Swift BAT Observations of the Be/X-ray Binary 4U 0728-25 (X 0726-260)

ATel #9823; Robin H. D. Corbet (UMBC/NASA GSFC/MICA), Joel B. Coley (NASA GSFC), Hans A. Krimm (USRA/NSF)
on 2 Dec 2016; 23:32 UT
Credential Certification: Robin Corbet (corbet@umbc.edu)

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Following the recent report of X-ray brightening from the Be/X-ray binary 4U 0728-25 (X 0726-260) by Nakajima et al. (2016, ATel #9820). we have investigated the Swift BAT light curve of this source. We used the BAT transient monitor light curve (15 - 50 keV, Krimm et al. 2013, ApJS, 209, 14) which covers times between MJD 53,416 to 57,724 (2005-02-15 to 2016-12-02). Individual outbursts on the 34.5 day period (Corbet & Peele 1997, ApJ, 489, L83) are difficult to discern by eye in the BAT light curve. We therefore have employed the power spectrum to investigate the source properties.

From the power spectrum of the total light curve we find very strong detection of the orbital period at 34.56 +/- 0.02 days, which is consistent with the Nakajima et al. period of 34.5446 +/- 0.0153 days. In addition, we detect the second harmonic of this period, which implies a period of 34.548 +/- 0.010 days.

We calculated power spectra of 400 day long sections of the light curve, offsetting by 100 days between sections. We find that the 34.5 day period is consistently detected in essentially all data sections, except for a few time intervals where BAT observations of the source were infrequent. We also investigated the relative strength of the peak in the power spectrum as a function of light curve length, starting with the first observation and increasing the light curve length in 100 day steps. This procedure showed a rather steady increase in the relative strength of modulation at 34.5 days as the light curve length was increased.

We therefore conclude that 4U 0728-25 appears to have exhibited rather steady modulated X-ray emission since 2005, and there does not appear to be any obvious recent change in source properties such as a strong brightening.