Localization and spectral properties of 4U 1901+03 with Swift/XRT and NuSTAR

ATel #12514; I. A. Mereminskiy(1), A. A. Lutovinov(1), S. S. Tsygankov(2, 1), A. N. Semena(1), A. E. Shtykovskiy(1) (1: IKI RAS, Moscow, Russia 2: Tuorla observatory, Turku)
on 18 Feb 2019; 10:56 UT
Credential Certification: Ilya Mereminskiy (i.a.mereminskiy@gmail.com)

Tweet

We report preliminary results of the analysis of observations of the transient X-ray pulsar 4U 1901+03, that recently went into outburst (GCN #23882, ATel #12498). Using a 500s-long observation with Swift/XRT in the imaging mode we significantly improved a position of the source - RA, Dec (J2000) = 285.91413, 3.20451 (=19h 03m 39.39s, +03° 12? 16.2??) with 2.3?? uncertainty (90% confidence radius).

Using two observations performed by Swift/XRT in the timing mode and the NuSTAR TOO observation on 17 Feb 2019, we extracted a broadband X-ray spectrum of the source. It can be described by an absorbed powerlaw with high-energy cutoff (tbabs*(bb+cutoffpl+ga)*gabs in XSPEC) and weak thermal component, the parameters are NH≈3.7e22, Gamma≈0.5, Ecut ≈7.1 keV, kT≈0.16 keV and the total flux is 7.1e-9 ergs/cm2/s in the 0.8-78 keV energy band. In the spectrum there is a prominent Fe line at ~6.5 keV and so-called "10-keV feature" (see Reig&Milonaki, 2016), which can be adequately described by a Gaussian absorption line with the energy Ec=11 keV, width of 3.9 keV and optical depth of 0.2.

There is no obvious UV counterpart in UVOT/UVW1 data inside the error region, which is understood, given the high absorption. We found a possible IR/optical candidate in UKIDS-DR6 catalogue (Lucas+2012) - J190339.39+031215.6 (0.6'' offset) with the K magnitude of 12.6. Note, that this star is detected also by the GAIA observatory with an estimated distance of 2.9 kpc (Bailer-Jones+2018). If this association is true, than the current X-ray luminosity of the source is about 7e36 ergs/s.

We thank the NuSTAR and Neil Gehrels Swift operations teams for rapid approval and execution of our Target of Opportunity proposals. Multi-wavelength observations of the source are strongly encouraged.