105 research outputs found
Secular evolution of compact binaries near massive black holes: gravitational wave sources and other exotica
The environment near super massive black holes (SMBHs) in galactic nuclei
contain a large number of stars and compact objects. A fraction of these are
likely to be members of binaries. Here we discuss the binary population of
stellar black holes and neutron stars near SMBHs and focus on the secular
evolution of such binaries, due to the perturbation by the SMBH. Binaries with
highly inclined orbits in respect to their orbit around the SMBH are strongly
affected by secular Kozai processes, which periodically change their
eccentricities and inclinations (Kozai-cycles). During periapsis approach, at
the highest eccentricities during the Kozai-cycles, gravitational wave emission
becomes highly efficient. Some binaries in this environment can inspiral and
coalesce at timescales much shorter than a Hubble time and much shorter than
similar binaries which do not reside near a SMBH. The close environment of
SMBHs could therefore serve as catalyst for the inspiral and coalescence of
binaries, and strongly affect their orbital properties. Such compact binaries
would be detectable as gravitational wave (GW) sources by the next generation
of GW detectors (e.g. advanced- LIGO). About 0.5% of such nuclear merging
binaries will enter the LIGO observational window while on orbit that are still
very eccentric (e>~0.5). The efficient gravitational wave analysis for such
systems would therefore require the use of eccentric templates. We also find
that binaries very close to the MBH could evolve through a complex dynamical
(non-secular) evolution leading to emission of several GW pulses during only a
few yrs (though these are likely to be rare). Finally, we note that the
formation of close stellar binaries, X-ray binaries and their merger products
could be induced by similar secular processes, combined with tidal friction
rather than GW emission as in the case of compact object binaries.Comment: 15 pages, 7 Figures. ApJ accepte
Astro2020 Science White Paper: The Local Relics of of Supermassive Black Hole Seeds
We have compelling evidence for stellar-mass black holes (BHs) of ~5-80 M_sun
that form through the death of massive stars. We also have compelling evidence
for so-called supermassive BHs (10^5-10^10 M_sun) that are predominantly found
in the centers of galaxies. We have very good reason to believe there must be
BHs with masses in the gap between these ranges: the first ~10^9 M_sun BHs are
observed only hundreds of millions of years after the Big Bang, and all
theoretically viable paths to making supermassive BHs require a stage of
"intermediate" mass. However, no BHs have yet been reliably detected in the
100-10}^5 M_sun mass range. Uncovering these intermediate-mass BHs of 10^3-10^5
M_sun is within reach in the coming decade. In this white paper we highlight
the crucial role that 30-m class telescopes will play in dynamically detecting
intermediate-mass black holes, should they exist.Comment: Science White Paper Submitted for the Astro2020 Decadal Survey on
Astronomy and Astrophysic
Mergers of Unequal Mass Galaxies: Supermassive Black Hole Binary Evolution and Structure of Merger Remnants
Galaxy centers are residing places for Super Massive Black Holes (SMBHs).
Galaxy mergers bring SMBHs close together to form gravitationally bound binary
systems which, if able to coalesce in less than a Hubble time, would be one of
the most promising sources of gravitational waves for the Laser Interferometer
Space Antenna (LISA). In spherical galaxy models, SMBH binaries stall at a
separation of approximately one parsec, leading to the "final parsec problem"
(FPP). On the other hand, it has been shown that merger-induced triaxiality of
the remnant in equal-mass mergers is capable of supporting a constant supply of
stars on so-called centrophilic orbits that interact with the binary and thus
avoid the FPP. In this paper, using a set of direct N-body simulations of
mergers of initially spherically symmetric galaxies with different mass ratios,
we show that the merger-induced triaxiality is able to drive unequal-mass SMBH
binaries to coalescence. The binary hardening rates are high and depend only
weakly on the mass ratios of SMBHs for a wide range of mass ratios q. The
hardening rates are significantly higher for galaxies having steep cusps in
comparison with those having shallow cups at centers. The evolution of the
binary SMBH leads to relatively shallower inner slopes at the centers of the
merger remnants. The stellar mass displaced by the SMBH binary on its way to
coalescence is ~ 1-5 times the combined mass of binary SMBHs. The coalescence
times for SMBH binary with mass ~ million solar masses are less than 1 Gyr and
for those at the upper end of SMBH masses (~ billion solar masses) are 1-2 Gyr
for less eccentric binaries whereas less than 1 Gyr for highly eccentric
binaries. SMBH binaries are thus expected to be promising sources of
gravitational waves at low and high redshifts.Comment: Accepted for publication in the Astrophysical Journal (ApJ). 14
pages, 8 figure
Secular Stellar Dynamics near a Massive Black Hole
The angular momentum evolution of stars close to massive black holes (MBHs)
is driven by secular torques. In contrast to two-body relaxation, where
interactions between stars are incoherent, the resulting resonant relaxation
(RR) process is characterized by coherence times of hundreds of orbital
periods. In this paper, we show that all the statistical properties of RR can
be reproduced in an autoregressive moving average (ARMA) model. We use the ARMA
model, calibrated with extensive N-body simulations, to analyze the long-term
evolution of stellar systems around MBHs with Monte Carlo simulations.
