Recent activity of the Rapid Burster (MXB 1730-335)

ATel #1398; E. Kuulkers (ESA/ESAC, Spain), S. Brandt (DNSC, Denmark), C. Markwardt (GSFC, USA), R. A. Remillard (MIT/MKI), S. Shaw (Southampton, UK/ ISDC, Switzerland), V. Beckmann (ISDC, Switzerland), J. Chenevez (DNSC, Denmark), T. J.-L. Courvoisier (ISDC, Switzerland), A. Domingo (LAEFF/INTA, Spain), K. Ebisawa (ISAS, Japan), P. Jonker (SRON, The Netherlands), P. Kretschmar (ESA/ESAC, Spain), T. Oosterbroek (ESA/ESTEC, The Netherlands), A. Paizis (INAF-IASF, Italy), D. Risquez (LAEFF/INTA, Spain), C. Sanchez-Fernandez (ESA/ESAC, Spain), R. Wijnands (UvA, The Netherlands)
on 26 Feb 2008; 16:40 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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Observations taken as part of the INTEGRAL Galactic bulge monitoring program (see ATel #1385) on 2008, February 23 13:32-17:13 (UT) showed bursting activity from the X-ray transient, the Rapid Burster (MXB 1730-335). During one of the 1800-sec pointings when the source was in the field-of-view of JEM-X a series of 6 X-ray bursts were observed, once every 250-350 sec, with durations of 30-60 sec and peak fluxes of about 0.6-0.9 Crab (3-10 keV). In between the bursts, no clear persistent emission is seen with the JEM-X, with upper limits of about 20 mCrab (3-10 keV). During another 1800-sec pointing, about 2 hours later, a single, triangular shaped burst was seen, lasting for up to about 500 sec, reaching about 0.6 Crab (3-10 keV). We note that such bursts are not uncommon for the Rapid Burster (see, e.g., Basinska et al. 1980, ApJ 241, 787). We ascribe these burst events as being of type II. The average 18-40 keV IBIS/ISGRI flux was 9+/-2 mCrab on February 23, while it was not detected during earlier INTEGRAL Galactic bulge monitoring observations taken on February 11 and 20 (with typical upper limits of about 10 mCrab, 18-40 keV).

During two 90-sec RXTE/ASM dwells on February 23 at 21:52 and February 24 09:14 (UT) high count rates of about 0.3 Crab (2-12 keV) were seen, interspersed with dwell non-detections. The 1-sec light curves within these bright dwells do not show obvious bursts, suggesting the events last longer than the dwell duration. The bright data points give rise to daily averages of 64+/-6 mCrab and 52+/-7 mCrab, on February 23 and 24, respectively. Between February 18-22 and on February 25 the source was not significantly detected in the daily averages with upper limits of up to about 20 mCrab. The current daily average on February 26 results in a detection at 42+/-7 mCrab.

During the most recent RXTE/PCA Galactic bulge scan on February 25 21:22 (UTC) the source is detected at about 19+/-2 mCrab (2-10 keV). An X-ray burst was also seen. No significant persistent emission was seen in the RXTE/PCA scans from the Rapid Burster since July 2006, with a typical upper limit of about 6 mCrab (2-10 keV).

The reported activity may be either the relic of an outburst which peaked near the end of November 2007 (as judged from the RXTE/ASM light curve; see also the source activity notes by the RXTE/ASM team at http://xte.mit.edu/XTE/xte_anno.html ), or it may signify the start of a new outburst. In the latter case, it is at odds with the fact that the last four outbursts occurred about every 170 days: the current activity is seen about 90 days after the start of the previous outburst. Note, however, that changes in the outburst recurrence time have been seen before in this source (e.g., Masetti, 2002, A&A 381, L45).