New Swift/BAT survey source Swift J045106.8-694803 is a HMXB with a 187 s X-ray period and 21.6 day optical period in the Large Magellanic Cloud.

ATel #1901; A. P. Beardmore (U. Leicester), M. J. Coe (U. Southampton), C. Markwadt (UMD/NASA), J. P. Osborne (U. Leicester), W. H. Baumgartner (UMBC & NASA/GSFC), J. Tueller, N. Gehrels (NASA/GSFC)
on 15 Jan 2009; 16:16 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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We report the discovery of a possible new Be/X-ray binary system Swift J045106.8-694803 in the Large Magellanic Cloud.

The source was initially detected by the BAT at 8.3 sigma, in the 36 months Swift/BAT hard X-ray survey (ATELs #1794, #1429) as Swift J0451.5-6949, with a position RA, Dec (J2000) = 72.875, -69.817 and an uncertainty of 5 arcminutes. The 14-195 keV flux was 2.8 +/- 0.3 x 10^-11 erg cm^-2 s^-1 and the spectrum could be fit with a powerlaw of photon index 2.5 +/- 0.4.

Subsequent follow-up observations with the Swift/XRT, consisting of 15.5 ks of Photon Counting mode data taken between 2008-Oct-23 to 2008-Nov-14, revealed a previously uncatalogued X-ray source within the BAT error circle at
RA (J2000) = 04:51:06.8
Dec (J2000) = -69:48:03.2
with an uncertainty of 3.5 arcseconds. The location of the X-ray source is consistent with the Vmag = 14.70 blue star (B-V = 0.00) [M2002] 9775 from the Massey (2002) UBVR CCD survey of the Magellanic clouds (2002 ApJS 141 81).

The 0.3-10 keV X-ray spectrum can be well fit by an absorbed powerlaw with a photon index of 0.96 +0.06 -0.04 and column density of 1.9+/-0.3 x 10²¹ cm^-2. The 0.3-10 keV observed X-ray flux is 1.68+/-0.11 x 10 ^-11 erg cm^-2 s^-1.

A Lomb-Scargle periodogram of the 0.3-10 keV X-ray light curve reveals the presence of a periodic signal with the maximum power occurring at 187 s. The modulation is approximately sinusoidal with a fractional amplitude of about 10 percent. However, strong secondary peaks occur at 181 s and 193 s which correspond to sidebands with Swift's 95 minute orbital period, and it is difficult to be sure of the correct period identification at this time.

A further 4.3 ks of Swift/XRT data were obtained on 2009-Jan-10. Even though the source intensity and spectral parameters are consistent with the earlier observation, the 187 s periodicity is not detected in this latest XRT observation, with an upper limit to the fractional amplitude of 4 percent.

We have also examined the MACHO archive data for this object, which has the MACHO catalogue ID 44.1741.17. The blue data show an optical period of 21.64 +/- 0.02 day, having a sinusoidal-like modulation of amplitude 0.02 mag.

The identification of Swift J045106.8-694803 with a blue star in the LMC, with periodicities of 21.6 day in the optical and 187 s in the X-ray, strongly suggest this is a new Be/X-ray pulsar binary system, especially considering its location near the edge of the Be/X-ray pulsar period/orbital period distribution of Corbet et al. (1999 ApJ 517, 956).

Further Swift observations are planned.