Swift Detects a New Outburst of the Millisecond Accreting Pulsar SAX J1808.4-3658
ATel #3733; C. B. Markwardt (NASA/GSFC), D. M. Palmer (LANL), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC), E. A. Hoversten (PSU), H. A. Krimm (USRA/GSFC), B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU) and B.-B. Zhang (PSU)
on 4 Nov 2011; 04:44 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Craig B. Markwardt (Craig.Markwardt@nasa.gov)
Subjects: X-ray, Request for Observations, Binary, Neutron Star, Transient, Pulsar
We report the detection of a new outburst of the accreting millisecond X-ray pulsar, SAX J1808.4-3658 with the Swift Burst Alert Telescope (BAT). This source is a 401 Hz pulsar in a binary orbit of period 2.01 hr. It has recurrent outbursts every 2-3 years, the last one occurring in October, 2008.
At 02:55:12 UT, BAT triggered and located a source (trigger=506961). The BAT on-board calculated location is RA, Dec 272.114, -36.991 which is, RA(J2000) = 18h 08m 27s, Dec(J2000) = -36d 59' 26", with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). This position is consistent with the known position of SAX J1808.4-3658 to within 0.7 arcmin.
BAT reported a persistent flux level over an 896 second exposure interval, indicating this BAT trigger was probably not due to an X-ray burst. The BAT Transient Monitor measurements shows that the source has been steadily rising in the 15-50 keV band from quiescence since about 2011-10-31, to a level of about 100 mCrab presently. This may indicate a stronger than usual outburst (since previous outbursts typically peaked at less than 100 mCrab). The most recent observation of the PCA bulge scan monitoring program occurred on 2011-10-30 and showed a 95% upper limit of about 0.2 mCrab (2-10 keV). Thus, SAX J1808.4-3658 has entered a new outburst phase.
Swift is not expected to slew to the target automatically, so no detailed Swift XRT or UVOT products are expected to be immediately available. Hard X-ray event data from BAT may be available after the following downlinks. Further observations are encouraged.
