A descriptive title

ATel #13208; V. Doroshenko (Institut fuer Astronomie und Astrophysik, Tuebingen, Germany), S. Tsygankov
on 21 Oct 2019; 12:04 UT
Credential Certification: Victor Doroshenko (doroshv@astro.uni-tuebingen.de)

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We report on observations of the X-ray transient Swift J1845.7-0037 (ATEL#4130, ATEL #13191, ATEL#13195) with NuSTAR observatory. The observation was triggered by recent detection of the source by Swift (ATEL#13195), where pulsations with period of ~200 s were suggested implying the NS BeXRB origin of the source. Significance of the pulsations was, however, formally low due to the short duration of the observation. Follow-up observations with NuSTAR on Oct 21 allowed to detect the source and highly significant pulsations with barycentric period of 200.44s thereby confirming neutron star origin of the source. The pulse profile is nearly sinusoidal, single peaked with the pulsed fraction in NuSTAR energy band of ~25%, which is typical for accreting pulsars. The NuSTAR and almost simultaneous Swift/XRT observations provide the first broadband view on the X-ray spectrum of the pulsar, which can be described with an absorbed non-thermal comptonization model (nthcomp in Xspec). There is no evidence for presence of possible cyclotron lines in NuSTAR band with this model. The best-fit temperature of 6.4(1) keV, asymptotic power-law photon index of 1.332(6), are similar to those observed in other BeXRBs. On the other hand, the absorption column of 8.0(4)e22 (assuming abundances by Wilms et al 2000, and TBabs model) is comparatively high, and might indicate intrinsic absorption in the system in excess of interstellar absorption of ~2e22 atoms/cm2 expected based on HI4PI survey data. This is not common for all BeXRBs, although can not be excluded. On the other hand, strong absorbtion is also indicative of a large distance to the system. The unabsorbed model source flux in 0.1-100 keV energy band is ~9e-10 erg/cm2/s, which for fiducial distance of 10kpc implies luminosity of ~1e37 erg/s typical for Type I Be outbursts. Detailed analysis of NuSTAR data is ongoing, observations in other bands, and particularly optical spectroscopy, are encouraged to clarify the origin of the counterpart and the distance to the system.