1,207 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
Luminous [O III] and [N II] from Tidally Disrupted Horizontal Branch Stars
We model the emission lines generated in the photoionised debris of a tidally
disrupted horizontal branch star. We find that at late times, the brightest
optical emission lines are [N II] \lambda\lambda 6548,6583 and [O III]
\lambda\lambda 4959,5007. Models of a red clump horizontal branch star
undergoing mild disruption by a massive (50 -- 100 M_\sun) black hole yield an
emission line spectrum that is in good agreement with that observed in the NGC
1399 globular cluster hosting the ultraluminous X-ray source CXOJ033831.8 -
352604. We make predictions for the UV emission line spectrum that can verify
the tidal disruption scenario and constrain the mass of the BH.Comment: 8 pages, 6 figures, 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
An X-ray emitting black hole in a globular cluster
We present optical and X-ray data for the first object showing strong
evidence for being a black hole in a globular cluster. We show the initial
X-ray light curve and X-ray spectrum which led to the discovery that this is an
extremely bright, highly variable source, and thus must be a black hole. We
present the optical spectrum which unambiguously identifies the optical
counterpart as a globular cluster, and which shows a strong, broad [O III]
emission line, most likely coming from an outflow driven by the accreting
source.Comment: 5 pages, 2 figures, to appear in the proceedings of IAUS 246,
"Dynamical Evolution of Dense Stellar Systems", ed. Vesperini, Giersz and
Sill
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