NICER and NuSTAR observations of the Be/X-ray binary pulsar 1A 0535+262 during the 2020 November giant outburst

Gaurava K. Jaisawal, Sachindra Naik*, K. C. Gendreau, Z. Arzoumanian, A. Jana, J. B. Coley, C. Malacaria, G. Vasilopoulos, F. Fuerst, D. Altamirano, K. Pottschmidt, J. Wilms, M. Wolff, D. J. K. Buisson

*Corresponding author for this work

Research output: Other contributionNet publication - Internet publicationResearch

Abstract

Following the detection of recent X-ray activity from the Be X-ray binary pulsar 1A 0535+262 on 2020 November 6 (ATel #14157), the Neutron Star Interior Composition Explorer (NICER) has observed the system at multiple epochs. The 0.5-10 keV source count rate continues to rise from 600 cps on November 7 to 2820 cps on November 14. To investigate the broadband emission of the pulsar, a 30 ks target-of-opportunity (ToO) observation was also carried out with the Nuclear Spectroscopic Telescope Array (NuSTAR) telescope on 2020 November 11-12. We detected an average count rate of 690 cps per module in the 3-79 keV range with NuSTAR. The current outburst is still in the rising phase (ATel #14171, #14173), and the pulsar is accreting above the 6.5 Crab level in the 15-50 keV range as seen by Swift/BAT around 2020 November 15 (e.g., see the BeXRB monitor page for 1A 0535+262 at http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_1A0535+262.html ). The X-ray source has now crossed the peak intensity level observed by BAT in the 2005 and 2009 giant outbursts. Initial timing analysis of the NICER and NuSTAR data shows a barycenter-corrected pulsation period at 103.57 s. The pulse shape is found to be complex - a broad profile with notches in soft X-rays evolving into a double-peaked profile in the 15-30 keV range, and further evolving into a broad and smooth profile in hard X-rays (>40 keV). The pulse fraction is negatively correlated with energy up to 20 keV, whereas a positive dependence between pulse fraction and energy is found in hard X-rays. A preliminary simultaneous spectral fit to one of the NuSTAR and NICER observations showed that the 0.7-79 keV spectrum can be described with an absorbed cutoff power-law model along with two thermal blackbody components, a Gaussian function for the 6.4 keV iron emission line, and a cyclotron absorption feature. The spectral parameters are: hydrogen column density N_H = (5.4+/-0.1)x10^21 cm^-2, soft black body temperature kT_1 = 0.42+/-0.01 keV, second black body temperature kT_2 = 1.57+/-0.01 keV, photon index \Gamma = 0.07+/-0.03, and cutoff energy 14.1+/-0.2 keV. The cyclotron line parameters are E_cyc = 44.3+/-0.5 keV and width W_cyc = 12+/-1 keV. The observed cyclotron line agrees with the values found in previous giant outbursts (see, e.g., Caballero et al. 2013). The best-fit reduced chi^2 is 1.34 for 2791 degrees of freedom. Quoted uncertainties are estimated at the 90% confidence level. The 0.5-79 keV unabsorbed source flux is derived to be (4.84+/-0.01)x10^{-8} erg/s/cm^2, corresponding to a source luminosity of 2.6x10^{37} erg/s at a distance of 2.13 kpc (Bailer-Jones et al. 2018). NICER is continuing to observe 1A 0535+262. Analysis of the full NICER and NuSTAR datasets is in progress. As the outburst is still rising we encourage additional observations of the source. NICER is a 0.2-12 keV X-ray telescope operating at the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA. We sincerely thank the NuSTAR team for promptly approving our ToO request.
Original languageEnglish
Publication date15 Nov 2020
Publication statusPublished - 15 Nov 2020
SeriesThe Astronomer's telegram
NumberATel #1417

Keywords

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

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