Swift J0549.7-6812: Swift detection of a pulse period and archival Chandra limits

ATel #5309; H. A. Krimm (CRESST/GSFC/USRA), J. M. Gelbord (Eureka Scientific), S. T. Holland (STScI) and J. A. Kennea (PSU)
on 21 Aug 2013; 21:33 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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We report on further Swift observations of Swift J0549.7-6812 (Krimm, Holland & Kennea, ATel #5293). A 1.1-ksec observation was carried out starting at 2013 August 20 21:03:43.819 UTC. This is the first of a set of approved monitoring observations at a 3-day cadence.

In the XRT Windowed Timing mode data, we find evidence for a 6.2-second period, which we interpret as the pulsation period of a pulsar in the system. More data are required to refine this period and determine the uncertainty in the measurement.

We also report a refined UVOT-enhanced XRT position based on the complete data set from the 2013 August 16 observation in Photon Counting mode, and using the method of Goad et al. (2007, A&A, 476, 1401). This position is:

RA: 05 50 06.47 (87.52694 deg)
Dec: -68 14 55.7 (-68.24881 deg)
90% Error radius: 1.4" (stat + syst)

This position is 0.9" from the XRT position reported in ATel #5293.

The current XRT count rate (0.3-10 keV) is consistent with the rate from the first observation (2013 August 16; ATel #5293) at 3.6 ct/sec. The spectral fit for the more recent observation was carried out using data and analysis based on Evans et al. (2009, MNRAS, 397, 1177). The data are well fitted by an absorbed power-law model (Cstat = 702.88 for 675 dof) with the following parameters:
N_H = 1.4 +/- 0.3 x 10^21 cm-2
Gamma = 1.15 +/- 0.07
Flux (0.3-10 keV; unabsorbed) = 2.50 +/- 0.09 x 10^-10 erg/s/cm-2

The refined XRT position lies within the field of view of a deep Chandra/ACIS observation (OBSID 3850; 67.5 ks observed on 2003 October 2). No X-ray source is detected within 20" of this position. We further note that the ROSAT counterpart 1RXS J055007.0-681451 is not consistent with any source detected by Chandra. This establishes a lower flux limit for the quiescent state of Swift J0549.7-6812 and indicates that this source was not quiescent when observed by ROSAT.

Within the 90% confidence XRT error circle, the ACIS upper limit count rate in the 0.3-8.0 keV band is 1.28 X 10^-4 count/sec (three sigma confidence according to the procedure of Kraft et al. 1991, ApJ 374, 344 and including a vignetting factor of 0.9). Adopting the current best-fitting XRT spectral model, this translates to an unabsorbed flux limit of < 2.7 X 10^-15 erg/cm^2/s in the 0.3-10 keV band. Thus, the flux observed by Swift on 2013 August 20 is at least a factor of 9.4 X 10^4 above the quiescent level and that measured by ROSAT was a factor of 570 higher than the upper limit in quiescence.

The UVOT observations were made in six bands and the results are shown below. The b magnitude is unchanged from the earlier observation (2013 August 16; ATel #5293).

--------------------
Filter Mag Err
--------------------
v 15.03 0.05
b 15.15 0.04
u 13.79 0.10
uvw1 13.37 0.10
uvm2 13.12 0.10
uvw2 13.16 0.10
--------------------

The b-v colour is (b-v) = +0.12 +/- 0.06 and (u-b) = -1.18 +/- 0.11. Corrected for E_{B-V} = 0.07 mag and transformed to B-V these become (B-V)_0 = +0.06 and (U-B)_0 = -1.10. These colors suggest a UV excess, although given the uncertainty in the u magnitude, they are marginally consistent with a blackbody.

The BAT (15-50 keV) rate has increased slightly since 2013 August 16 and is now at 0.009 +/- 0.003 ct/s/cm^2 (~40 mCrab).

Scaled Map Transient Analysis for Swift J0549.7-6812