Disk Reflection and Winds in a NuSTAR Observation of the Black Hole Candidate EXO 1846-031

ATel #13012; J. M. Miller, A. Zoghbi (Univ. of Michigan), P. Gandhi, J. Paice (Univ. of Southampton)
on 9 Aug 2019; 14:32 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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EXO 1846-031 is an X-ray binary and black hole candidate. After 34 years of quiescence, it has recently started a new outburst (ATEL #12968, #12969, #12976, #12977, #12992).

We observed EXO 1846-031 using NuSTAR. The net exposure of 22 ks was obtained, starting on 2019-08-03 at 02:01:09 UT. The source is clearly detected across the full NuSTAR band (3-79 keV), with an average count rate of 280 c/s (FPMA and FPMB combined). The light curve shows no bursts indicative of a neutron star.

In general terms, the largely non-thermal spectrum can be described with a cool disk (kT = 0.45 keV), an intermediate power-law index (Gamma = 1.93), and a relatively high cut-off energy (E ~ 115 keV). These parameters are broadly consistent with a bright hard state, or hard intermediate state. A flux of 9.9 E-9 erg/cm2/s is measured in the 3-79 keV band. An unabsorbed flux of 3.6 E-8 erg/cm2/s over the 0.5-79.0 keV band is implied. The measured column density along the line of sight to EXO 1846-031 appears to be relatively high, between 5-11 E+22 cm^-2 depending on the model, abundances, and cross sections employed.

Fits to the spectrum with simple continuum models leave strong residuals indicative of relativistic disk reflection. We therefore explored two families of fits with "relxill" (Garcia et al. 2014, Dauser et al. 2014): one set with the basic "relxill," wherein nothing is assumed about the nature of the corona, and a second set assuming a "lamppost" geometry for the corona. Good fits are obtained with both, but those made without prior assumptions about the corona are statistically far superior. Solar abundances are acceptable in both cases.

Although the reflection fits are preliminary, they strongly prefer an accretion disk at the innermost stable circular orbit (ISCO), a near-maximal black hole spin parameter (a > 0.9 at the 5 sigma level of confidence), and a fairly high inclination value (75 deg). Equatorial disk winds are often detected in stellar-mass black holes viewed at high inclinations; in the NuSTAR spectrum, a highly ionized absorber is required to model a broad absorption feature centered at or above 7 keV. When fit with a simple Gaussian, we find that E = 7.01(1) keV, sigma ~ 0.1 keV, EW = 11(2) eV. Fits with "zxipcf" imply a high ionization (log xi = 4.4) and a high outflow velocity (v = 0.01c), making it a possible "ultra-fast outflow" or UFO.

A full analysis will be summarized in a journal paper led by a Univ. of Michigan graduate student. We thank NuSTAR for quickly executing this TOO/DDT observation.

Dauser, T., et al., 2014, MNRAS, 444, L100
Garcia, J., et al., 2014, ApJ, 782, 76