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Swift BAT Observations of IGR J01217-7257

ATel #8312; Robin H. D. Corbet (UMBC/NASA GSFC), Joel B. Coley (NASA GSFC), Hans A. Krimm (USRA/NASA GSFC)
on 20 Nov 2015; 17:19 UT
Credential Certification: Robin Corbet (corbet@umbc.edu)

Subjects: X-ray, Binary, Neutron Star, Transient

IGR J01217-7257 in the Small Magellanic Cloud has recently been found to be in outburst with INTEGRAL (Coe et al. 2015; ATel #8246) and XMM (Haberl et al. 2015; ATel #8305). The XMM observations revealed pulsations at a period of 2.16501(1) s, suggesting an identification with XTE J0119-731 (Corbet et al. 2003; IAUC # 8064 ). Photometry of the proposed optical counterpart showed a likely orbital period of 84 days (Coe et al. 2014; ATel #5806).

We have examined Swift BAT transient monitor data (15 - 50 keV; Krimm et al., 2013, ApJSS 209,14) obtained between MJD 56,679 to 57,346 (2014-01-22 to 2015-11-20). The power spectrum of the light curve shows substantial low-frequency noise, perhaps related to long-term flux changes. The strongest peak in the power spectrum is at a period of 83.4 +/- 1 day, consistent with the optical period reported by Coe et al. 2014. There is also a smaller peak at a period consistent with the second harmonic of this period at 41.3 +/- 0.5 day. The statistical significance of the 83.4 day modulation is difficult to determine because of the presence of the low-frequency variability. However, its consistency with the optical period is suggestive that it is indeed the orbital period. The BAT light curve folded on the 83.4 day period shows a profile with a sharp rise to maximum with a slower decline. The flux maximum is at an epoch of approximately MJD 56995. The flux near maximum, averaged over the entire light curve, is approximately 0.0013 cts/cm^2/s (~6 mCrab).

This ephemeris gives a time of flux maximum of MJD 57,329 (2015-11-03) for the most recent expected outburst, which is between the INTEGRAL and XMM observations. The next predicted flux maximum is MJD 57,412 (2016-01-25).