NICER detection of QPOs from EXO 1846-031

Referred to by ATel #: 12977, 12992, 13012, 13036, 13037, 13255, 13344 Following reports of renewed activity from the X-ray transient EXO 1846-031 (ATel #12969, ATel #12968), NICER executed rapid follow-up observations. Pointed observations with NICER began on 2019 July 31 14:10 UTC, collecting a total exposure of 3.4 ks.

We clearly detect the source at an average 0.5-10 keV count rate of ~210 c/s, well over the estimated background rate of 0.5 c/s in the same energy band. A preliminary spectral analysis indicates the source spectrum could be reasonably well described using an absorbed power-law model with a column density of N_H = (5.8 +/- 0.1)E22 cm^-2 and photon-index of Gamma = 1.52 +/- 0.01.

The unabsorbed 0.5-10 keV source flux is (4.7 +/- 0.1)E-9 erg cm^-2 s^-1. We further find systematic residuals near 6 keV, indicating a Fe K reflection component is likely present. We computed the power spectrum by averaging the Fourier transforms of 128-s duration 0.5-10 keV light curve segments. Superimposed on a ~25% fractional rms band-limited low-frequency noise structure (integrated between 0.1-10 Hz), the power spectrum shows a sharp QPO at 0.26 Hz with an amplitude of (7.0 +/- 0.5)% fractional rms, and a broader (9.8 +/- 1.4)% fractional rms amplitude QPO at a centroid frequency of 0.61 Hz.

The X-ray spectral and variability properties are consistent with a hard-state black hole X-ray binary, which suggests the QPOs can be identified as the harmonics of a low-frequency (Type C) QPO. We note, however, that a neutron star nature cannot be firmly ruled out at this time. Further NICER monitoring of this source is underway.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.