SRG/ART-XC discovery of SRGA J204318.2+443815: Towards the complete population of faint X-ray pulsars

Creators: Lutovinov, A. A.; Tsygankov, S. S.; Mereminskiy, I. A.; Molkov, S. V.; Semena, A. N.; Arefiev, V. A.; Bikmaev, I. F.; Djupvik, A. A.; Gilfanov, M. R.; Karasev, D. I.; Lapshov, I. Yu.; Medvedev, P. S.; Shtykovsky, A. E.; Sunyaev, R. A.; Tkachenko, A. Yu.; Anand, S.; Ashley, M. C. B.; De, K.; Kasliwal, M. M.; Kulkarni, S. R.; van Roestel, J.; Yao, Y.

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Abstract

We report the discovery of the new long-period X-ray pulsar SRGA J204318.2+443815/SRGe J204319.0+443820 in a Be binary system. The source was found in the second all-sky survey by the Mikhail Pavlinsky ART-XC telescope on board the SRG mission. The follow-up observations with XMM-Newton, NICER, and NuSTAR allowed us to discover a strong coherent signal in the source light curve with a period of ~742 s. The pulsed fraction was found to depend on an increase in energy from ~20% in soft X-rays to >50% at high energies, as is typical for X-ray pulsars. The source has a quite hard spectrum with an exponential cutoff at high energies and a bolometric luminosity of L_X ≃ 4 × 10³⁵ ergs⁻¹. The X-ray position of the source is found to be consistent with the optical transient ZTF18abjpmzf, located at a distance of ~8.0 kpc. Dedicated optical and infrared observations with the RTT-150, NOT, Keck, and Palomar telescopes revealed a number of emission lines (Ha, He I, and the Paschen and Braket series) with a strongly absorbed continuum. According to the SRG scans and archival XMM-Newton data, the source flux is moderately variable (by a factor of 4-10) on timescales of several months and years. All this suggests that SRGA J204318.2+443815/SRGe J204319.0+443820 is a new quasi-persistent low-luminosity X-ray pulsar in a distant binary system with a Be-star of the B0-B2e class. Thus the SRG observatory allowed us to unveil a hidden population of faint objects, including a population of slowly rotating X-ray pulsars in Be systems.

Additional Information

© ESO 2022. Received 24 June 2021; Accepted 22 November 2021; Published online 18 May 2022. The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG mission. This work is based on observations with the Mikhail Pavlinsky ART-XC, and eROSITA X-ray telescopes aboard the SRG observatory. The SRG observatory was built by Roskosmos in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI) in the framework of the Russian Federal Space Program, with the participation of the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The ART-XC team thanks the Russian Space Agency, Russian Academy of Sciences and State Corporation Rosatom for the support of the ART-XC telescope design and development. The SRG/eROSITA X-ray telescope was built by a consortium of German Institutes led by MPE, and supported by DLR. The SRG spacecraft was designed, built, launched and is operated by the Lavochkin Association and its subcontractors. The science data are downlinked via the Deep Space Network Antennae in Bear Lakes, Ussurijsk, and Baykonur, funded by Roskosmos. The eROSITA data used in this work were processed using the eSASS software system developed by the German eROSITA consortium and proprietary data reduction and analysis software developed by the Russian eROSITA Consortium. We also would like to thank the XMM-Newton, NuSTAR, NICER and Swift/XRT teams for organising prompt observations. This research has made use of data, software and/or web tools obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), a service of the Astrophysics Science Division at NASA/GSFC and of the Smithsonian Astrophysical Observatory's High Energy Astrophysics Division. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. Authors (IFB, MRG, RAS) are grateful to TUBITAK, the Space Research Institute, the Kazan Federal University for their partial support in using RTT-150 (the Russian-Turkish 1.5-m telescope in Antalya). The work is partly based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. This work was supported by the Russian Science Foundation (grant 19-12-00423).

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