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

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.