NICER detection of QPOs from EXO 1846-031

P. M. Bult*, K. C. Gendreau, Z. Arzoumanian, T. E. Strohmayer, P. S. Ray, S. Guillot, W. Iwakiri, J. Homan, D. Altamirano, G. K. Jaisawal, J. M. Miller

*Corresponding author for this work

Research output: Other contributionNet publication - Internet publicationResearch


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.
Original languageEnglish
Publication date1 Aug 2019
Publication statusPublished - 1 Aug 2019
SeriesThe Astronomer's telegram
NumberATel #12976


  • Binary
  • Black Hole
  • Transient


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