First characterization of Swift J1845.7-0037 with NuSTAR

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 similar to 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 greater than or similar to 10(13) G at a large distance of d similar to 10 kpc. The latter is consistent with the nondetection of a cyclotron line in the NuSTAR energy band.