Monitoring the X-ray spectra and pulse profile evolution of the Be X-ray binary EXO 2030+375 during the giant outburst of 2021

ATel #14931; P. Pradhan (MIT), P. Thalhammer (Remeis-Observatory & ECAP, FAU Erlangen-Nuremberg), R. Ballhausen (UMD & NASA-GSFC/CRESST), K. Pottschmidt (UMBC & NASA-GSFC/CRESST), P. Kretschmar (ESA-ESAC), F. Fuerst (ESA-ESAC), G. K. Jaisawal (DTU Space), C. Malacaria (USRA), J. B. Coley (Howard University & NASA-GSFC/CRESST), M. T. Wolff (NRL), C. Ferrigno (ISDC), C. A. Wilson-Hodge (NASA-MSFC), J. Wilms (Remeis-Observatory & ECAP), P. Jenke (UAH), J. Kennea (PSU), G. Raman (PSU)
on 21 Sep 2021; 17:39 UT
Credential Certification: Katja Pottschmidt (katja@umbc.edu)

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EXO 2030+375 is a Be X-ray binary discovered during a giant outburst in 1985. Apart from normal Type-I outbursts associated with its periastron passage every ~46 days (Wilson et al. 2008), the source exhibited two giant outbursts in 1985 and 2006. Since 20 July 2021, EXO 2030+375 showed an increase in flux indicative of the onset of another giant Type II outburst as reported by MAXI/GSC in July 2021 (ATel #14809) and as also seen in the Swift/BAT lightcurves. Following this, NICER started monitoring the source and found that from 28 July 2021 to 24 August 2021 the 0.5-10 keV flux rose from 1.3e-09 erg/cm^2/s to 5.7e-09 erg/cm^2/s (ATel #14911). The NICER observations also show that the pulsar in EXO 2030+375 is spinning at a period of ~41.3 seconds (ATel #14911).

Since 25 August 2021, EXO 2030+375 has also been monitored with Swift/XRT. Modeling the XRT spectra of these observations with an absorbed power law indicates that the (absorbed) flux in the 0.5-10 keV band increased from 5.9e-09 erg/cm^2/s on 25 August 2021 to 6.9e-09 erg/cm^2/s on 05 September 2021. Thereafter, the flux remained nearly constant at ~6.7e-09 erg/cm^2/s in subsequent observations on 11 September 2021, 15 September 2021, and 16 September 2021. The X-ray spectra are almost unchanged during these observations. with the photon index ~1.47 and the equivalent hydrogen column density ~(3.3-3.7)e22 cm^-2, consistent with the earlier NICER findings.

EXO 2030+375 is the first pulsar in which a dramatic flip in its pulse profile was observed with changes in luminosity. As the luminosity decreased, the dominant X-ray beaming pattern changed from a fan-beam to a pencil-beam (Parmar, White, and Stella, 1989, ApJ 338, 373). For the current outburst we find that the Swift/XRT pulse profiles are triple-peaked with the main peak becoming more dominant as luminosity increases, as has also been reported for the NICER pulse profiles.

Additional NICER monitoring of the source from 25 August 2021 to 16 September 2021, i.e., contemporaneous with the Swift/XRT monitoring described above, continues to show spectral parameters and pulse profiles similar to that reported in ATel #14911.

The Fermi/GBM accreting pulsar monitor reveals that as of 18 September 2021 (59475.5 MJD), the spin frequency has increased to 24.2499(6) mHz and the RMS pulsed flux has risen to and plateaued at (2.82 +/- 0.02)e-09 erg/cm^2/s in the 12-50 keV range. Currently EXO 2030+375 is spinning up at 0.065 cycles/day/day. Fermi/GBM has been monitoring the RMS pulsed flux and frequency of EXO 2030+375 since 2008.

We will continue to monitor EXO 2030+375 during this outburst and strongly encourage observations at other wavelengths.