MeerKAT

Abstract:

We present results from the first successful millimetre (combined 33 and 35GHz) observations of the neutron star X-ray binary Circinus X-1, using the Australia Telescope Compact Array. The source was clearly detected in all three observing epochs. We see strong evidence for a periastron flare beginning at MJD 55519.9 ± 0.04 with estimated peak flux densities of up to 50mJy and which proceeds to decline over the following 4d. We directly resolve jet structures on sub-arcsecond scales. Flux density variability and distance from the core of nearby components suggest recent shock re-energization, though we are unable to directly connect this with the observed flare. We suggest that, if the emission is powered by an unseen outflow, then a phase delay exists between flare onset and subsequent brightening of nearby components, with flows reaching mildly relativistic velocities. Given resolved structure positions, in comparison to past observations of Cir X-1, we find evidence that jet direction may vary with distance from the core, or the source's precession parameters have changed. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

We present results from the first radio observations of a complete orbit (~17d) of the neutron star X-ray binary CircinusX-1 using the Australia Telescope Compact Array Broadband Backend, taken while the system was in an historically faint state. We have captured the rapid rise and decline of a periastron passage flare, with flux densities for 9d prior to the event stable at ~1mJy at 5.5GHz and ~0.5mJy at 9GHz. The highest flux densities of 43.0 ± 0.5mJy at 5.5GHz and 29.9 ± 0.6mJy at 9GHz were measured during the flare's decline (MJD 55206.69) which continues towards pre-flare flux densities over the following 6d. Imaging of pre-flare data reveals steady structure including two stable components within 15arcsec of the core which we believe may be persistent emission regions within the system's outflows, one of which is likely associated with the system's counter jet. Unlike past observations carried out in the system's brighter epochs, we observe no significant structural variations within ≈3arcsec of the core's position. Model subtraction and difference mapping provide evidence for variations slightly further from the core: up to 5arcsec away. If related to the observed core flare, then these variations suggest very high outflow velocities with Γ > 35, though this can be reduced significantly if we invoke phase delays of at least one orbital period. Interestingly, the strongest structural variations appear to the north-west of the core, opposite to the strongest arcsec-scale emission historically. We discuss the implications of this behaviour, including the possibility of precession or a kinked approaching jet. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

In this paper we report on Expanded Very Large Array radio and Chandra and Swift X-ray observations of the outburst decay of the transient black hole candidate MAXI J1659-152 in 2011. We discuss the distance to the source taking the high inclination into account and conclude that the source distance is probably 6 ± 2kpc. The lowest observed flux corresponds to a luminosity of ergs-1. This, together with the orbital period of 2.4h reported in the literature, suggests that the quiescent X-ray luminosity is higher than predicted on the basis of the orbital period-quiescent X-ray luminosity relationship. It is more in line with that expected for a neutron star, although the outburst spectral and timing properties reported in the literature strongly suggest that MAXI J1659-152 harbours a black hole. This conclusion is subject to confirmation of the lowest observed flux as the quiescent flux. The relation between the accretion and ejection mechanisms can be studied using the observed correlation between the radio and X-ray luminosities as these evolve over an outburst. We determine the behaviour of MAXI J1659-152 in the radio-X-ray diagram at low X-ray luminosities using the observations reported in this paper and at high X-ray luminosities using values reported in the literature. At high X-ray luminosities, the source lies closer to the sources that follow a correlation index steeper than 0.6-0.7. However, when compared to other sources that follow a steeper correlation index, the X-ray luminosity in MAXI J1659-152 is also lower. The latter can potentially be explained by the high inclination of MAXI J1659-152 if the X-ray emission comes from close to the source and the radio emission is originating in a more extended region. However, it is probable that the source was not in the canonical low-hard state during these radio observations and this may affect the behaviour of the source as well. At intermediate X-ray luminosities, the source makes the transition from the radio underluminous sources in the direction of the relation traced by the 'standard' correlation similar to what has been reported for H 1743-322 in the literature. However, MAXI J1659-152 remains underluminous with respect to this 'standard' correlation. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Abstract:

We present optical, X-ray and radio observations of the black hole transient (BHT) XTE J1752-223 towards and in quiescence. Optical photometry shows that the quiescent magnitude of XTE J1752-223 is fainter than 24.4mag in the i′ band. A comparison with measurements of the source during its 2009-2010 outburst shows that the outburst amplitude is more than 8 mag in the i′ band. Known X-ray properties of the source combined with the faintness of the quiescence optical counterpart and the large outburst optical amplitude point towards a short orbital-period system (Porb≲ 6.8h) with an M type (or later) mass donor, at a distance of 3.5 ≲d≲ 8kpc. Simultaneous X-ray and radio data were collected with Chandra and the Expanded Very Large Array (EVLA), allowing constraints to be placed on the quiescent X-ray and radio flux of XTE J1752-223. Furthermore, using data covering the final stage of the outburst decay, we investigated the low-luminosity end of the X-ray-radio correlation for this source and compared it with other BHTs. We found that XTE J1752-223 adds to the number of outliers with respect to the 'standard' X-ray-radio luminosity relation. Furthermore, XTE J1752-223 is the second source, after the BHT H1743-322, that shows a transition from the region of the outliers towards the 'standard' correlation at low luminosity. Finally, we report on a faint, variable X-ray source we discovered with Chandra at an angular distance of ~2.9arcsec to XTE J1752-223 and at a position angle consistent with that of the radio jets previously observed from the BHT. We discuss the possibility that we detected X-ray emission associated with a jet from XTE J1752-223. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Abstract:

We present the first resolved imaging of the milliarcsecond-scale jets in the neutron star X-ray binary Circinus X-1, made using the Australian Long Baseline Array. The angular extent of the resolved jets is ∼20mas, corresponding to a physical scale of ∼150au at the assumed distance of 7.8kpc. The jet position angle is relatively consistent with previous arcsecond-scale imaging with the Australia Telescope Compact Array. The radio emission is symmetric about the peak, and is unresolved along the minor axis, constraining the opening angle to be <20°. We observe evidence for outward motion of the components between the two halves of the observation. Constraints on the proper motion of the radio-emitting components suggest that they are only mildly relativistic, although we cannot definitively rule out the presence of the unseen, ultrarelativistic (Γ > 15) flow previously inferred to exist in this system. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.