Andrew J. Gosling

A young massive stellar population around the intermediate-mass black hole ESO 243-49 HLX-1

We present Hubble Space Telescope and simultaneous Swift X-ray Telescope observations of the strongest candidate intermediate-mass black hole (IMBH) ESO 243-49 HLX-1. Fitting the spectral energy distribution from X-ray to near-infrared wavelengths showed that the broadband spectrum is not consistent with simple and irradiated disk models, but is well described by a model comprised of an irradiated accretion disk plus a ~106 M ☉ stellar population. The age of the population cannot be uniquely constrained, with both young and old stellar populations allowed. However, the old solution requires excessive disk reprocessing and an extremely small disk, so we favor the young solution (~13 Myr). In addition, the presence of dust lanes and the lack of any nuclear activity from X-ray observations of the host galaxy suggest that a gas-rich minor merger may have taken place less than ~200 Myr ago. Such a merger event would explain the presence of the IMBH and the young stellar population.

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

Using VLT/ISAAC, we have detected two candidate counterparts to the bursting pulsar GRO J1744-28, one bright and one faint, both within the X-ray error circles found using XMM-Newton and Chandra. In determining the spectral types of the counterparts we applied three different extinction corrections; one for an all-sky value, one for a Galactic bulge value and one for a local value. We find that 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 the bright and faint counterparts that are consistent with a small range of spectral types, and in the case of the bright counterpart are also consistent with the spectroscopic identification. Photometry of the fainter candidate then indicates that it is a K7/M0 V star at a distance of 3.75 +/- 1kpc. Such a star would require a very low inclination angle (i = 15° a stripped giant, or a main-sequence M3+V star would be 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. In this case, the derived magnetic field strength of 2.4 × 1011 G is sufficient to inhibit accretion of captured material by the propeller effect, hence the quiescent state of the system.

A New 626 s periodic X-ray source in the direction of the Galactic center

Aims. Here we report the detection of a 626 s periodic modulation from the X-ray source 2XMM J174016.0−290337 located in the direction of the Galactic center. Methods. We present temporal and spectral analyses of archival XMM-Newton data and photometry of archived near-infrared data in order to investigate the nature of this source. Results. We find that the X-ray light curve shows a strong modulation at 626 ± 2 s with a confidence level >99.9% and a pulsed fraction of 54%. Spectral fitting demonstrates that the spectrum is consistent with an absorbed power law. No significant spectral variability was observed over the 626 s period. We have investigated the possibility that the 626 s period is orbital in nature (either that of an ultra-compact X-ray binary or an AM CVn) or related to the spin of a compact object (either an accretion powered pulsar or an intermediate polar). Conclusions. The X-ray properties of the source and the photometry of the candidate near-infrared counterparts are consistent with an accreting neutron star X-ray binary on the near-side of the Galactic bulge, where the 626 s period is most likely indicative of the pulsar spin period. However, we cannot rule out an ultra-compact X-ray binary or an intermediate polar with the data at hand. In the former case, if the 626 s modulation is the orbital period of an X-ray binary, it would be the shortest period system known. In the latter case, the modulation would be the spin period of a magnetic white dwarf. However, we find no evidence for absorption dips over the 626 s period, a low temperature black body spectral component, or Fe Kα emission lines. These features are commonly observed in intermediate polars, making 2XMM J174016.0−290337 a rather unusual member of this class if confirmed. Based on the slow period and the photometry of the near-infrared counterparts, we instead suggest that 2XMM J174016.0−290337 could be a new addition to the emerging class of symbiotic X-ray binaries.

An infrared imaging survey of the faint Chandra sources near the Galactic Centre

We present near-infrared imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra Advanced CCD Imaging Spectrometer (ACIS)-I survey towards the Galactic Centre (GC) by Wang et al. These ∼800 discrete sources represent an important and previously undetected population within the Galaxy. From our Very Large Telescope observations of 77 X-ray sources, we identify candidate K-band counterparts to 75 per cent of the Chandra sources in our sample. The near-infrared magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely to be accreting binaries at or near the GC.

Near Infrared Extinction at the Galactic Centre

We present new results from UKIDSS on the Nuclear Bulge of our Galaxy that shows the previous determinations of the extinction parameter need revising.

A Billion Stars: The Near-IR View of the Galaxy with the UKIDSS Galactic Plane Survey

The scope and status of the UKIDSS Galactic Plane Survey is described. Some issues regarding automatic processing of crowded fields and comparisons with other surveys are identified and some early results are presented. The detection of more than 100 new clusters and 16 new high amplitude variables are reported.

Counterparts to the Nuclear Bulge X‐ray Source Population

We present an initial matching of the source positions of the Chandra Nuclear Bulge X‐ray sources to the new UKIDSS‐GPS near‐infrared survey of the Nuclear Bulge. This task is made difficult by the extremely crowded nature of the region; despite this, we find candidate counterparts to ∼50% of the X‐ray sources. We show that detection in the J‐band for a candidate counterpart to an X‐ray source preferentially selects those candidate counterparts in the foreground whereas candidate counterparts with only detections in the H and K‐bands are more likely to be Nuclear Bulge sources. We discuss the planned follow‐up for these candidate counterparts.

The Nuclear Bulge Extinction

We present a new, high resolution (5″ per pixel) near‐infrared extinction map of the Nuclear Bulge using data from the UKIDSS‐GPS. Using photometry from the J, H and K‐bands we show that the extinction law parameter α is also highly variable in this region on similar scales to the absolute extinction. We show that only when this extinction law variation is taken into account can the extinction be measured consistently at different wavelengths.

The Path to Buried Treasure: Paving the Way to the FLAMINGOS-2 Galactic Center Survey with IR and X-ray Observations

I describe the IR and X‐ray campaign we have undertaken to determine the nature of the faint discrete X‐ray source population discovered by Chandra in the Galactic Center. These results will provide the input to the FLAMINGOS‐2 Galactic Center Survey (F2GCS). With FLAMINGOS‐2s multi‐object IR spectrograph we will obtain 1000s of IR spectra of candidate X‐ray source counterparts, allowing us to efficiently identify the nature of these sources, and thus dramatically increase the number of known X‐ray binaries and CVs in the Milky Way.

Exploring a New Population of Compact Objects: X‐ray and IR Observations of the Galactic Centre

I describe the IR and X-ray observational campaign we have undertaken for the purpose of determining the nature of the faint discrete X-ray source population discovered by Chandra in the Galactic Center (GC). Data obtained for this project includes a deep Chandra survey of the Galactic Bulge; deep, high resolution IR imaging from VLT/ISAAC, CTIO/ISPI, and the UKIDSS Galactic Plane Survey (GPS); and IR spectroscopy from VLT/ISAAC and IRTF/SpeX. By cross-correlating the GC X-ray imaging from Chandra with our IR surveys, we identify candidate counterparts to the X-ray sources via astrometry. Using a detailed IR extinction map, we are deriving magnitudes and colors for all the candidates. Having thus established a target list, we will use the multi-object IR spectrograph FLAMINGOS-2 on Gemini-South to carry out a spectroscopic survey of the candidate counterparts, to search for emission line signatures which are a hallmark of accreting binaries. By determining the nature of these X-ray sources, this FLAMINGOS-2 Galactic Center Survey will have a dramatic impact on our knowledge of the Galactic accreting binary population.