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A 49 Day Period in IGR J14488-5942/Swift J1448.4-5945 from Swift/BAT Observations

ATel #2598; R. H.D. Corbet (UMBC/NASA GSFC), S. D. Barthelmy (NASA GSFC), W. H. Baumgartner (UMBC/NASA GSFC), H. A. Krimm (USRA/NASA GSFC), C. B. Markwardt (UMCP/NASA GSFC), G. K. Skinner (UMCP/NASA GSFC), J. Tueller (NASA GSFC)
on 5 May 2010; 04:47 UT
Credential Certification: Robin Corbet (Robin.Corbet@nasa.gov)

Subjects: X-ray, Binary, Black Hole, Neutron Star, Star, Variables

IGR J14488-5942 (Swift J1448.4-5945) is listed in the 4th INTEGRAL/IBIS Survey Catalogue (Bird et al. 2010, ApJ Supp, 186, 1) as a variable source. It is not present in the BAT 22 month survey (Tueller et al. 2010, ApJ Supp, 186, 376).

We have analyzed the Swift BAT 58 month survey (Baumgartner et al. 2010, HEAD, 11, 1305) light curve of this source. The light curve covers the interval 2004-12-16 through 2009-09-30 (MJD 53,355 to 55,104) and we used an energy range of 15 - 100 keV. The power spectrum of this light curve reveals highly significant modulation (false alarm probability < 1e^-7) at a period near 49 days.

In order to characterize the modulation we fitted a sine wave to the light curve and derived:
Tmax = MJD 54,318.1 (+/- 1.2) + n x 49.51 (+/- 0.12)
where Tmax is the time of maximum flux.

The mean flux is 2.8e-5 cts/s/detector element, equivalent to approximately 0.7 mCrab, and the flux modulation (semi-amplitude/mean) on the 49 day period is approximately 100%. Examination of the light curve suggests an increase in source flux at approximately MJD 53,700 and the strength of the peak in the power spectrum at 49 days increases if only data after that time are used.

We note that Landi et al. (2009, ATel #2355) performed Swift XRT observations covering the location of IGR J14488-5942 and found two sources: "N1" and "N2". They proposed that the brighter and spectrally harder source, N2, was the counterpart of IGR J14488-5942. These observations were obtained on 2009-09-25 (MJD 55,099) at a phase = 0.93 when IGR J14488-5942 would be expected to be bright. We extracted XRT light curves and searched for pulsations in both sources. No pulsations were seen in N2. For the fainter source, N1, a marginal signal was seen at a period of 33.4 seconds.

The length of the period found in the BAT data, which is expected to be the orbital period of the system, together with the low Galactic latitude of -0.13 degrees, suggests that Swift J1448.4-5945 may be a high-mass X-ray binary with a Be star primary (Corbet 1986, MNRAS, 220, 1047). The possible long-term variability would also be consistent with a Be star system. Although 33.4 seconds would be consistent with the pulse period expected for a Be system with a 49 day orbital period, it would be surprising for N1 to be the counterpart of IGR J14488-5942 as it was both fainter and spectrally softer than N2. The XRT light curve we extracted of N1 also contained only 72 photons. Additional observations of both N1 and N2 are therefore needed to perform more sensitive pulsation searches.