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We present the first results on the new black hole candidate, MAXI J1305-704, observed by MAXI/GSC. The new X-ray transient, named as MAXI J1305-704, was first detected by the MAXI-GSC all-sky survey on 2012 April 9 in the direction to the outer Galactic bulge at (l,b)=(304.2deg,-7.6deg). The Swift/XRT follow-up observation confirmed the uncatalogued point source and localized to the position at (13h06m56s.44,-70d274.91). The source continued the activity for about five months until 2012 August. The MAXI/GSC light curve in the 2--10 keV band and the variation of the hardness ratio of the 4-10 keV to the 2-4 keV flux revealed the hard-to-soft state transition on the the sixth day (April 15) in the brightening phase and the soft-to-hard transition on the ~60th day (June 15) in the decay phase. The luminosity at the initial hard-to-soft transition was significantly higher than that at the soft-to-hard transition in the decay phase. The X-ray spectra in the hard state are represented by a single power-law model with a photon index of ~2.0, while those in the soft state need such an additional soft component as represented by a multi-color disk blackbody emission with an inner disk temperature ~0.5--1.2 keV. All the obtained features support the source identification of a Galactic black-hole binary located in the Galactic bulge.
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We report on a Chandra/HETG X-ray spectrum of the black hole candidate MAXI J1305-704. A rich absorption complex is detected in the Fe L band, including density-sensitive lines from Fe XX, XXI, and XXII. Spectral analysis over three bands with photoionization models generally requires a gas density of n > 1 E+17 cm^-3. Assuming a luminosity of L = 1 E+37 erg/s, fits to the 10-14 A band constrain the absorbing gas to lie within r = 3.9(7) E+3 km from the central engine, or about r = 520 +/- 90 (M/5 Msun) r_g, where r_g = GM/c^2. At this distance from the compact object, gas in Keplerian orbits should have a gravitational red-shift of z = v/c ~ 3 +/- 1 E-3 (M/5 Msun), and any tenuous inflowing gas should have a free-fall velocity of v/c ~ 6 +/- 1 E-2 (M/5 Msun)^1/2. The best-fit single-zone photoionization models measure a red-shift of v/c = 2.6-3.2 E-3. Models with two zones provide significantly improved fits; the additional zone is measured to have a red-shift of v/c =4.6-4.9 E-2 (models including two zones suggest slightly different radii and may point to lower densities). Thus, the shifts are broadly consistent with the photoionization radius. The results may be explained in terms of a failed wind like those predicted in some numerical simulations. We discuss our results in the context of accretion flows across the mass scale, and the potential role of failed winds in black hole state transitions.
MAXI J1305-704 has been proposed as a high-inclination candidate black hole X-ray binary in view of its X-ray properties and dipping behaviour during outburst. We present photometric and spectroscopic observations of the source in quiescence that allow us to reveal the ellipsoidal modulation of the companion star and absorption features consistent with those of an early K-type star (Teff = 4610 +130 -160 K). The central wavelengths of the absorption lines vary periodically at Porb = 0.394 +- 0.004 d with an amplitude of K2 = 554 +- 8 km/s . They imply a mass function for the compact object of f(M1) = 6.9 +- 0.3 Msun, confirming its black hole nature. The simultaneous absence of X-ray eclipses and the presence of dips set a conservative range of allowed inclinations 60 deg < i < 82 deg, while modelling of optical light curves further constrain it to i = 72 +5 -8 deg. The above parameters together set a black hole mass of M1 = 8.9 +1.6 -1.0 Msun and a companion mass of M2=0.43 +- 0.16 Msun, much lower than that of a dwarf star of the observed spectral type, implying it is evolved. Estimates of the distance to the system (d = 7.5 +1.8 -1.4 kpc) and space velocity (vspace = 270 +- 60 km/s ) place it in the Galactic thick disc and favour a significant natal kick during the formation of the BH if the supernova occurred in the Galactic Plane.
We present the first results on the black hole candidate XTE J1752-223 from the Gas Slit Camera (GSC) on-board the Monitor of All-sky X-ray Image (MAXI) on the International Space Station. Including the onset of the outburst reported by the Proportional Counter Array on-board the Rossi X-ray Timing Explorer on 2009 October 23, the MAXI/GSC has been monitoring this source approximately 10 times per day with a high sensitivity in the 2-20 keV band. XTE J1752-223 was initially in the low/hard state during the first 3 months. An anti-correlated behavior between the 2-4 keV and 4-20 keV bands were observed around January 20, 2010, indicating that the source exhibited the spectral transition to the high/soft state. A transient radio jet may have been ejected when the source was in the intermediate state where the spectrum was roughly explained by a power-law with a photon index of 2.5-3.0. The unusually long period in the initial low/hard state implies a slow variation in the mass accretion rate, and the dramatic soft X-ray increase may be explained by a sudden appearance of the accretion disk component with a relatively low innermost temperature (0.4-0.7 keV). Such a low temperature might suggest that the maximum accretion rate was just above the critical gas evaporation rate required for the state transition.
We report on the detection and subsequent X-ray monitoring of the new Galactic black hole candidate MAXI J1535-571 with the MAXI/GSC. Afterthe discovery on 2017 September 2 made independently with MAXI and the Swift/BAT, the source brightened gradually, and in a few weeks, reached the peak intensity of ~5 Crab, or ~1.6 x 10^{-7} erg cm^{-2} s^{-1} in terms of the 2--20 keV flux. On the initial outburst rise, the X-ray spectrum was described by a power-law model with a photon index of ~2, while after a hard-to-soft transition which occurred on September 18, the spectrum required a disk blackbody component in addition. At around the flux peak, the 2-8 keV and 15-50 keV light curves showed quasi-periodic and anti-correlated fluctuations with an amplitude of 10--20%, on a time scale of ~1-day. Based on these X-ray properties obtained with the MAXI/GSC, we discuss the evolution of the spectral state of this source, and give constraints on its system parameters.
MAXI J1813-095 is an X-ray transient discovered during an outburst in 2018. We report on X-ray and optical observations obtained during this event, which indicate that the source is a new low-mass X-ray binary. The outburst lasted ~70 d and peaked at Lx(0.5-10keV)~7.6 x 10^36 erg s-1, assuming a distance of 8 kpc. Swift/XRT follow-up covering the whole activity period shows that the X-ray emission was always dominated by a hard power-law component with a photon index in the range of 1.4-1.7. These values are consistent with MAXI J1813-095 being in the hard state, in agreement with the ~30 per cent fractional root-mean-square amplitude of the fast variability (0.1-50 Hz) inferred from the only XMM-Newton observation available. The X-ray spectra are well described by a Comptonization emission component plus a soft, thermal component (kT ~0.2 keV), which barely contributes to the total flux (<8 per cent). The Comptonization y-parameter (~1.5), together with the low temperature and small contribution of the soft component supports a black hole accretor. We also performed optical spectroscopy using the VLT and GTC telescopes during outburst and quiescence, respectively. In both cases the spectrum lack emission lines typical of X-ray binaries in outburst. Instead, we detect the Ca II triplet and H_alpha in absorption. The absence of velocity shifts between the two epochs, as well as the evolution of the H_alpha equivalent width, strongly suggest that the optical emission is dominated by an interloper, likely a G-K star. This favours a distance >3 kpc for the X-ray transient.
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