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Register a new userThe face-on disk of MAXI J1836-194
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T. D. Russell
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We present Very Large Telescope optical spectra of the black hole candidate X-ray binary MAXI J1836-194 at the onset of its 2011 outburst. Although the spectrum was taken at the beginning of the outburst and contains a significant contribution from the optically-thin synchrotron emission that originates in the radio jet, we find that the accretion disk was already large and bright. Single-peaked, narrow H$alpha$ and He II $lambda$4686 lines imply the most face-on accretion disk observed in a black hole low-mass X-ray binary to date, with an inclination angle between 4$^{circ}$ and 15$^{circ}$, assuming a black hole mass of between 5 M$_odot$ and 12 M$_odot$, for distances of between 4 and 10 kpc. We use New Technology Telescope observations of the system in quiescence to place strong upper limits on the mass and radius of the donor star and the orbital period. The donor is a main sequence star with a mass < 0.65 M$_{odot}$ and a radius < 0.59 R$_{odot}$ with an orbital period of < 4.9 hours. From those values and Roche lobe geometry constraints we find that the compact object must be >1.9 M$_{odot}$ if the system is located 4 kpc away and >7.0 M$_{odot}$ at 10 kpc.
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The X-ray transient MAXI J1836-194 is a newly-identified Galactic black hole binary candidate. As most X-ray transients, it was discovered at the beginning of an X-ray outburst. After the initial canonical X-ray hard state, the outburst evolved into a hard intermediate state and then went back to the hard state. The existing RATAN-600 radio monitoring observations revealed that it was variable on a timescale of days and had a flat or inverted spectrum, consistent with optically thick synchrotron emission, possibly from a self-absorbed jet in the vicinity of the central compact object. We observed the transient in the hard state near the end of the X-ray outburst with the European VLBI Network (EVN) at 5 GHz and the Chinese VLBI Network (CVN) at 2.3 and 8.3 GHz. The 8.3 GHz observations were carried out at a recording rate of 2048 Mbps using the newly-developed Chinese VLBI data acquisition system (CDAS), twice higher than the recording rate used in the other observations. We successfully detected the low-declination source with a high confidence level in both observations. The source was unresolved (<=0.5 mas), which is in agreement with an AU-scale compact jet.
We present a detailed spectral analysis of the black hole candidate MAXI J1836-194. The source was caught in the intermediate state during its 2011 outburst by Suzaku and RXTE. We jointly fit the X-ray data from these two missions using the relxill model to study the reflection component, and a steep inner emissivity profile indicating a compact corona as the primary source is required in order to achieve a good fit. In addition, a reflection model with a lamp-post configuration (relxilllp), which is normally invoked to explain the steep emissivity profile, gives a worse fit and is excluded at 99% confidence level compared to relxill. We also explore the effect of the ionization gradient on the emissivity profile by fitting the data with two relativistic reflection components, and it is found that the inner emissivity flattens. These results may indicate that the ionization state of the disc is not constant. All the models above require a supersolar iron abundance higher than 4.5. However, we find that the high-density version of reflionx can describe the same spectra even with solar iron abundance well. A moderate rotating black hole (a* = 0.84-0.94) is consistently obtained by our models, which is in agreement with previously reported values.
We study transient Galatic black hole candidate MAXI~J1836-194 during its 2011 outburst using RXTE/PCA archival data. 2.5-25~keV spectra are fitted with Two Component Advective Flow (TCAF) model fits file as an additive table local model in XSPEC. From TCAF model spectral fits, physical parameters such as Keplerian disk rate, sub-Keplerian halo rate, shock location and compression ratio are extracted directly for better understanding of accretion processes around the BHC during this outburst. Low frequency quasi-periodic oscillation (QPO) are observed sporadically during the entire epoch of the outburst, with a general trend of increasing frequency during rising and decreasing frequency during declining phases of the outburst, as in other transient BHCs. The nature of the variation of the accretion rate ratio (ratio of halo and disk rates) and QPOs (if observed), allows us to properly classify entire epoch of the outburst into following two spectral state, such as hard (HS), hard-intermediate (HIMS). These states are observed in the sequence of HS (Ris.) $rightarrow$ HIMS (Ris.) $rightarrow$ HIMS (Dec.) $rightarrow$ HS (Dec.). This outburst of MAXI~J1836-194 could be termed as `failed outburst, since no observation of soft (SS) and soft-intermediate (SIMS) spectral state are found during the entire outburst.
MAXI J1836-194 is a Galactic black hole candidate X-ray binary that was discovered in 2011 when it went into outburst. In this paper, we present the full radio monitoring of this system during its `failed outburst, in which the source did not complete a full set of state changes, only transitioning as far as the hard intermediate state. Observations with the Karl G. Jansky Very Large Array (VLA) and Australia Telescope Compact Array (ATCA) show that the jet properties changed significantly during the outburst. The VLA observations detected linearly polarised emission at a level of ~1% early in the outburst, increasing to ~3% as the outburst peaked. High-resolution images with the Very Long Baseline Array (VLBA) show a ~15 mas jet along the position angle $-21 pm 2^circ$, in agreement with the electric vector position angle found from our polarisation results ($-21 pm 4^circ$), implying that the magnetic field is perpendicular to the jet. Astrometric observations suggest that the system required an asymmetric natal kick to explain its observed space velocity. Comparing quasi-simultaneous X-ray monitoring with the 5 GHz VLA observations from the 2011 outburst shows an unusually steep hard-state radio/X-ray correlation of $L_{rm R} propto L_{rm X}^{1.8pm0.2}$, where $L_{rm R}$ and $L_{rm X}$ denote the radio and X-ray luminosities, respectively. With ATCA and Swift monitoring of the source during a period of re-brightening in 2012, we show that the system lay on the same steep correlation. Due to the low inclination of this system, we then investigate the possibility that the observed correlation may have been steepened by variable Doppler boosting.
We report striking changes in the broadband spectrum of the compact jet of the black hole transient MAXI J1836-194 over state transitions during its discovery outburst in 2011. A fading of the optical-infrared (IR) flux occurred as the source entered the hard-intermediate state, followed by a brightening as it returned to the hard state. The optical-IR spectrum was consistent with a power law from optically thin synchrotron emission, except when the X-ray spectrum was softest. By fitting the radio to optical spectra with a broken power law, we constrain the frequency and flux of the optically thick/thin break in the jet synchrotron spectrum. The break gradually shifted to higher frequencies as the source hardened at X-ray energies, from ~ 10^11 to ~ 4 x 10^13 Hz. The radiative jet luminosity integrated over the spectrum appeared to be greatest when the source entered the hard state during the outburst decay (although this is dependent on the high energy cooling break, which is not seen directly), even though the radio flux was fading at the time. The physical process responsible for suppressing and reactivating the jet (neither of which are instantaneous but occur on timescales of weeks) is uncertain, but could arise from the varying inner accretion disk radius regulating the fraction of accreting matter that is channeled into the jet. This provides an unprecedented insight into the connection between inflow and outflow, and has implications for the conditions required for jets to be produced, and hence their launching process.
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