Using X-ray continuum-fitting to estimate the spin of MAXI J1305-704

TL;DR

This study estimates the black hole spin in MAXI J1305-704 using X-ray continuum-fitting, revealing a moderate spin value with high confidence, based on stable thermal-state spectra.

Contribution

First measurement of the black hole spin in MAXI J1305-704 using continuum-fitting with high-quality X-ray spectra.

Findings

Black hole spin estimated as a* = 0.87_{-0.13}^{+0.07} at 68.3% confidence

Identified 13 optimal spectra for continuum-fitting analysis

First spin measurement for this transient X-ray binary

Abstract

MAXI J1305-704 is a transient X-ray binary with a black hole primary. It was discovered on April 9, 2012, during its only known outburst. MAXI J1305-704 is also a high inclination low-mass X-ray binary with prominent dip features in its light curves, so we check the full catalog of 92 \emph{Swift}/XRT continuous observations of MAXI J1305-704, focusing only on the stable spectra. We select 13 ``gold" spectra for which the root mean square RMS <0.075 and the coronal scattered fraction $f_{\mathrm{sc}} \lesssim 25 \%$. These ``gold" data are optimal thermal-state observations for continuum-fitting modeling, in which the disk extends to the innermost-stable circular orbit and is geometrically thin. The black hole spin was unknown for this object before. By utilizing the X-ray continuum fitting method with the relativistic thin disk model \texttt{kerrbb2} and supplying the known dynamical binary system parameters, we find MAXI J1305-704 has a moderate spin ($a_{*}=0.87_{-0.13}^{+0.07}$) at a 68.3\% confidence level. This is the first determination of MAXI J1305-704's spin.