral variability of bright sources in the 15-150 keV energy range. Detailed tests of the capabilities of our procedure were performed focusing, in particular, on the reliability of spectral measurements over the entire BAT FOV. First, we analyzed a large set of Crab observations, spread over ~7 months. Next, we studied the case of GRO J1655-40, a strongly variable source, which experienced a 9-month long outburst, beginning on February 2005. Such an outburst was systematically monitored with the well-calibrated PCA and HEXTE instruments on board the RXTE mission. Thanks to the good BAT temporal coverage of the source, we have been able to cross-check BAT light-curves with simultaneous HEXTE ones. The Crab tests have shown that our procedure recovers both the flux and the source spectral shape over the whole FOV of the BAT instrument. Moreover, by cross-checking GRO J1655-40 light-curves obtained by BAT and HEXTE, we found the spectral and flux evolution of the outburst to be in very good agreement. Using our procedure, BAT reproduces HEXTE fluxes within a 10-15% uncertainty with a 3sigma sensitivity of ~20 mCrab for an on-axis source, thus establishing its capability to monitor the evolution of relatively bright hard X-rays sources.

The Swift γ-ray burst MIDEX mission

The Swift satellite, a NASA Medium-Class Explorer (MIDEX) mission scheduled for launch in July, 2004, is a multi-wavelength observatory for rapid-response observations of astrophysical transients. Swift is composed of three co-aligned telescopes covering an energy range from 7 eV (170 nm) to 150 keV. The Burst Alert Telescope (BAT) is a coded aperture hard X-ray imager, which serves as the γ-ray burst (GRB) trigger for Swift. The Narrow Field Instruments (NFIs) are the Ultra-Violet/Optical Telescope (UVOT) and the X-ray Telescope (XRT). The Swift satellite has the ability to rapidly maneuver to point all there instruments at a bursting source within a minute of detection. Rapid burst alerts will be distributed through the GRB Coordinates Network (GCN) and all Swift data will become public as soon as it is processed. Secondary science for Swift includes detecting astrophysical transients on time scales of from 5 min to several hours and immediately alerting the astrophysical community of such events. Furthermore, the BAT instrument will be used for a high sensitivity hard X-ray all-sky survey which is expected to detect as many as 400 Active Galactic Nuclei (AGN).