We show that for a single-mass system in steady-state, a depression is carved
out near an MBH as a result of tidal disruptions. Using Galactic center
parameters, the extent of the depression is about 0.1 pc, of similar order to
but less than the size of the observed "hole" in the distribution of bright
late-type stars. We also find that the velocity vectors of stars around an MBH
are locally not isotropic. In a second application, we evolve the highly
eccentric orbits that result from the tidal disruption of binary stars, which
are considered to be plausible precursors of the "S-stars" in the Galactic
center. We find that RR predicts more highly eccentric (e > 0.9) S-star orbits
than have been observed to date.Comment: 24 pages, 31 figures; final version as published in Ap
Localizing Sagittarius A* and M87 on Microarcsecond Scales with Millimeter VLBI
With the advent of the Event Horizon Telescope (EHT), a
millimeter/sub-millimeter very-long baseline interferometer (VLBI), it has
become possible to image a handful of black holes with sub-horizon resolutions.
However, these images do not translate into microarcsecond absolute positions
due to the lack of absolute phase information when an external phase reference
is not used. Due to the short atmospheric coherence time at these wavelengths,
nodding between the source and phase reference is impractical. However, here we
suggest an alternative scheme which makes use of the fact that many of the VLBI
stations within the EHT are arrays in their own right. With this we show that
it should be possible to absolutely position the supermassive black holes at
the centers of the Milky Way (Sgr A*) and M87 relative to nearby objects with
precisions of roughly 1 microarcsecond. This is sufficient to detect the
perturbations to Sgr A*'s position resulting from interactions with the stars
and stellar-mass black holes in the Galactic cusp on year timescales, and
severely constrain the astrophysically relevant parameter space for an orbiting
intermediate mass black hole, implicated in some mechanisms for producing the
young massive stars in the Galactic center. For M87, it allows the registering
of millimeter images, in which the black hole may be identified by its
silhouette against nearby emission, and existing larger scale radio images,
eliminating present ambiguities in the nature of the radio core and
inclination, opening angle, and source of the radio jet.Comment: 18 pages, 4 figures, Accepted for publication in Ap
Dissipationless Formation and Evolution of the Milky Way Nuclear Star Cluster
Abridged: In one widely discussed model for the formation of nuclear star
clusters (NSCs), massive globular clusters spiral into the center of a galaxy
and merge to form the nucleus. It is now known that at least some NSCs coexist
with supermassive black holes (SBHs); this is the case, for instance, in the
Milky Way (MW). In this paper, we investigate how the presence of a SMBH at the
center of the MW impacts the merger hypothesis for the formation of its NSC.
Starting from a model consisting of a low-density nuclear stellar disk and the
SMBH, we use N-body simulations to follow the successive inspiral and merger of
globular clusters. The clusters are started on circular orbits of radius 20 pc,
and their initial masses and radii are set up in such a way as to be consistent
with the galactic tidal field at that radius. The total accumulated mass by ~10
clusters is about 1.5x10^7 Solar masses. Each cluster is disrupted by the SMBH
at a distance of roughly one parsec. The density profile that results after the
final inspiral event is characterized by a core of roughly this radius, and an
envelope with density that falls off as 1/r^2. These properties are similar to
those of the MW NSC, with the exception of the core size, which in the MW is a
little smaller. But by continuing the evolution of the model after the final
inspiral event, we find that the core shrinks substantially via gravitational
encounters in a time (when scaled to the MW) of 10 Gyr as the stellar
distribution evolves toward a Bahcall-Wolf cusp. We also show that the
luminosity function of the MW NSC is consistent with the hypothesis that a
large fraction of the mass comes from (~10Gyr) old stars, brought in by
globular clusters. We conclude that a model in which a large fraction of the
mass of the MW NSC arose from infalling globular clusters is consistent with
existing observational constraints.Comment: 15 pages, 13 figures. ApJ accepte
Fermi Large Area Telescope Gamma-Ray Detection of the Radio Galaxy M87
We report the Fermi-LAT discovery of high-energy (MeV/GeV) gamma-ray emission
positionally consistent with the center of the radio galaxy M87, at a source
significance of over 10 sigma in ten-months of all-sky survey data. Following
the detections of Cen A and Per A, this makes M87 the third radio galaxy seen
with the LAT. The faint point-like gamma-ray source has a >100 MeV flux of 2.45
(+/- 0.63) x 10^-8 ph cm^-2 s^-1 (photon index = 2.26 +/- 0.13) with no
significant variability detected within the LAT observation. This flux is
comparable with the previous EGRET upper limit (< 2.18 x 10^-8 ph cm^-2 s^-1, 2
sigma), thus there is no evidence for a significant MeV/GeV flare on decade
timescales. Contemporaneous Chandra and VLBA data indicate low activity in the
unresolved X-ray and radio core relative to previous observations, suggesting
M87 is in a quiescent overall level over the first year of Fermi-LAT
observations. The LAT gamma-ray spectrum is modeled as synchrotron self-Compton
(SSC) emission from the electron population producing the radio-to-X-ray
emission in the core. The resultant SSC spectrum extrapolates smoothly from the
LAT band to the historical-minimum TeV emission. Alternative models for the
core and possible contributions from the kiloparsec-scale jet in M87 are
considered, and can not be excluded.Comment: ApJ, accepted, 6 pages, 4 figures. Corresponding authors: C.C.
