56 research outputs found
Continued monitoring of LMXBs with the Faulkes Telescopes
The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). It has two 2-metre robotic telescopes, located at Haleakala on Maui (FT North) and Siding Spring in Australia (FT South). It is planned for these telescopes to be complemented by a research network of eighteen 1-metre telescopes, along with an educational network of twenty-eight 0.4-metre telescopes, providing 24 hour coverage of both northern and southern hemispheres.
We have been conducting a monitoring project of 13 low-mass X-ray binaries (LMXBs) using FT North since early 2006. The introduction of FT South has allowed us to extend this to monitor a total of 30 LMXBs (see target list, Section 4). New instrumentation will allow us to expand this project to include both infrared wavelengths (z and y band) and spectroscopy. Brighter targets (~ 16 - 18 mag.) are imaged weekly in V, R and iâ bands (SNR ~ 50), while fainter ones (> 18 mag.) are observed only in iâ band (SNR ~ 20). We alter this cadence in response to our own analysis or Astronomers Telegrams (ATels)
The infrared counterpart of the Z source GX5-1
We have obtained UKIRT infrared observations of the field of the bright
Galactic Z source GX5-1. From an astrometric plate solution tied to Tycho-ACT
standards we have obtained accurate positions for the stars in our field which,
combined with an accurate radio position, have allowed us to identify the
probable infrared counterpart of GX5-1. Narrow-band photometry marginally
suggests excess Br-gamma emission in the counterpart, supporting its
association with an accretion-disc source. No significant variability is
observed in a limited number of observations. We compare the H and K magnitudes
with those of other Z sources, and briefly discuss possible sources of infrared
emission in these systems.Comment: 5 pages, 2 figures, accepted by MNRA
Monitoring LMXBs with the Faulkes Telescopes
We have been undertaking a monitoring project of 13 low-mass X-ray binaries
(LMXBs) using FT North since early 2006. The introduction of FT South has
allowed us to extend this monitoring to include 15 southern hemisphere LMXBs.
With new instrumentation, we also intend to expand this monitoring to include
both infrared wavelengths and spectroscopy.Comment: Conference proceedings from 'A Population Explosion: The Nature and
Evolution of X-ray Binaries in Diverse Environments', 28 Oct - 2 Nov, St.
Petersburg Beach, FL. 3 pages, 3 figure
Assessing luminosity correlations via cluster analysis: Evidence for dual tracks in the radio/X-ray domain of black hole X-ray binaries
[abridged] The radio:X-ray correlation for hard and quiescent state black
hole X-ray binaries is critically investigated in this paper. New observations
of known sources, along with newly discovered ones, have resulted in an
increasingly large number of outliers lying well outside the scatter about the
quoted best-fit relation. Here, we employ and compare state of the art data
clustering techniques in order to identify and characterize different data
groupings within the radio:X-ray luminosity plane for 18 hard and quiescent
state black hole X-ray binaries with nearly simultaneous multi-wavelength
coverage. Linear regression is then carried out on the clustered data to infer
the parameters of a relationship of the form {ell}_{r}=alpha+beta {ell}_x
through a Bayesian approach (where {ell} denotes log lum). We conclude that the
two cluster model, with independent linear fits, is a significant improvement
over fitting all points as a single cluster. While the upper track slope
(0.63\pm0.03) is consistent, within the errors, with the fitted slope for the
2003 relation (0.7\pm0.1), the lower track slope (0.98\pm0.08) is not
consistent with the upper track, nor it is with the widely adopted value of
~1.4 for the neutron stars. The two luminosity tracks do not reflect systematic
differences in black hole spins as estimated either from reflection, or
continuum fitting method. These results are insensitive to the selection of
sub-samples, accuracy in the distances, and to the treatment of upper limits.
