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Register a new userA Transient Sub-Eddington Black Hole X-ray Binary Candidate in the Dust Lanes of Centaurus A
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We report the discovery of a bright X-ray transient, CXOU J132527.6-430023, in the nearby early-type galaxy NGC 5128. The source was first detected over the course of five Chandra observations in 2007, reaching an unabsorbed outburst luminosity of 1-2*10^38 erg/s in the 0.5-7.0 keV band before returning to quiescence. Such luminosities are possible for both stellar-mass black hole and neutron star X-ray binary transients. Here, we attempt to characterize the nature of the compact object. No counterpart has been detected in the optical or radio sky, but the proximity of the source to the dust lanes allows for the possibility of an obscured companion. The brightness of the source after a >100 fold increase in X-ray flux makes it either the first confirmed transient non-ULX black hole system in outburst to be subject to detailed spectral modeling outside the Local Group, or a bright (>10^38 erg/s) transient neutron star X-ray binary, which are very rare. Such a large increase in flux would appear to lend weight to the view that this is a black hole transient. X-ray spectral fitting of an absorbed power law yielded unphysical photon indices, while the parameters of the best-fit absorbed disc blackbody model are typical of an accreting ~10 Msol black hole in the thermally dominant state.
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We report the discovery of a bright transient X-ray source, CXOU J132518.2-430304, towards Centaurus A (Cen A) using six new Chandra X-Ray Observatory observations in 2007 March--May. Between 2003 and 2007, its flux has increased by a factor of >770. The source is likely a low-mass X-ray binary in Cen A with unabsorbed 0.3-10 keV band luminosities of (2-3) x 10^{39} erg s^-1 and a transition from the steep-power law state to the thermal state during our observations. CXOU J132518.2-430304 is the most luminous X-ray source in an early-type galaxy with extensive timing information that reveals transience and a spectral state transition. Combined with its luminosity, these properties make this source one of the strongest candidates to date for containing a stellar-mass black hole in an early-type galaxy. Unless this outburst lasts many years, the rate of luminous transients in Cen A is anomalously high compared to other early-type galaxies.
We present a broadband radio study of the transient jets ejected from the black hole candidate X-ray binary MAXI J1535-571, which underwent a prolonged outburst beginning on 2 September 2017. We monitored MAXI J1535-571 with the Murchison Widefield Array (MWA) at frequencies from 119 to 186 MHz over six epochs from 20 September to 14 October 2017. The source was quasi-simultaneously observed over the frequency range 0.84-19 GHz by UTMOST (the upgraded Molonglo Observatory Synthesis Telescope), the Australian Square Kilometre Array Pathfinder, the Australia Telescope Compact Array (ATCA), and the Australian Long Baseline Array (LBA). Using the LBA observations from 23 September 2017, we measured the source size to be $34pm1$ mas. During the brightest radio flare on 21 September 2017, the source was detected down to 119 MHz by the MWA, and the radio spectrum indicates a turnover between 250 and 500 MHz, which is most likely due to synchrotron self-absorption (SSA). By fitting the radio spectrum with a SSA model and using the LBA size measurement, we determined various physical parameters of the jet knot (identified in ATCA data), including the jet opening angle (= $4.5pm1.2^{circ}$) and the magnetic field strength (= $104^{+80}_{-78}$ mG). Our fitted magnetic field strength agrees reasonably well with that inferred from the standard equipartition approach, suggesting the jet knot to be close to equipartition. Our study highlights the capabilities of the Australian suite of radio telescopes to jointly probe radio jets in black hole X-ray binaries (BH-XRBs) via simultaneous observations over a broad frequency range, and with differing angular resolutions. This suite allows us to determine the physical properties of XRB jets. Finally, our study emphasizes the potential contributions that can be made by the low-frequency part of the Square Kilometre Array (SKA-Low) in the study of BH-XRBs.
AT2019wey is a Galactic low mass X-ray binary with a candidate black hole accretor first discovered as an optical transient by ATLAS in December 2019. It was then associated with an X-ray source discovered by SRG in March 2020. After observing a brightening in X-rays in August 2020, VLA observations of the source revealed an optically thin spectrum that subsequently shifted to optically thick, as the source continued to brighten in radio. This motivated observations of the source with the VLBA. We found a resolved source that we interpret to be a steady compact jet, a feature associated with black hole X-ray binary systems in the hard X-ray spectral state. The jet power is comparable to the accretion-disk X-ray luminosity. Here, we summarize the results from these observations.
We report the first half-year monitoring of the new Galactic black hole candidate MAXI J1348-630, discovered on 2019 January 26 with the Gas Slit Camera (GSC) on-board MAXI. During the monitoring period, the source exhibited two outburst peaks, where the first peak flux (at T=14 day from the discovery of T =0) was ~4 Crab (2-20 keV) and the second one (at T =132 day) was ~0.4 Crab (2-20 keV). The source exhibited distinct spectral transitions between the high/soft and low/hard states and an apparent q-shape curve on the hardness-intensity diagram, both of which are well-known characteristics of black hole binaries. Compared to other bright black hole transients, MAXI J1348-630 is characterized by its low disk-temperature (~0.75 keV at the maximum) and high peak flux in the high/soft state. The low peak-temperature leads to a large innermost radius that is identified as the Innermost Stable Circular Orbit (ISCO), determined by the black hole mass and spin. Assuming the empirical relation between the soft-to-hard transition luminosity (Ltrans) and the Eddington luminosity (LEdd), Ltrans/LEdd ~ 0.02, and a face-on disk around a non-spinning black hole, the source distance and the black hole mass are estimated to be D ~ 4 kpc and ~7 (D/4 kpc) Mo, respectively. The black hole is more massive if the disk is inclined and the black hole is spinning. These results suggest that MAXI J1348-630 may host a relatively massive black hole among the known black hole binaries in our Galaxy.
We identify SDSS J153636.22+044127.0, a QSO discovered in the Sloan Digital Sky Survey, as a promising candidate for a binary black hole system. This QSO has two broad-line emission systems separated by 3500 km/sec. The redder system at z=0.3889 also has a typical set of narrow forbidden lines. The bluer system (z=0.3727) shows only broad Balmer lines and UV Fe II emission, making it highly unusual in its lack of narrow lines. A third system, which includes only unresolved absorption lines, is seen at a redshift, z=0.3878, intermediate between the two emission-line systems. While the observational signatures of binary nuclear black holes remain unclear, J1536+0441 is unique among all QSOs known in having two broad-line regions, indicative of two separate black holes presently accreting gas. The interpretation of this as a bound binary system of two black holes having masses of 10^8.9 and 10^7.3 solar masses, yields a separation of ~ 0.1 parsec and an orbital period of ~100 years. The separation implies that the two black holes are orbiting within a single narrow-line region, consistent with the characteristics of the spectrum. This object was identified as an extreme outlier of a Karhunen-Loeve Transform of 17,500 z < 0.7 QSO spectra from the SDSS. The probability of the spectrum resulting from a chance superposition of two QSOs with similar redshifts is estimated at 2X10^-7, leading to the expectation of 0.003 such objects in the sample studied; however, even in this case, the spectrum of the lower redshift QSO remains highly unusual.
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