Referred to by ATel #: 12704
We report on a NuSTAR observation of the Be X-ray binary 4U 1901+03 taken on 2019 April 11.95 (MJD 58584.95; ObsID 90502307004). It was performed during the decline of the source's current outburst, which began around 2019-02-08 (ATel #12498, GCN #23882).
We describe the 3-80 keV spectrum with a cutoff power-law ("cutoffpl") with a photon index 0.43+/-0.04 and a high-energy cutoff of 6.7±0.2. The continuum is modified by a neutral absorption column of (1.8±0.2)x10^22 cm^-2 and a prominent "10-keV feature" in absorption, with centroid energy, width, and optical depth of 10.19 (+0.10, -0.09) keV, 3.3±0.2 keV, and 1.54±0.2, respectively. This is similar to the feature reported by Reig & Milonaki (2016, A&A, 594, A45) using RXTE observations. Prominent emission lines are detected at 6.4 keV and 6.7 keV, which we interpret to be neutral Fe K-alpha and Fe XXV lines. The unabsorbed X-ray flux is (4.57±0.02)x10^-9 erg cm^-2 s^-1, which is roughly half the flux at the peak of the outburst. Based on the 12 kpc lower limit on the distance from ATel #12554, this corresponds to a 3-50 keV luminosity of (7.86±0.03)x10^37 erg s^-1.
Negative residuals were found near 30 keV, indicating the possible presence of a narrow absorption feature. A possible interpretation is a Cyclotron Resonant Scattering Feature (CRSF). The absorption feature can be well described with a multiplicative absorption line with a Gaussian optical depth profile ("gabs" in XSPEC), with an energy of 31.0 (+1.4, -0.9) keV, width of 3.1 (+1.1, -0.8) keV, and optical depth of 1.1 (+0.7, -0.4). The addition of this line reduces the χ^2/d.o.f. from 1794.74/1457 to 1619.76/1454. To estimate its significance, we simulated 10^4 spectra using a Monte Carlo analysis (following Protassov et al. 2002, ApJ 571, 545). We find the significance of the feature to be >99.99%. Interestingly, this feature was not significantly detected in the first two NuSTAR observations performed at higher fluxes (see ATel #12514).
We thank the NuSTAR Operations, Software and Calibration teams for scheduling and the execution of these observations. Follow-up observations in particular in the hard X-ray band are encouraged.
|Publication date||1 Apr 2019|
|Publication status||Published - 1 Apr 2019|
|Series||The Astronomer's telegram|