GRO J1744-28, search for the counterpart: infrared photometry and spectroscopy

Using VLT/ISAAC, we detected 2 candidate counterparts to the bursting pulsar GRO J1744-28, one bright and one faint, within the X-ray error circles of XMM-Newton and Chandra. In determining the spectral types of the counterparts we applied 3 different extinction corrections; one for an all-sky value, one for a Galactic Bulge value and one for a local value. We find the local value, with an extinction law of alpha = 3.23 +- 0.01 is the only correction that results in colours and magnitudes for both bright and faint counterparts consistent with a small range of spectral types, and for the bright counterpart, consistent with the spectroscopic identification. Photometry of the faint candidate indicates it is a K7/M0 V star at a distance of 3.75 +- 1 kpc. This star would require a very low inclination angle (i < 9deg) to satisfy the mass function constraints; however it cannot be excluded as the counterpart without follow-up spectroscopy to detect emission signatures of accretion. Photometry and spectroscopy of the bright candidate indicate it is most likely a G/K III star. The spectrum does not show Br-gamma emission, a known indicator of accretion. The bright star's magnitudes are in agreement with the constraints placed on a probable counterpart by the calculations of Rappaport & Joss (1997) for an evolved star that has had its envelope stripped. The mass function indicates the counterpart should have M < 0.3 Msol for an inclination of i >= 15deg; a stripped giant, or a main sequence M3+ V star are consistent with this mass-function constraint. In both cases mass-transfer, if present, will be by wind-accretion as the counterpart will not fill its Roche lobe given the observed orbital period. The derived magnetic field of 2.4 x 10^{11} G will inhibit accretion by the propeller effect, hence its quiescent state.Comment: 12 pages, 6 figures, 4 table, MNRAS accepted Changes to the content and an increased analysis of the Galactic centre extinctio

Jet evolution, flux ratios and light-travel time effects

Studies of the knotty jets in both quasars and microquasars frequently make use of the ratio of the intensities of corresponding knots on opposite sides of the nucleus in order to infer the product of the intrinsic jet speed (beta) and the cosine of the inclination angle of the jet-axis (cos{theta}), via the formalism I_{a}/I_{r} = ((1+beta cos{theta})/(1-beta cos{theta}))^{3+alpha}, where alpha relates the intensity I_{nu} as a function of frequency nu as I_{nu} propto nu^{-alpha}. Where beta cos{theta} is determined independently, the intensity ratio of a given pair of jet to counter-jet knots is over-predicted by the above formalism compared with the intensity ratio actually measured from radio images. As an example in the case of Cygnus X-3 the original formalism predicts an intensity ratio of about 185, whereas the observed intensity ratio at one single epoch is about 3. Mirabel and Rodriguez (1999) have refined the original formalism, and suggested measuring the intensity ratio of knots when they are at equal angular separations from the nucleus. This method is only applicable where there is sufficient time-sampling with sufficient physical resolution to interpolate the intensities of the knots at equal distances from the nucleus, and can therefore be difficult to apply to microquasars and is impossible to apply to quasars. Accounting for both the light-travel time between the knots and the simple evolution of the knots themselves reconciles this over-prediction and renders the original formalism obsolete.Comment: 5 pages, no figures, to be published in ApJ Letter

Infrared interferometry to spatially and spectrally resolve jets in X-ray binaries

Infrared interferometry is a new frontier for precision ground based observing, with new instrumentation achieving milliarcsecond (mas) spatial resolutions for faint sources, along with astrometry on the order of 10 microarcseconds. This technique has already led to breakthroughs in the observations of the supermassive black hole at the Galactic centre and its orbiting stars, AGN, and exo-planets, and can be employed for studying X-ray binaries (XRBs), microquasars in particular. Beyond constraining the orbital parameters of the system using the centroid wobble and spatially resolving jet discrete ejections on mas scales, we also propose a novel method to discern between the various components contributing to the infrared bands: accretion disk, jets and companion star. We demonstrate that the GRAVITY instrument on the Very Large Telescope Interferometer (VLTI) should be able to detect a centroid shift in a number of sources, opening a new avenue of exploration for the myriad of transients expected to be discovered in the coming decade of radio all-sky surveys. We also present the first proof-of-concept GRAVITY observation of a low-mass X-ray binary transient, MAXI J1820+070, to search for extended jets on mas scales. We place the tightest constraints yet via direct imaging on the size of the infrared emitting region of the compact jet in a hard state XRB.Comment: 12 Pages, 3 figures, accepted for publication in MNRA

Simultaneous Chandra and VLA Observations of the Transitional Millisecond Pulsar PSR J1023+0038: Anti-correlated X-Ray and Radio Variability

