First characterization of Swift J1845.7–0037 with NuSTAR

¹ Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany
e-mail: doroshv@astro.uni-tuebingen.de
² Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³ Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Str. 84/32, Moscow 117997, Russia
⁴ Moscow Institute of Physics and Technology, Moscow Region, Dolgoprudnyi, Russia
⁵ Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049, PR China
⁶ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, PR China

Received: 1 November 2019
Accepted: 15 January 2020

Abstract

The hard X-ray transient source Swift J1845.7–0037 was discovered in 2012 by Swift/BAT. However, at that time, no dedicated observations of the source were performed. In October 2019, the source became active again, and X-ray pulsations with a period of ∼199 s were detected with Swift/XRT. This triggered follow-up observations with NuSTAR. Here, we report on the timing and spectral analysis of the source properties using NuSTAR and Swift/XRT. The main goal was to confirm pulsations and search for possible cyclotron lines in the broadband spectrum of the source to probe its magnetic field. Despite highly significant pulsations with period of 207.379(2) s being detected, no evidence for a cyclotron line was found in the spectrum of the source. We therefore discuss the strength of the magnetic field based on the source flux and the detection of the transition to the “cold-disc” accretion regime during the 2012 outburst. Our conclusion is that the source is most likely a highly magnetized neutron star with B ≳ 10¹³ G at a large distance of d ∼ 10 kpc. The latter is consistent with the nondetection of a cyclotron line in the NuSTAR energy band.