Title: Detection of a 31.6 s pulse period for the supernova impostor SN 2010da in NGC 300, observed in ULX state

ATel #11158; S. Carpano, F. Haberl, C. Maitra (MPE-MPG, Germany)
on 12 Jan 2018; 11:05 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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The supernova impostor SN 2010da located in NGC 300, later identified as a likely Supergiant B[e] High-mass X-ray binary (Lau et al. 2016, ApJ, 830, 142 and Villar et al. 2016, ApJ, 830, 11), was observed in outburst during two long (139 and 82 ks) XMM-Newton observations performed on 2016 December 17 to 20. We report the discovery of a strong periodic modulation in the X-ray flux with a pulse period of 31.6 s and a very rapid spin-up, and confirm therefore that the compact object is a neutron star. The spin period is changing from 31.69 s to 31.54 s from the start of the first observation to the end of the second one (251.3 ks). The spin-up during this period is -5.4e-7s s^-1 with additional sinusoidal variations with 6.7e-3 s amplitude and 2.23 d period that we associate with the orbital period. The spectrum is described by a powerlaw+black body model, leading to a 0.3-10 keV unabsorbed luminosity of 2.9 and 3.1×10³⁹erg s^-1 for the first and second observation, respectively. The source is much brighter than during the outburst of 2010, when the X-ray luminosity was 6×10³⁸erg s^-1 in the SWIFT XRT observation of May 2010 (Immler et al 2010, Atel#2639) and 2×10³⁷erg s^-1 in the Chandra observation of September 2010 (Binder et al. 2011, ApJ, 739L, 51). We conclude that the source is the fourth case for the new class of ULX pulsars that already includes M82 X-2, NGC 7793 P13 and NGC 5907 ULX1. A paper describing the results in more detail is in preparation by S. Carpano et al.