Black Hole-Neutron Star Mergers in Globular Clusters

We model the formation of black hole-neutron star (BH-NS) binaries via dynamical interactions in globular clusters. We find that in dense, massive clusters, 16-61% of the BH-NS binaries formed by interactions with existing BH binaries will undergo mergers driven by the emission of gravitational radiation. If the BHs are retained by the cluster after merging with a NS, the BHs acquire subsequent NS companions and undergo several mergers. Thus, the merger rate depends critically upon whether or not the BH is retained by the cluster after the merger. Results from numerical relativity suggest that kick imparted to a ~7 M_sun BH after it merges with a NS will greatly exceed the cluster's escape velocity. In this case, the models suggest that the majority of BH-NS mergers in globular clusters occur within 4 Gyrs of the cluster's formation and would be unobservable by Advanced LIGO. For more massive BHs, on the other hand, the post merger kick is suppressed and the BH is retained. Models with 35 M_sun BHs predict Advanced LIGO detection rates in the range 0.04 - 0.7 per year. On the pessimistic end of this range, BH-NS mergers resulting from binary-single star interactions in globular clusters could account for an interesting fraction of all BH-NS mergers. On the optimistic end, this channel may dominate the rate of detectable BH-NS mergers.Comment: 13 pages, 7 figures, 3 tabels, accepted for publication in MNRA

Dynamically formed black hole+millisecond pulsar binaries in globular clusters

The discovery of a binary comprising a black hole (BH) and a millisecond pulsar (MSP) would yield insights into stellar evolution and facilitate exquisitely sensitive tests of general relativity. Globular clusters (GCs) are known to harbor large MSP populations and recent studies suggest that GCs may also retain a substantial population of stellar mass BHs. We modeled the formation of BH+MSP binaries in GCs through exchange interactions between binary and single stars. We found that in dense, massive clusters most of the dynamically formed BH+MSP binaries will have orbital periods of 2 to 10 days, regardless of the mass of the BH, the number of BHs retained by the cluster, and the nature of the GC's binary population. The size of the BH+MSP population is sensitive to several uncertain parameters, including the BH mass function, the BH retention fraction, and the binary fraction in GCs. Based on our models, we estimate that there are $0.6\pm0.2$ dynamically formed BH+MSP binaries in the Milky Way GC system, and place an upper limit on the size of this population of $\sim 10$. Interestingly, we find that BH+MSP binaries will be rare even if GCs retain large BH populations.Comment: 15 pages, 5 figures, 2 tables, accepted for publication in MNRAS, updated to match published versio

Action Search: Spotting Actions in Videos and Its Application to Temporal Action Localization

State-of-the-art temporal action detectors inefficiently search the entire video for specific actions. Despite the encouraging progress these methods achieve, it is crucial to design automated approaches that only explore parts of the video which are the most relevant to the actions being searched for. To address this need, we propose the new problem of action spotting in video, which we define as finding a specific action in a video while observing a small portion of that video. Inspired by the observation that humans are extremely efficient and accurate in spotting and finding action instances in video, we propose Action Search, a novel Recurrent Neural Network approach that mimics the way humans spot actions. Moreover, to address the absence of data recording the behavior of human annotators, we put forward the Human Searches dataset, which compiles the search sequences employed by human annotators spotting actions in the AVA and THUMOS14 datasets. We consider temporal action localization as an application of the action spotting problem. Experiments on the THUMOS14 dataset reveal that our model is not only able to explore the video efficiently (observing on average 17.3% of the video) but it also accurately finds human activities with 30.8% mAP.Comment: Accepted to ECCV 201

Siciak-Zahariuta extremal functions, analytic discs and polynomial hulls

We prove two disc formulas for the Siciak-Zahariuta extremal function of an arbitrary open subset of complex affine space. We use these formulas to characterize the polynomial hull of an arbitrary compact subset of complex affine space in terms of analytic discs. Similar results in previous work of ours required the subsets to be connected

A Panchromatic Study of the Globular Cluster NGC 1904. I: The Blue Straggler Population

By combining high-resolution (HST-WFPC2) and wide-field ground based (2.2m ESO-WFI) and space (GALEX) observations, we have collected a multi-wavelength photometric data base (ranging from the far UV to the near infrared) of the galactic globular cluster NGC1904 (M79). The sample covers the entire cluster extension, from the very central regions up to the tidal radius. In the present paper such a data set is used to study the BSS population and its radial distribution. A total number of 39 bright ($m_{218}\le 19.5$) BSS has been detected, and they have been found to be highly segregated in the cluster core. No significant upturn in the BSS frequency has been observed in the outskirts of NGC 1904, in contrast to other clusters (M 3, 47 Tuc, NGC 6752, M 5) studied with the same technique. Such evidences, coupled with the large radius of avoidance estimated for NGC 1904 ($r_{avoid}\sim 30$ core radii), indicate that the vast majority of the cluster heavy stars (binaries) has already sunk to the core. Accordingly, extensive dynamical simulations suggest that BSS formed by mass transfer activity in primordial binaries evolving in isolation in the cluster outskirts represent only a negligible (0--10%) fraction of the overall population.Comment: ApJ accepte

Synthesis and characterization of RNA containing a rigid and nonperturbing cytidine-derived spin label.

