980 research outputs found
Gaia DR2 Distances and Peculiar Velocities for Galactic Black Hole Transients
We report on a first census of Galactic black hole X-ray binary (BHXRB)
properties with the second data release (DR2) of {\em Gaia}, focusing on
dynamically confirmed and strong candidate black hole transients. DR2 provides
five-parameter astrometric solutions including position, parallax and proper
motion for 11 of a sample of 24 systems. Distance estimates are tested with
parallax inversion as well as Bayesian inference. We derive an empirically
motivated characteristic scale length of =2.170.12 kpc for this BHXRB
population to infer distances based upon an exponentially decreasing space
density prior. Geometric DR2 parallaxes provide new, independent distance
estimates, but the faintness of this population in quiescence results in
relatively large fractional distance uncertainties. Despite this, DR2 estimates
generally agree with literature distances. The most discrepant case is BW Cir,
for which detailed studies of the donor star have suggested a distant location
at >~25 kpc. A large DR2 measured parallax and relatively high proper motion
instead prefer significantly smaller distances, suggesting that the source may
instead be amongst the nearest of XRBs. However, both distances create problems
for interpretation of the source, and follow-up data are required to resolve
its true nature. DR2 also provides a first distance estimate to one source,
MAXI J1820+070, and novel proper motion estimates for 7 sources. Peculiar
velocities relative to Galactic rotation exceed 50 km s for the
bulk of the sample, with a median system kinetic energy of peculiar motion of
5 10 erg. BW Cir could be a new high-velocity BHXRB if
its astrometry is confirmed. A putative anti-correlation between peculiar
velocity and black hole mass is found, as expected in mass-dependent BH kick
formation channels, but this trend remains weak in the DR2 data.Comment: MNRAS in pres
Calcium-Rich Gap Transients: Tidal Detonations of White Dwarfs?
We hypothesize that at least some of the recently discovered class of
calcium-rich gap transients are tidal detonation events of white dwarfs (WDs)
by black holes (BHs) or possibly neutron stars. We show that the properties of
the calcium-rich gap transients agree well with the predictions of the tidal
detonation model. Under the predictions of this model, we use a follow-up X-ray
observation of one of these transients, SN 2012hn, to place weak upper limits
on the detonator mass of this system that include all intermediate-mass BHs
(IMBHs). As these transients are preferentially in the stellar haloes of
galaxies, we discuss the possibility that these transients are tidal
detonations of WDs caused by random flyby encounters with IMBHs in dwarf
galaxies or globular clusters. This possibility has been already suggested in
the literature but without connection to the calcium-rich gap transients. In
order for the random flyby cross-section to be high enough, these events would
have to be occurring inside these dense stellar associations. However, there is
a lack of evidence for IMBHs in these systems, and recent observations have
ruled out all but the very faintest dwarf galaxies and globular clusters for a
few of these transients. Another possibility is that these are tidal
detonations caused by three-body interactions, where a WD is perturbed toward
the detonator in isolated multiple star systems. We highlight a number of ways
this could occur, even in lower-mass systems with stellar-mass BHs or neutron
stars. Finally, we outline several new observational tests of this scenario,
which are feasible with current instrumentation.Comment: 10 pages, 1 figure, accepted for publication in MNRA
Some Constraints On the Effects of Age and Metallicity on the Low Mass X-ray Binary Formation Rate
We have studied the low mass X-ray binary (LMXB) populations within and
outside globular clusters (GC) in NGC 4365 and NGC 3115. Using published age
and metallicity constraints from optical and IR observations of their GCs, we
do not find any evidence for an increase in the LMXB formation rate in the
intermediate age cluster population of NGC 4365, as has been proposed in some
scenarios of dynamical LMXB formation in GCs. The old, metal-rich, red
population of GCs in NGC 3115 on the other hand is {\it at least} three times
as efficient at creating LMXBs as the old, metal-poor, blue clusters. These
data suggest that the higher formation efficiency of LMXBs in the red GC
subsystems of many galaxies is largely a consequence of their higher
metallicity. A comparison of the densities of field LMXBs in different galaxies
does not reveal an obvious correlation with the age of the field stars as
predicted by models in which the LMXB formation rate in the field drops
monotonically with time after an initial burst. This suggests that either a
significant fraction of the field LMXBs are created in GCs and subsequently
injected into the field, or the LMXB formation rate has a more complex time
evolution pattern.Comment: Accepted for publication in ApJ Letters. 7 pages including 5 figure
The Atmospheric Monitoring Strategy for the Cherenkov Telescope Array
The Imaging Atmospheric Cherenkov Technique (IACT) is unusual in astronomy as
the atmosphere actually forms an intrinsic part of the detector system, with
telescopes indirectly detecting very high energy particles by the generation
and transport of Cherenkov photons deep within the atmosphere. This means that
accurate measurement, characterisation and monitoring of the atmosphere is at
the very heart of successfully operating an IACT system. The Cherenkov
Telescope Array (CTA) will be the next generation IACT observatory with an
ambitious aim to improve the sensitivity of an order of magnitude over current
facilities, along with corresponding improvements in angular and energy
resolution and extended energy coverage, through an array of Large (23m),
Medium (12m) and Small (4m) sized telescopes spread over an area of order
~km. Whole sky coverage will be achieved by operating at two sites: one in
the northern hemisphere and one in the southern hemisphere. This proceedings
will cover the characterisation of the candidate sites and the atmospheric
calibration strategy. CTA will utilise a suite of instrumentation and analysis
techniques for atmospheric modelling and monitoring regarding pointing
forecasts, intelligent pointing selection for the observatory operations and
for offline data correction.Comment: 6 pages. To appear in the proceedings of the Adapting to the
Atmosphere conference 201
Wind, jet, hybrid corona and hard X-ray flares: multiwavelength evolution of GRO J1655-40 during the 2005 outburst rise
We have investigated the complex multiwavelength evolution of GRO J1655-40
during the rise of its 2005 outburst. We detected two hard X-ray flares, the
first one during the transition from the soft state to the ultra-soft state,
and the second one in the ultra-soft state. The first X-ray flare coincided
with an optically thin radio flare. We also observed a hint of increased radio
emission during the second X-ray flare. To explain the hard flares without
invoking a secondary emission component, we fit the entire data set with the
eqpair model. This single, hybrid Comptonization model sufficiently fits the
data even during the hard X-ray flares if we allow reflection fractions greater
than unity. In this case, the hard X-ray flares correspond to a Comptonizing
corona dominated by non-thermal electrons. The fits also require absorption
features in the soft and ultra-soft state which are likely due to a wind. In
this work we show that the wind and the optically thin radio flare co-exist.
Finally, we have also investigated the radio to optical spectral energy
distribution, tracking the radio spectral evolution through the quenching of
the compact jet and rise of the optically thin flare, and interpreted all data
using state transition models.Comment: 16 pages, 11 figure
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