Swift and RXTE report the detection of a new transient pulsar Swift J0513.4-6547 in the Large Magellanic Cloud

ATel #2011; H. A. Krimm (CRESST/GSFC/USRA), R. H. D. Corbet (CRESST/GSFC/UMBC), P. A. Evans (Leicester Univ.), S. T. Holland (CRESST/GSFC/USRA), C. B. Markwardt (CRESST/GSFC/UMD), T. E. Strohmayer (GSFC), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. R. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (CRESTT/GSFC/UMBC), G. Skinner (CRESST/GSFC/UMBC), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on 14 Apr 2009; 20:38 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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The hard X-ray transient monitor of the Swift Burst Alert Telescope (BAT) has detected a previously unknown transient source. This source, Swift J0513.4-6547, was first detected at a daily average rate of 0.0018 ± 0.0014 counts/sec/cm^2 (8 mcrab) in the 15-50 keV band on 2009 April 2 (MJD 54923). It has continued at nearly the same brightness for the past two weeks. Examination of archived data reveals that the source was first detectable around 2009 Mar 4 and reached a peak brightness of ~40 mCrab on 2009 March 6.

Observing constraints prevented an observation with the Swift X-Ray Telescope (XRT) until 2009 April 11. A bright source was found in a 4.66 ks observation beginning at 01:34:47 (UT). The position was determined to be:

RA (J2000): 05 13 28.05 (78.36688 deg)
DEC (J2000): -65 47 20.0 (-65.78888 deg)

The estimated error is 1.9 arcsec (radius, 90% confidence).

RXTE observed Swift J0513.4-6547 for 1.2 ksec beginning on 2009 April 14 at 10:08:32 (UT). We detect strong pulsations from the source with a barycentric period of 27.28 seconds. The pulse profile is double peaked, and the amplitude (max - min / max + min) is high at about 85%. The average count rate was approximately 6 ct/s/PCU, corresponding to a flux of about 3 mCrab (2 - 20 keV). Additional observations with RXTE are planned.

Examination of the DSS images reveals a bright star within the XRT error radius. The star is 2MASS 05132826-6547187 and is located at coordinates, RA=78.367757, dec=-65.788536. The star has reported magnitudes B=15.3, R=15.5 (USNO-B1.0), J=15.2, H=15.1, K=14.8 (2MASS). We propose that this star is the counterpart of the X-ray transient. This source is at galactic coordinates L=275.98685, B=-34.55446. The brightness of the optical source suggests that this system is a high-mass X-Ray binary.

The star was also detected in the Swift UltraViolet Telescope (UVOT) at a position (J2000):

RA (J2000): 05:13:28.28 = 78.36783 (deg)
Dec (J2000): -65:47:18.4 = -65.78843 (deg)

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence, statistical + systematic). This is 2.14 arcsec from the center of the XRT error circle. The magnitude of the source is u = 13.7 ± 0.1. The magnitude error is fairly large because this is a crowded field. The Galactic extinction in this direction is E_{B-V} = 0.15 mag. Comparison with the catalog magnitudes above suggests that this source either has a very blue U-B color, or that it is variable. Further UVOT observations in multiple filters have been requested.

The Swift location is consistent with the source being located in the Large Magellanic Cloud. This is somewhat unusual given that the LMC hosts relatively few HMXB X-ray pulsars, compared to the SMC. Liu, van Paradijs and van den Heuvel, A&A 442, 1135 (2005) note that there are 92 HMXBs in the SMC and 36 in the LMC.

The XRT light curve varies between 0.7 and 1.0 ct/s (0.3-10 keV). The source spectrum can be fit to a power law with photon index 1.111 (+0.078, -0.063), and nH (intrinsic) = 7.6 (+28.5, -7.6) x 10^19 cm^-2 (nH (galactic) = 9.8 X 10^20 cm^-2. The unabsorbed flux (0.3-10 keV) is 5.6 x 10^-11 erg cm-2 s-1. At the distance of the LMC (50 kpc), this corresponds to a luminosity of 1.8 X 10^37 erg s^-1. The source is too faint in the BAT for spectral analysis.

Swift/BAT Hard X-ray Monitor light curves for SWIFT J0513.4-6547