First characterization of Swift J1845.7-0037 with NuSTAR

TL;DR

This study presents the first detailed timing and spectral analysis of Swift J1845.7-0037 using NuSTAR and Swift/XRT, confirming pulsations and estimating a high magnetic field, but finding no cyclotron lines.

Contribution

First comprehensive timing and spectral characterization of Swift J1845.7-0037, including pulsation confirmation and magnetic field estimation without cyclotron line detection.

Findings

Detected pulsations with a period of ~207s.

No cyclotron lines found in the spectrum.

Estimated magnetic field strength of ~10^13 G.

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. On Oct 2019 the source became active again, and X-ray pulsations with a period of ~199s 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) were 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, most likely, the source is a highly magnetized neutron star with B 1e13G at a large distance of d~10 kpc. The latter one consistent with the non-detection of a cyclotron line in the NuSTAR energy band.