NICER observations during the decline of the current giant outburst of EXO 2030+375: Beginning spectral and pulse profile change

The Be X-ray binary EXO 2030+375 is currently going through the third, type II outbursts (ATel #14809, #14911, #14931) since its discovery in 1985. The current outburst is weaker than the first two, with a peak flux of ~550 mCrab in the Swift/BAT monitor, roughly half of the previous peak fluxes. We report on the recent changes in both the spectrum and the pulse profile of EXO 2030+375 as observed by NICER.

During the rise of the current outburst the ongoing NICER monitoring showed a clear change in the source behavior, i.e., a softening of the spectrum and a flip in the pulse profile with two out of the three major peaks in the profile shrinking while the third became dominant (ATel #14911). The observed changes in the pulse profile mirrored those seen during previous type II outbursts (Parmar, White, and Stella, 1989, ApJ 338, 373; Epili, Naik, Jaisawal, Gupta, 2017, MNRAS, 472, 3455).

These changes with increased flux have been interpreted as signs for a reconfiguration in the accretion geometry from a pencil to a fan beam emission pattern. During the current decline of the outburst similar changes in both pulse profile and spectral shape are starting to be visible. The spectral evolution is marked by a hardening towards lower luminosities.

A fit to the spectrum with an absorbed black body and power-law model shows a decreasing photon index from 1.515+/-0.014 during the peak of the outburst on 14 September 2021 to 1.291+/-0.024 during the most recent NICER observations on 25 October 2021. The black body temperature, on the other hand, has remained constant at around 90 eV, while the hydrogen column density varied between (2.9-3.3)e22 cm^-2.

The relative strength of the black body component compared to the power law increases with decreasing flux, again mirroring the behavior during the rise. Since 14 October 2021 we see one of the weaker peaks in the pulse profile slowly growing, indicating the ongoing state transition back to the state early in the rise of the outburst.

The transition began as the 0.5-10 keV flux reached ~4e-9 erg/s/cm^2. The flux continues to decline. Its evolution is, however, more gradual than during the rise. The NICER monitoring will continue to catch any further changes at even lower luminosities, although until November 4th only very limited observations will be possible due to visibility constraints.

Follow-up observations with other instruments are therefore encouraged as the source transitions to this new accretion regime.