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RXTE PCA Detections of Transient Activity of X-ray Bursters in the Galactic Center Region

ATel #255; C. B. Markwardt (U. Maryland & NASA/GSFC), J. H. Swank (NASA/GSFC)
on 20 Mar 2004; 02:16 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

Subjects: X-ray, Binary, Globular Cluster, Neutron Star, Transient

Referred to by ATel #: 256, 262, 567, 637, 2425, 2729, 5319

This report is a summary of recent transient activity in the galactic center region, as measured by scanning observations by the RXTE PCA instrument. The specific sources are: SAX J1747.0-2853, GRS 1747-312, RX J170930.2-263927, 4U 1711-34 (= 2S 1711-339), and 4U 1728-34 (= GX 354-0).

SAX J1747.0-2853 was detected in outburst on 19.81 Mar 2004 (MJD 53083.81). The previous outburst was seen in September 2001, although it is debatable whether this source entered true "quiescence" or maintained a low level of activity. The current flux (220 mCrab) is the highest seen since the bulge scans started in Feb 1999.

The bursting and eclipsing source GRS 1747-312 in the globular cluster Terzan 6 has entered one of its quasi-regular outbursts. The flux has risen from 9 mCrab on Mar 13.01 to 32 mCrab on Mar 19.77 (MJD 53077.01 to 53083.77). Based on previous experience, this outburst should last 3-4 weeks. The apparent peak flux for this outburst is typical.

The X-ray burster RX J170930.2-263927, in the globular cluster Terzan 5, has entered into an outburst starting before Mar 6 (MJD 53070). The flux has steadily increased from 6 mCrab, as measured on that date, to 32 mCrab on Mar 19.83 (MJD 53083.83). The previous outburst of this source was in Feb 2002, suggesting a mean recurrence period of 2-3 years.

The position of the potential quiescent counterpart to 4U 1711-34 (= 2S 1711-339) has recently been reported by Torres et al. in X-rays (ATEL #233, ATEL #238) and in R-band (Tomsick, ATEL #244). We can report that this source has recently undergone a brief outburst (2.5 weeks duration). Since this source is at the edge of the PCA scanning region, the X-ray flux precision depends on the scanning direction, which alternates every ~3.5 days. We estimate that the peak flux was between 13-29 mCrab between Mar 3-6 (MJD 53067-53070).

We confirm the recent transient increase of 4U 1728-34 (= GX354-0) as reported by Zurita et al. (ATEL #248). The source reached a peak flux of 385 mCrab on Mar 10.04 (MJD 53074.04), and has since declined to a flux of 120 mCrab on Mar 19.76 (MJD 53083.76), which is now within the typical range for this source since the start of monitoring by the PCA in 1999.