Super-critical accretion onto the Be/X-ray binary pulsar 1A 0535+262 during its 2020 giant X-ray outburst

G. K. Jaisawal*, J. B. Coley, J. Wilms, G. Vasilopoulos, S. Naik, K. C. Gendreau, I. Caballero, A. Jana, C. Malacaria, Michael T. Wolff, P. Kretschmar, K. Pottschmidt, F. Fuerst

*Corresponding author for this work

Research output: Other contributionNet publication - Internet publicationResearch

Abstract

The Be X-ray binary pulsar 1A 0535+262 has recently been observed in outburst with Swift/BAT and MAXI (ATel #14157, #14173). Since then the pulsar has been rapidly evolving in X-rays. It reached an intensity level of 11 Crab in the Swift/BAT 15-50 keV band around 2020 November 19. The current outburst is, therefore, one of the rare giant X-ray outbursts of this source. Before that, the brightest outburst, with a flux of ~8 Crab (20-40 keV range), was recorded by BATSE in 1994 (Wilson et al. 1994, Camero-Arranz et al. 2012). Considering the exceptional activity of the pulsar in the present outburst, we are monitoring the source in soft and hard X-rays with NICER and NuSTAR. Initial results from the first NuSTAR observation have already been reported (ATel #14179). Our second NuSTAR ToO observation was performed on 2020 November 19 for 20 ks. We detected the source at a count rate of 3150 c/s in each NuSTAR detector module.

Our preliminary timing analysis of the barycenter-corrected light curves yields a pulsation period at 103.469 s. The spin period of the pulsar has changed by ~0.1 s over a period of 7 days, providing evidence of spin up from the accretion, also evident in Fermi/GBM monitoring. The pulse profiles, as in the earlier NuSTAR observation (ATel #14179), are complex due to the appearance and disappearance of peculiar features as a function of energy. A broad feature with a dip is observed in soft X-rays that further evolves into a double-peaked structure in the 20-30 keV range. The pulse profile then becomes smooth in hard X-rays above 40 keV. The pulse fraction varies significantly as a function of energy as in the previous observation (ATel #14179).

The 3-79 keV NuSTAR spectrum can be approximated by an absorbed cutoff power-law with a blackbody component. Using a Gaussian absorption model (GABS), a cyclotron scattering feature is clearly detected at 43.8+/-1.1 keV with a width of 9.6+/-1.2 keV. Despite the luminosity having changed by an order of magnitude since our last NuSTAR observation on 2020 November 11-12 (ATel #14179), the cyclotron line energy is independent of luminosity (within errors). A 6.45 keV iron emission line is also observed. Though the fitting was acceptable (reduced chi^2 of 1.36 for 2188 d.o.f), we noticed the presence of residuals at around 10 keV. The spectral fitting was further improved by adding a 10 keV Gaussian absorption feature. We find the energy, width, and strength of this ``10 keV'' feature to be 9.8+/-0.4 keV, 3.6+/-0.4 keV, and 0.8+/-0.2, respectively. This feature was detected marginally later in the reanalysis of our first NuSTAR observation. The spectral shape of the pulsar has changed effectively since our last observation (ATel #14179). We observe spectral hardening below ~20 keV in the present data. The other spectral parameters with the 90% errors from the study are: N_H = (1.6+/-1.2) x 10^22 cm^-2 (Wilms abundance), black body temperature kT = 0.45+/-0.04 keV, photon index = 0.01+/-0.02, cutoff energy = 12+/-0.2 keV, Fe-emission line energy E_line = 6.45+/-0.02 keV and width E_width=0.26+/- 0.03 keV. The best-fit reduced chi^2 is 1.11 for 2185 d.o.f. The 0.5-79 keV unabsorbed flux is derived to be 2.59x10^{-7} erg/s/cm^2. The source luminosity is estimated to be 1.4x10^{38} erg/s assuming a distance of 2.13 kpc (Bailer-Jones et al. 2018), suggesting super-critical accretion onto the pulsar just below the spherical Eddington limit for a canonical 1.4 M_sun neutron star..

NICER continues to monitor the source as the outburst is ongoing. The current observation schedule is available on https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html. Analysis of the full NICER and NuSTAR datasets is in progress. We encourage multiwavelength observations of the system.

We thank the NuSTAR team for rapidly approving the ToO request.
Original languageEnglish
Publication date27 Nov 2020
Publication statusPublished - 27 Nov 2020
SeriesThe Astronomer's telegram
NumberATel #14227

Keywords

  • X-ray
  • Binary
  • Neutron Star
  • Transient
  • Pulsar

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