Swift and RXTE observations of MXB 0656-072

ATel #1293; J. A. Kennea (PSU), P. Romano (INAF-OAB & Univ Bicocca), K. Pottschmidt (CRESST-UMBC/GSFC), J. Wilms (Dr. Remeis-Observatory Bamberg), J. Cummings (CRESST-UMBC/GSFC), P. Evans (U Leicester) and D. N. Burrows (PSU)
on 20 Nov 2007; 15:19 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 17:40:37 UT on November 16th, 2007, Swift/BAT triggered on the transient source MXB 0656-072 (Burrows et al., GCN #7097), which has recently been reported to be in outburst by INTEGRAL (ATEL #1281). At 17:53:55 UT XRT began a 1.5ks observation of the field of MXB 0656-072, and revealed a bright variable point source detection at a position coincident with the optical counterpart of MXB 0656-072 reported by Pakull et al. (ATEL #202). AT 20:39:51 UT, XRT performed a 2.3ks observation with XRT forced into Photon Counting mode in order to obtain an accurate X-ray localization of the source, which was followed by a 3.5ks observation in Window Timing Mode starting at 22:13:57 UT.  From analysis of the PC mode data, the following position was found for MXB 0656-072:

RA(J2000) = 06h 58m 16.8s
Dec(J2000) = -07d 12m 37.8s

with an estimated uncertainty of 4.1 arc-seconds radius (90% containment). We note that this position is 5.5 arc-seconds away from the GSC2.2 S100221215, reported by Pakull et al. (ATEL #202) to be a Be star counterpart of MXB 0656-072 (although the spectral type of the star is actually O9.7Ve), so the optical counterpart lies outside of the XRT error circle. Therefore without a firm positional coincidence, we cannot rule out the possibility that the previously reported counterpart of MXB 0656-072 is an unrelated to the X-ray transient.

As with RXTE, Swift/XRT data shows a strong modulation in the X-ray light-curve with a period of ~160 seconds. In addition to this, the lightcurve shows strong non-periodic variability, with the source varying by almost an order of magnitude in flux during the observations. The average flux of the source over the entire observation was 1.6 x 10^-9 erg/s/cm² (0.3-10 keV) or ~80 mCrab. Fitting the WT mode data to an absorbed power-law model to the data gives the following spectral parameters: N_H = 0.6 (+/-0.05) x 10²² cm-2, Photon Index = 0.86 +/- 0.04. The addition of a gaussian line to the spectral model did not significantly improve the fit.

In a continuation of the monitoring of the source (see Pottschmidt et al., ATEL 1283), MXB0656-072 was observed with RXTE on 2007-11-17 starting at 18:13UT (MJD 54420.943) for 1.5 ks and on 2007-11-18 at 23:24UT (MJD 54422.976) for 0.9 ks. In both cases, the data can be described by an absorbed (N_H=7.28 x 10²¹ cm², fixed) and exponentially cutoff power law plus a strong iron line at 6.4 keV (equivalent width around 390 eV for both observations at a width of 0.5keV). In the first observation the photon index was 0.91 +/- 0.04 and the cutoff energy was 15.5 +/- 1.5 keV at a 2-10 keV flux of 1.51 x 10^-9 erg/s/cm2 (75 mCrab), i.e., 1.5 times brighter than in the observations reported in ATEL #1283. The source had slightly brightened to a 2-10keV flux of 1.58 x 10^-9 erg/s/cm2 in the second observation, the spectrum is well fit by a cut-off power law model with photon index 1.00 +/- 0.02, E_cut = 20 +/- 1 keV. The spectral parameters correspond to 0.3-10 keV fluxes of 1.63 x 10^-9 erg/s/cm2 and 1.71 x 10^-9 erg/s/cm2 for the two observations, i.e., they are consistent with the Swift results. The RXTE ASM indicates a further brightening of the source.

We will continue the monitoring of MXB0656 with both, RXTE and Swift. Observations at other wavebands are encouraged.