eRASSU J050810.4-660653 in the LMC: Discovery of X-ray pulsations with XMM-Newton and NuSTAR

ATel #15133; F. Haberl (MPE), A. Salganik (St. Petersburg State University, IKI), C. Maitra (MPE), V. Doroshenko, L. Ducci (IAAT Tuebingen), D. Kaltenbrunner (MPE), I. Kreykenbohm (Remeis-Observatory/ECAP, FAU Erlangen), A. Lutovinov (IKI), P. Maggi (Universite de Strasbourg), I. Mereminskiy, S. Molkov (IKI), A. Rau (MPE), A. Semena (IKI), S. Tsygankov (IKI, University of Turku), G. Vasilopoulos (Universite de Strasbourg), P. Weber, J. Wilms (Remeis-Observatory/ECAP, FAU Erlangen)
on 22 Dec 2021; 21:05 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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eRASSU J050810.4-660653 is a new Be/X-ray binary in the LMC, which was discovered during the beginning of the first all-sky survey of the eROSITA instrument on board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission (ATel#13609). During the end of the second survey (eRASS2) the source was bright again and we triggered one of our anticipated target of opportunity (TOO) observations with XMM-Newton.

The XMM-Newton observation was performed on 17 Dec, 2020 for a total exposure of about 32 ks. The derived X-ray source position of RA (J2000) = 05:08:09.99, DEC (J2000) = -66:06:51.6 with an estimated uncertainty of 0.5" radius (including systematics) is within 0.6" of the position of the proposed optical counterpart.

A power spectrum of the EPIC-pn data revealed X-ray pulsations with a period of about 40.6 s, together with five harmonics. Using a Bayesian approach (Gregory & Loredo 1996) we determined the fundamental period to 40.60254(8) s. The pulse profile is characterized by a deep narrow dip which causes the large number of harmonics and a pulsed fraction of about 25%.

During the ongoing all-sky survey we have detected an enhanced X-ray activity from the source with the Mikhail Pavlinsky ART-XC telescope. The source was registered on 16-17 Dec, 2021 at the level of ~2 mCrab (3×10^-11 erg cm^-2 s^-1) in the 4-12 keV band. For LMC distance this corresponds to a luminosity of about 10³⁷ erg s^-1, that is an order of magnitude higher, than at the moment of its discovery with eROSITA in 2019 (ATel#13609).

We have triggered a NuSTAR TOO observation, which allowed us to independently detect the pulse period at ~40.578 s, a value similar to that measured one year ago. The spectrum is typical for X-ray pulsars and could be described by a cutoff power-law.

We are grateful to the NuSTAR team for approving and rapid scheduling of the follow-up observation.