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MAXI J1631-479 is a new X-ray transient

ATel #12340; Hiromasa Miyasaka (Caltech), John A. Tomsick (SSL/UCB), Yanjun Xu (Caltech), and Fiona A. Harrison (Caltech)
on 30 Dec 2018; 08:16 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 12396, 12418, 12421, 12438, 12440, 12504

On 2018 December 21, MAXI/GSC detected a bright hard X-ray transient source, MAXI J1631-479, in the Norma region (Kobayashi et al., ATEL#12320). The X-ray pulsar AX J1631.9-4752/IGR J16320-4751 is within the MAXI error circle and was suggested to be the source producing the bright transient emission. However, the Swift/BAT position reported for the bright source (GCN#23550) is 8.4 arcminutes away from the XMM position of AX J1631.9-4752. As this is significantly more than the 3 arcminute uncertainty (90% confidence) in the Swift/BAT position, we obtained two NuSTAR observations on December 28: a 14 ks observation pointed at the Swift/BAT position and a 12 ks observation pointed at AX J1631.9-4752.

In the 14 ks NuSTAR observation, we found a very bright source at R.A. = 247.806 deg, Decl. = -47.805 deg (J2000) with an uncertainty of 15 arcsec. This position is consistent with the Swift/BAT position and is not consistent with the position of AX J1631.9-4752. In addition, a search of the SIMBAD database does not show any known source consistent with the NuSTAR position. Thus, we conclude that MAXI J1631-479 is a new X-ray transient.

The NuSTAR observations were performed when the source was 34 degrees away from the sun. As the sun is close to the FOV, the star tracker used for aspect reconstruction was only available approximately for 2.6 ks, but we were able to obtain the source position with NuSTAR nominal astrometric accuracy due to the brightness of the source.

The source is detected across the 3-79 keV NuSTAR bandpass with a count rate in excess of 600 c/s (FPMA and FPMB combined). An absorbed disk-blackbody plus power-law model provides a reasonably good description of the continuum with a reduced chi2 of 1.25 for 893 degrees of freedom. The column density is (2.9+/-0.2)e22 atoms/cm2 (using wilm abundances), the temperature of the disk-blackbody is 1.12+/-0.01 keV, and the power-law photon index is 2.39+/-0.02 (90% confidence errors). The 3-79 keV and 2-10 keV absorbed fluxes are 1.8e-8 erg/cm2/s and 1.7e-8 erg/cm2/s, respectively. The residuals show clear evidence for an iron Kalpha emission line, and adding a broad gaussian with an equivalent width of 90 eV improves the fit significantly. This demonstrates that we are seeing a reflection component in the spectrum.

The spectral properties of MAXI J1631-479 indicate that it is very likely to be an accreting black hole in the soft state. In the future, further work on the MAXI and Swift/BAT data may provide more information about the overall evolution of the current outburst. The source is still in outburst, and multi-wavelength observations are encouraged. Based on the behavior of other black hole transients, the outburst may last for another month or more.