NICER observations during the rise and extended bright phase of the current giant outburst of EXO 2030+375

ATel #14911; 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), P. Pradhan (MIT), 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)
on 12 Sep 2021; 21:51 UT
Credential Certification: Katja Pottschmidt (katja@umbc.edu)

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The Be X-ray binary system EXO 2030+375 is currently undergoing its first giant outburst since 2006 (ATEL #14809). We report on the results of recent NICER observations of EXO 2030+375, performed during the rise of the outburst and its current extended bright phase, NICER started monitoring on 28 July 2021, with daily monitoring starting 24 August 2021. Until 2 September 2021 it has accumulated a total exposure of 35 ks.

The 0.5-10 keV flux initially rose from 1.3e-09 erg/cm^2/s to 5.7e-09 erg/cm^2/s on 24 August 2021. Since then the flux of EXO 2030+375 remained on a high level with only slight variations.

The spectrum of these observations can be described by a partially absorbed powerlaw with a weak blackbody component and a narrow iron emission line. Weak residuals around 2 keV are detected that might be calibration artifacts due to the quick-look nature of the data. A detailed investigation is ongoing.

During the rise of the outburst the spectral shape changed notably. The spectrum in the NICER energy band became softer with the photon index increasing from 1.048(22) to 1.460(6), while the black-body temperature decreased from 175(10) eV to 87.5(27) eV. The equivalent hydrogen column density remained constant during all observations, with NH of 3.0--3.2*10^22 cm^-2, a typical value for this source. During the later phase, the overall spectral shape remained remarkably constant except for slight flux changes.

In all observations strong pulsations with a period of ~41.3 s are present. At low fluxes, early in the outburst the pulse profiles are complex and triple-peaked, but constant with energy. During the extended high-flux period later in the outburst the pulse profile becomes simpler and is more clearly dominated by a single peak.

NICER, as well as Swift/XRT, will continue monitoring the giant outburst of EXO 2030+375 in the soft X-ray band. Further observations of this rare phenomenon, in particular in other wavebands, are encouraged.