Cheung, W. McConvill
A stacked search for intermediate-mass black holes in 337 extragalactic star clusters
Forbes et al. recently used the Hubble Space Telescope to localize hundreds
of candidate star clusters in NGC 1023, an early-type galaxy at a distance of
11.1 Mpc. Old stars dominate the light of 92% of the clusters and
intermediate-age stars dominate the light of the remaining 8%. Theory predicts
that clusters with such ages can host intermediate-mass black holes (IMBHs)
with masses M_BH \lesssim 10^5 M_sun. To investigate this prediction, we used
264 s of 5.5 GHz data from the Karl G. Jansky Very Large Array (VLA) to search
for the radiative signatures of IMBH accretion from 337 candidate clusters in
an image spanning 492 arcsec (26 kpc) with a resolution of 0.40 arcsec (22 pc).
None of the individual clusters are detected, nor are weighted-mean image
stacks of the 311 old clusters, the 26 intermediate-age clusters, and the 20
clusters with stellar masses M_star \gtrsim 7.5 x 10^5 M_sun. The clusters thus
lack radio analogs of HLX-1, a strong IMBH candidate in a cluster in the
early-type galaxy ESO 243-49. This suggests that HLX-1 is accreting gas related
to its cluster's light-dominating young stars. Alternatively, the HLX-1
phenomenon could be so rare that no radio analog is expected in NGC 1023. Also,
using a formalism heretofore applied to star clusters in the Milky Way, the
radio-luminosity upper limit for the massive-cluster stack corresponds to a
mean 3 IMBH mass of M_BH(massive) < 2.3 x 10^5 M_sun, suggesting mean
black-hole mass fractions of M_BH(massive)/M_star < 0.05-0.29.Comment: 19 pages; 6 figures; accepted by A
Pulsar-Black Hole Binaries in the Galactic Center
Binaries consisting of a pulsar and a black hole (BH) are a holy grail of
astrophysics, both for their significance for stellar evolution and for their
potential application as probes of strong gravity. In spite of extensive
surveys of our Galaxy and its system of globular clusters, no pulsar-black hole
(PSR-BH) binary has been found to date. Clues as to where such systems might
exist are therefore highly desirable. We show that if the central parsec around
Sgr A* harbors a cluster of ~25,000 stellar BHs (as predicted by mass
segregation arguments) and if it is also rich in recycled pulsar binaries (by
analogy with globular clusters), then 3-body exchange interactions should
produce PSR-BHs in the Galactic center. Simple estimates of the formation rate
and survival time of these binaries suggest that a few PSR-BHs should be
present in the central parsec today. The proposed formation mechanism makes
unique predictions for the PSR-BH properties: 1) the binary would reside within
~1 pc of Sgr A*; 2) the pulsar would be recycled, with a period of ~1 to a few
tens of milliseconds, and a low magnetic field B<~10^10 G; 3) the binary would
have high eccentricity, e~0.8, but with a large scatter; and 4) the binary
would be relatively wide, with semi-major axis a_b~0.1 - >~3 AU. The potential
discovery of a PSR-BH binary therefore provides a strong motivation for deep,
high-frequency radio searches for recycled pulsars toward the Galactic center.Comment: 13 pages, 1 figur
Changes in Body Weight and Psychotropic Drugs: A Systematic Synthesis of the Literature
<div><h3>Introduction</h3><p>Psychotropic medication use is associated with weight gain. While there are studies and reviews comparing weight gain for psychotropics within some classes, clinicians frequently use drugs from different classes to treat psychiatric disorders.</p> <h3>Objective</h3><p>To undertake a systematic review of all classes of psychotropics to provide an all encompassing evidence-based tool that would allow clinicians to determine the risks of weight gain in making both intra-class and interclass choices of psychotropics.</p> <h3>Methodology and Results</h3><p>We developed a novel hierarchical search strategy that made use of systematic reviews that were already available. When such evidence was not available we went on to evaluate randomly controlled trials, followed by cohort and other clinical trials, narrative reviews, and, where necessary, clinical opinion and anecdotal evidence. The data from the publication with the highest level of evidence based on our hierarchical classification was presented. Recommendations from an expert panel supplemented the evidence used to rank these drugs within their respective classes. Approximately 9500 articles were identified in our literature search of which 666 citations were retrieved. We were able to rank most of the psychotropics based on the available evidence and recommendations from subject matter experts. There were few discrepancies between published evidence and the expert panel in ranking these drugs.</p> <h3>Conclusion</h3><p>Potential for weight gain is an important consideration in choice of any psychotropic. This tool will help clinicians select psychotropics on a case-by-case basis in order to minimize the impact of weight gain when making both intra-class and interclass choices.</p> </div
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