Swift Discovery of a Luminous New X-ray Source in M101 near SN2011fe

ATel #3696; Derek B. Fox (Penn State)
on 21 Oct 2011; 00:19 UT
Credential Certification: Derek B. Fox (dfox@astro.psu.edu)

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I have analyzed 133.4 ksec of Swift XRT observations of SN2011fe / PTF11kly (Target ID 32081; Nugent et al., ATEL #3581), using the "Swift-XRT products" generator (Evans et al. 2009, MNRAS 397, 1177) at the UK Swift Science Data Centre at the University of Leicester to generate a single unified, filtered, and astrometrically-registered event file for analysis (ObsIDs 32081001 to 32081077).

Examining the resulting dataset, I identify a luminous new X-ray source, proximate to but distinguished from SN2011fe, that has brightened by a factor of >10 since the Chandra observation of 27.4 Aug 2011 UT (see Hughes, Soderberg & Slane, ATEL #3602; Horesh et al., arXiv:1109.2912).

Using a weighted-centroiding approach informed by the known XRT point spread function, I determine a best-fit source position of 14:03:08.0, +54:16:20 (J2000), with a positional uncertainty of r_90 = 5.1 arcsec (90%-confidence radius). The source, denoted Swift J140308.0+541620, is thus located 20 arcsec, or 8.5*r_90, distant from the position of SN2011fe.

Given the small number of observed X-ray counts, the source light curve is not well constrained, but can be adequately characterized as quiescent until a sudden brightening at Sep 23.0 UT, with an aperture-corrected excess count rate of 0.4 cts ksec^-1 since that time yielding ~18 counts in a 9"-radius aperture, by comparison to the ~2 counts expected on the basis of the ~0.05 cts ksec^-1 count rate seen in previous Swift observations, and the ~0.03 cts ksec^-1 upper limit inferred from non-detection in the Chandra observation (using a 5:1 ratio of effective areas, and assuming <8 total expected counts at the time of the 50 ksec Chandra ACIS exposure). A source count rate of 0.4 cts ksec^-1 corresponds to an X-ray luminosity of 7.3e+37 erg s^-1 (0.3-10 keV) at the 6.4 Mpc distance of M101 (Shappee & Stanek 2011, ApJ, 733, 124), for an assumed photon index alpha=2 power-law spectrum and negligible intrinsic absorption. The properties of Swift J140308.0+541620 are thus consistent with its interpretation as a newly active X-ray binary in M101.

No Chandra counterparts to Swift J140308.0+541620 are identified from the catalog of 325 X-ray sources detected in Chandra archival observations of M101 by J. Liu (2011, ApJS, 192, 10).