Fading outbursts of EXO 2030+375

ATel #8835; Felix Fuerst (Caltech), Colleen A. Wilson-Hodge (NASA-MSFC), Peter Kretschmar, Jari Kajava (ESA/ESAC), Matthias Kuehnel (Remeis-Observatory & ECAP)
on 17 Mar 2016; 23:16 UT
Credential Certification: Felix Fuerst (ff@caltech.edu)

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The Be X-ray binary pulsar EXO 2030+375, first detected in 1985 (Parmar et al., IAUC 4066 ), has shown a significant detected X-ray outburst at nearly every periastron passage of its 46-day orbit for the past ~25 years. However, since early 2015, this regularity has been decreasing with first every second outburst much fainter than usual and more recently hardly any outburst activity at all. At the same time, the monotonic increase of the X-ray pulse frequency of the neutron star (spin-up), as monitored first by RXTE and then the Fermi/GBM, has flattened to almost zero.

This situation is reminiscent of a torque reversal accompanied by a sudden drop in fluxes observed with BATSE in 1994/1995 as reported in Wilson-Hodge (2015, 6th Fermi Symposium).

In order to explore the source behavior in the poorly explored low-flux state with a possible transition to a state of centrifugal inhibition of accretion we have obtained a brief Swift/XRT monitoring program with five observations centered on the outburst peak as predicted from previous data, with roughly daily spacing beginning on MJD 57455.67.

EXO 2030+375 is detected in all five XRT snapshots, but the source flux in 2—8 keV is low around (2.10 +/- 0.25)e-11 erg/s/cm^2 for the first three observations (MJD 57455-57457) and then decreased further to (1.48 +/- 0.20)e-11 erg/s/cm^2 on MJD 57458 and (1.05 +/- 0.22)e-11 erg/s/cm^2 in the last observation (MJD 57459). The observed luminosity on the order of 5e34 erg/s for a distance of 7kpc is low enough that accretion directly from the stellar wind can account for it, indicating that the neutron star is not accreting from the Be disk.

A preliminary spectral fit finds parameters very similar to those observed in outbursts (e.g. Naik et al. 2013, ApJ 764, 158), i.e., a hard power law with Gamma = 1.4+/-0.25 absorbed with an equivalent hydrogen column of N_H = (4.3 +/- 0.9)e22 cm^-2. The spectral similarity is remarkable despite more than 2 orders of magnitude difference in flux.

We detect significant pulsations around 41.3s in the observations performed on MJD 57456 and 57457. The pulse period is in agreement with the last Fermi/GBM measurement. The pulsed fraction, defined as (Max-Min)/(Max+Min), is around 0.36, somewhat lower than observed during outburst with INTEGRAL (Klochkov et al., 2008, A&A 491, 833). The pulse profile looks sinusoidal, but due to the low count-rate and corresponding coarse resolution, detailed comparisons of its shape to previous data are not informative.

The fading outbursts of EXO 2030+375 is a new and unusual phenomenon for that source and indicates significant changes in the physical conditions of the system. Follow-up observations at all wavelengths and higher sensitivity are encouraged, in particular in the optical to study the companion and its Be disk.

We thank the Swift PI and team for making these observations possible.