Besides introducing a further level of complexity in understanding the
interplay between synchrotron and Comptonised emission from black hole X-ray
binaries, the existence of two tracks in the radio:X-ray domain underscores
that a high level of caution must be exercised when employing black hole
luminosity relations for the purpose of estimating a third parameter, such as
distance or mass.Comment: MNRAS, in press (10 pages, 7 figures
Polarised infrared emission from X-ray binary jets
Near-infrared (NIR) and optical polarimetric observations of a selection of
X-ray binaries are presented. The targets were observed using the Very Large
Telescope and the United Kingdom Infrared Telescope. We detect a significant
level (3 sigma) of linear polarisation in four sources. The polarisation is
found to be intrinsic (at the > 3 sigma level) in two sources; GRO J1655-40 (~
4-7% in H and Ks-bands during an outburst) and Sco X-1 (~ 0.1-0.9% in H and K),
which is stronger at lower frequencies. This is likely to be the signature of
optically thin synchrotron emission from the collimated jets in these systems,
whose presence indicates a partially-ordered magnetic field is present at the
inner regions of the jets. In Sco X-1 the intrinsic polarisation is variable
(and sometimes absent) in the H and K-bands. In the J-band (i.e. at higher
frequencies) the polarisation is not significantly variable and is consistent
with an interstellar origin. The optical light from GX 339-4 is also polarised,
but at a level and position angle consistent with scattering by interstellar
dust. The other polarised source is SS 433, which has a low level (0.5-0.8%) of
J-band polarisation, likely due to local scattering. The NIR counterparts of
GRO J0422+32, XTE J1118+480, 4U 0614+09 and Aql X-1 (which were all in or near
quiescence) have a linear polarisation level of < 16% (3 sigma upper limit,
some are < 6%). We discuss how such observations may be used to constrain the
ordering of the magnetic field close to the base of the jet in such systems.Comment: Accepted to be published in MNRAS; 13 pages, 6 figure
Chandra Detections of Two Quiescent Black Hole X-Ray Transients
Using the Chandra X-ray Observatory, we have detected the black hole
transients V4641 Sgr and XTE J1859+226 in their low luminosity, quiescent
states. The 0.3-8 keV luminosities are (4.0^(+3.3)_(-2.4))E31 (d/7 kpc)^2 erg/s
and (4.2^(+4.8)_(-2.2))E31 (d/11 kpc)^2 erg/s for V4641 Sgr and XTE J1859+226,
respectively. With the addition of these 2 systems, 14 out of the 15 transients
with confirmed black holes (via compact object mass measurements) now have
measured quiescent luminosities or sensitive upper limits. The only exception
is GRS 1915+105, which has not been in quiescence since its discovery in 1992.
The luminosities for V4641 Sgr and XTE J1859+226 are consistent with the median
luminosity of 2E31 erg/s for the systems with previous detections. Our analysis
suggests that the quiescent X-ray spectrum of V4641 Sgr is harder than for the
other systems in this group, but, due to the low statistical quality of the
spectrum, it is not clear if V4641 Sgr is intrinsically hard or if the column
density is higher than the interstellar value. Focusing on V4641 Sgr, we
compare our results to theoretical models for X-ray emission from black holes
in quiescence. Also, we obtain precise X-ray positions for V4641 Sgr and XTE
J1859+226 via cross-correlation of the X-ray sources detected near our targets
with IR sources in the 2 Micron All-Sky Survey catalog.Comment: 4 pages, Accepted by ApJ Letter
Identifying transient and variable sources in radio images
With the arrival of a number of wide-field snapshot image-plane radio transient surveys, there will be a huge influx of images in the coming years making it impossible to manually analyse the datasets. Automated pipelines to process the information stored in the images are being developed, such as the LOFAR Transients Pipeline, outputting light curves and various transient parameters. These pipelines have a number of tuneable parameters that require training to meet the survey requirements. This paper utilises both observed and simulated datasets to demonstrate different machine learning strategies that can be used to train these parameters. We use a simple anomaly detection algorithm and a penalised logistic regression algorithm. The datasets used are from LOFAR observations and we process the data using the LOFAR Transients Pipeline; however the strategies developed are applicable to any light curve datasets at different frequencies and can be adapted to different automated pipelines. These machine learning strategies are publicly available as PYTHON tools that can be downloaded and adapted to different datasets (https://github.com/AntoniaR/TraP_ML_tools)
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