The nitroxide-containing nucleoside Çm is reported as the first rigid spin label for paramagnetic modification of RNA by solid-phase synthesis. The spin label is well accommodated in several RNA secondary structures as judged by its minor effect on the thermodynamic stability of hairpin and duplex RNA. Electron paramagnetic resonance (EPR) spectroscopic characterization of mono-, bi-, and trimolecular RNA structures shows that Çm will be applicable for advanced EPR studies to elucidate structural and dynamic aspects of folded RNA

Double Blue Straggler sequences in GCs: the case of NGC 362

We used high-quality images acquired with the WFC3 on board the HST to probe the blue straggler star (BSS) population of the Galactic globular cluster NGC 362. We have found two distinct sequences of BSS: this is the second case, after M 30, where such a feature has been observed. Indeed the BSS location, their extension in magnitude and color and their radial distribution within the cluster nicely resemble those observed in M 30, thus suggesting that the same interpretative scenario can be applied: the red BSS sub-population is generated by mass transfer binaries, the blue one by collisions. The discovery of four new W UMa stars, three of which lying along the red-BSS sequence, further supports this scenario. We also found that the inner portion of the density profile deviates from a King model and is well reproduced by either a mild power-law (\alpha -0.2) or a double King profile. This feature supports the hypothesis that the cluster is currently undergoing the core collapse phase. Moreover, the BSS radial distribution shows a central peak and monotonically decreases outward without any evidence of an external rising branch. This evidence is a further indication of the advanced dynamical age of NGC 362: in fact, together with M 30, NGC 362 belongs to the family of dynamically old clusters (Family III) in the "dynamical clock" classification proposed by Ferraro et al. (2012). The observational evidence presented here strengthens the possible connection between the existence of a double BSS sequence and a quite advanced dynamical status of the parent cluster.Comment: Accepted for publication by ApJ; 39 pages, 16 figures, 1 tabl

The Triple Pulsar System PSR B1620-26 in M4

The millisecond pulsar PSR B1620-26, in the globular cluster M4, has a white dwarf companion in a half-year orbit. Anomalously large variations in the pulsar's apparent spin-down rate have suggested the presence of a second companion in a much wider orbit. Using timing observations made on more than seven hundred days spanning eleven years, we confirm this anomalous timing behavior. We explicitly demonstrate, for the first time, that a timing model consisting of the sum of two non-interacting Keplerian orbits can account for the observed signal. Both circular and elliptical orbits are allowed, although highly eccentric orbits require improbable orbital geometries. The motion of the pulsar in the inner orbit is very nearly a Keplerian ellipse, but the tidal effects of the outer companion cause variations in the orbital elements. We have measured the change in the projected semi-major axis of the orbit, which is dominated by precession-driven changes in the orbital inclination. This measurement, along with limits on the rate of change of other orbital elements, can be used to significantly restrict the properties of the outer orbit. We find that the second companion most likely has a mass m~0.01 Msun --- it is almost certainly below the hydrogen burning limit (m<0.036 Msun, 95% confidence) --- and has a current distance from the binary of ~35 AU and orbital period of order one hundred years. Circular (and near-circular) orbits are allowed only if the pulsar magnetic field is ~3x10^9 G, an order of magnitude higher than a typical millisecond pulsar field strength. In this case, the companion has mass m~1.2x10^-3 Msun and orbital period ~62 years.Comment: 12 pages, 6 figures, 3 tables. Very minor clarifications and rewording. Accepted for publication in the Astrophys.

Two serendipitous low-mass LMC clusters discovered with HST

We present V and I photometry of two open clusters in the LMC down to V ~ 26. The clusters were imaged with the Wide Field and Planetary Camera - 2 on board of the Hubble Space Telescope, as part of the Medium Deep Survey Key-Project. Both are low luminosity (Mv ~ -3.5), low mass systems (M ~ 10^3 Msolar). The chance discovery of these two clusters in two parallel WFPC2 fields suggests a significant incompleteness in the LMC cluster census near the bar. One of the clusters is roughly elliptical and compact, with a steep light profile, a central surface brightness Mu_v(0) 20.2 mag/arcsec2, half-light radius Rhl ~ 0.9 pc (total visual major diameter D ~ 3 pc) and an estimated mass M ~ 1500 Msolar. From the colour-magnitude diagram and isochrone fits we estimate its age as t ~ 2-5 10^8 years. Its mass function has a fitted slope of Gamma = \Delta log \phi (M) / \Delta log M = -1.8 +/- 0.7 in the range probed (0.9 < M/Msolar < 4.5). The other cluster is more irregular and sparser, having shallower density and surface brightness profiles. We obtain Gamma = -1.2 +/- 0.4, and estimate its mass as M ~ 400 Msolar. A derived upper limit for its age is t < 5 10^8 years. Both clusters have mass functions with slopes similar to that of R136, a massive LMC cluster, for which HST results indicate Gamma ~ -1.2. They also seem to be relaxed in their cores and well contained in their tidal radii.Comment: 16 pages plus 9 figures (2 large figs not included