We present coordinated Chandra X-ray Observatory and Karl G. Jansky Very Large Array observations of the transitional millisecond pulsar PSR J1023+0038 in its low-luminosity accreting state. The unprecedented five hours of strictly simultaneous X-ray and radio continuum coverage for the first time unambiguously show a highly reproducible, anti-correlated variability pattern. The characteristic switches from the X-ray high mode into a low mode are always accompanied by a radio brightening with a duration that closely matches the X-ray low mode interval. This behavior cannot be explained by a canonical inflow/outflow accretion model where the radiated emission and the jet luminosity are powered by, and positively correlated with, the available accretion energy. We interpret this phenomenology as alternating episodes of low-level accretion onto the neutron star during the X-ray high mode that are interrupted by rapid ejections of plasma by the active rotation-powered pulsar, possibly initiated by a reconfiguration of the pulsar magnetosphere, that cause a transition to a less X-ray luminous mode. The observed anti-correlation between radio and X-ray luminosity has an additional consequence: transitional MSPs can make excursions into a region of the radio/X-ray luminosity plane previously thought to be occupied solely by black hole X-ray binary sources. This complicates the use of this luminosity relation for identifying candidate black holes, suggesting the need for additional discriminants when attempting to establish the true nature of the accretor

An HI absorption distance to the black hole candidate X-ray binary MAXI J1535-571

With the Australian Square Kilometre Array Pathfinder (ASKAP) we monitored the black hole candidate X-ray binary MAXI J1535--571 over seven epochs from 21 September to 2 October 2017. Using ASKAP observations, we studied the HI absorption spectrum from gas clouds along the line-of-sight and thereby constrained the distance to the source. The maximum negative radial velocities measured from the HI absorption spectra for MAXI J1535--571 and an extragalactic source in the same field of view are $-69\pm4$ km s$^{-1}$ and $-89\pm4$ km s$^{-1}$, respectively. This rules out the far kinematic distance ($9.3^{+0.5}_{-0.6}$ kpc), giving a most likely distance of $4.1^{+0.6}_{-0.5}$ kpc, with a strong upper limit of the tangent point at $6.7^{+0.1}_{-0.2}$ kpc. At our preferred distance, the peak unabsorbed luminosity of MAXI J1535--571 was $>78$ per cent of the Eddington luminosity, and shows that the soft-to-hard spectral state transition occurred at the very low luminosity of 1.2 -- 3.4 $\times$ 10$^{-5}$ times the Eddington luminosity. Finally, this study highlights the capabilities of new wide-field radio telescopes to probe Galactic transient outbursts, by allowing us to observe both a target source and a background comparison source in a single telescope pointing.Comment: Revised after favorable referee report from MNRAS Letter

The Ghana Community-based Health Planning and Services Initiative: Fostering evidence-based organizational change and development in a resource-constrained setting

Research projects demonstrating ways to improve health services often fail to have an impact on what national health programs actually do. An approach to evidence-based policy development has been launched in Ghana that bridges the gap between research and program implementation. The Community-based Health Planning and Services (CHPS) initiative employs strategies tested in the successful Navrongo experiment to guide national health reforms that mobilize volunteers, resources, and cultural institutions to support community-based primary health care. Over the 1999 to 2002 period, 100 out of the 110 districts in Ghana adopted a CHPS initiative. This paper reviews features of the initiative that explain its success and constrain future progress

The spectral energy distribution of quiescent black hole X-ray binaries: new constraints from Spitze

Among the various issues that remain open in the field of accretion onto black hole X-ray binaries (BHBs) is the question of how gas accretes at very low Eddington ratios, in the so-called quiescent regime. While there is general agreement that X-rays are produced by a population of high-energy electrons near the BH, there is controversy concerning the modeling of the contributions of inflowing versus outflowing particles and their relative energy budget. Recent Spitzer observations of three quiescent BHBs have shown evidence for excess emission with respect to the Rayleigh-Jeans tail of the companion star between 8-24 μm. We suggest that synchrotron emission from a partially self-absorbed outflow might be responsible for the observed mid-IR excess, in place of, or in addition to, thermal emission from circumbinary material. If so, then the jet synchrotron luminosity, integrated from radio to near-IR frequencies, exceeds the measured 2-10 keV luminosity by a factor of a few in these systems. In turn, the mechanical power stored in the jet exceeds the bolometric X-ray luminosity by at least 4 orders of magnitude. We compile the broadband spectral energy distribution (SED) of A0620-00, the lowest Eddington-ratio stellar mass BH with a known radio counterpart, by means of simultaneous radio, optical, and X-ray observations, and the archival Spitzer data. We are able to fit the SED of A0620-00 with a maximally jet-dominated model, in which the radio through the soft X-rays are dominated by synchrotron emission, while the hard X-rays are dominated by inverse Compton at the jet base. The fitted parameters land in a range of values reminiscent of the Galactic center supermassive black hole Sgr A*. Most notably, the inferred ratio of the jet acceleration rate to local cooling rates is 2 orders of magnitude weaker than higher luminosity, hard-state sources