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A 160 day Period in the Be star X-ray Binary IGR J01363+6610 from Swift BAT Observations

ATel #3079; R. H.D. Corbet (UMBC/NASA GSFC), H. A. Krimm (USRA/NASA GSFC)
on 15 Dec 2010; 20:56 UT
Credential Certification: Robin Corbet (Robin.Corbet@nasa.gov)

Subjects: Binary, Neutron Star, Star, Transient, Variables

The hard X-ray source IGR J01363+6610 has been proposed to be a Be star X-ray binary (e.g. Reig et al. 2005, A&A, 440, 637; Tomsick et al. 2010, arXiv:1012.2817v1). We have analyzed the Swift BAT 15 - 50 keV light curve of IGR J01363+6610 obtained between MJD 53,414 to 55,544 (2005-02-13 to 2010-12-14) to search for periodic modulation that could reveal the orbital period of the system. From a power spectrum we find an apparently significant peak (false alarm probability ~2E-7 for a search for periods longer than 2 days) at approximately 160 days. However, an examination of sub-sections of the light curve indicates that this modulation is only present in recent data. The time of onset of the modulation is difficult to determine due to the faintness of the source and the length of the period. However, it appears to occur between approximately MJD 54,500 to 55,000.

From the data obtained after MJD 54,750, a parameterization of the modulation from a sine wave fit yields:
Tmax = MJD 55,291 +/- 3 + n x 159 +/- 2
where Tmax is the time of maximum flux.

The mean fluxes before and after MJD 54,750 are -0.00001(2) and 0.00013(3) cts/cm^2/s respectively, where the errors are statistical only. The mean flux after MJD 54,750 is equivalent to approximately 0.6 mCrab. However, when the folded light curve is examined, negative count rates are obtained around phase 0.5, suggesting a systematic underestimation of the source flux, and the mean flux near phase 0 is approximately 0.0005 cts/cm^2/s (2.3 mCrab). The length of this period, if it is the orbital period of the system, and the transition between active and inactive states are both consistent with a Be star classification for IGR J01363+6610. We caution, however, that only a small number of 160 day cycles are covered during the apparent active state, and additional observations are required to confirm that this is a persistent property of the light curve.

The apparent transition from an inactive to active state between approximately MJD 54,500 to 55,000 is consistent with the X-ray results summarized by Tomsick et al. (2010): The source was discovered with INTEGRAL on MJD 53,114, before the launch of Swift, but not detected in subsequent INTEGRAL observations. Chandra observations on MJD 54,259 did not detect the source, but XMM observations on 55,043 did. In addition, IGR J01363+6610 is not detected in the BAT 58 month catalog (Baumgartner et al., 2010, HEAD, 11, 1305) which only covers observations up to MJD 55,104

The next predicted maximum is near MJD 55,609 (2011-02-17). If IGR J01363+6610 remains in an active state, X-ray observations near this time may be able to detect pulsations.

Scaled Map Transient Analysis for IGR J01363+6610