RXTE observations of MXB 0656-072

ATel #1283; K. Pottschmidt (CRESST, UMBC/NASA-GSFC), V. A. McBride (Univ. Southampton), S. Suchy (CASS, UCSD), I. Kreykenbohm (IAA Tuebingen/ISDC), J. Wilms (Bamberg Observatory, Univ. Erlangen-Nuremberg), R. E. Rothschild (CASS, UCSD), P. Kretschmar (ESAC), G. Schoenherr (AIP), I. Caballero, E. Kendziorra, R. Staubert (IAA Tuebingen), J. H. Swank (NASA-GSFC)
on 16 Nov 2007; 15:04 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Katja Pottschmidt (katja@milkyway.gsfc.nasa.gov)

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RXTE is currently performing a series of snapshot observations of the recurrent X-ray transient pulsar MXB 0656-072 (= XTE J0658-073), following the report of the detection of a new outburst of this source with INTEGRAL (ATEL #1281). Here we report on the detection of the source in the first two snapshot observations which were performed on 2007 November 14 (starting MJD 54418.233) and 2007 November 15 (starting MJD 54419.488). RXTE-PCA spectra and light curves were extracted from realtime data, resulting in exposures of 592 seconds (with one PCU) and 960 seconds (with four PCUs) for the first and second observation, respectively. We performed preliminary spectral fits using an absorbed cutoff power-law plus a Gaussian iron line, with the hydrogen column density set to the canonical value of 7.28 x 10^21 cm^-2 (McBride et al., 2006, A&A, 451, 267). The 3 to 30 keV spectra from the two observations are very similar with spectral indices of 1.18 +0.08/-0.04 and 1.26 +0.03/-0.03 and cutoff energies of 17 +3/-2 keV and 21 +2/-2 keV. Note that the recent INTEGRAL-ISGRI measurements (ATEL #1281) also indicate a softer spectrum above 20 keV. A strong iron K_alpha line with an equivalent width around 300 to 400 eV is detected. The PCA light curves confirm that the source is strongly pulsating with a period of 160 to 170 seconds and a pulsed fraction of 50-60%. With a 2 to 10 keV flux of 50 mCrab in both RXTE observations the source did not become significantly brighter compared to the ISGRI observations on 2007 November 10-12. Further RXTE snapshot observations are scheduled over the next week in order to follow the evolution of this new outburst of MXB 0656-072.