NuSTAR observations reveal relativistic reflection and strong absorption features in the spectrum of 1A 1744-361

ATel #15429; Sean N. Pike (Caltech), Amruta Jaodand (Caltech), R. M. Ludlam (Caltech), John A. Tomsick (SSL/UC Berkeley)
on 10 Jun 2022; 06:21 UT
Credential Certification: Sean Pike (spike@caltech.edu)

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We present preliminary spectral results from NuSTAR observations (OBSID 90801312001) of the accreting neutron star 1A 1744-361 in outburst (Atels #15407, #15408, #15410, #15424). NuSTAR observed the source between 2022-06-08 19:55:48.750 UTC and 2022-06-09 16:09:59.025 UTC for a total exposure of 31.5 ks. We find that in addition to a power law with a high energy cutoff, the spectrum exhibits broad iron Kalpha emission as well as clear absorption features. We fitted the time-averaged spectrum in the energy range 3-40 keV, and we found that the spectrum is well-described by a combination of relativistic disk reflection of a power law with a high energy cutoff, represented by the Xspec models cutoffpl and relxill (Garcia et al., 2014; Dauser et al., 2014), with an intervening warm absorber (Xspec model zxipcf) with ionization log(xi) = 2.8+/-0.1, narrow absorption lines at 6.95+/-0.02 keV and 8.01+/-0.03 keV, as well as a neutral absorption column with nH=(4.8+/-0.3)x10^22 cm^-2. We achieved a fit statistic of Cstat/d.o.f. = 325/254 with this model, and we measure a bolometric flux (0.1-100 keV) of (6.5+/-0.5)x10^-9 erg/s/cm^2, which corresponds to a luminosity of about 5x10^37erg/s if we assume a distance of 8 kpc. We hypothesize that the warm absorber component and the narrow absorption lines are the result of an ionized outflow. Although type-I X-ray bursts have previously been observed from the source (Bhattacharyya et al. 2006), we do not observe any type-I X-ray bursts in the NuSTAR light-curve. However we do note clear variability throughout the observation and moderate flaring during one NuSTAR orbit. Given the complexity of the emission and absorption features exhibited by the source, as well as its variability, we encourage further observations in the soft X-ray and other wavelengths.