GW170104 and the origin of heavy, low-spin binary black holes via classical isolated binary evolution [HEAP]

The Advanced LIGO has observed four BH-BH mergers thus far. The effective spin of GW170104 has an 82 percent probability of being negative, which would indicate spin-orbit misalignment. It has been suggested that LIGO’s detections favor a population of binaries with frequent and significant spin-orbit misalignment, hence supporting dynamical formation over classical isolated binary evolution. However, it is also well known that massive stars can have efficient transport of angular momentum within the star and have strong winds that carry away substantial angular momentum. The progenitors of the heaviest stellar-mass BHs (M>30Msun) are expected to efficiently reduce their angular momentum, producing BHs with low spin (consistent with existing LIGO constraints). A stellar evolution model that incorporates rotation is used to determine the natal spins of the BHs produced by core collapse. These natal spins are incorporated into our population synthesis code which follows the evolution of Population I/II stars across cosmic time and predicts the BH-BH population in the universe. Our classical isolated binary evolution model produces a BH-BH population that is consistent with the observed LIGO population. In particular, it produces systems that are consistent with the measured properties of GW170104; total mass, mass ratio, effective spin and the overall BH-BH merger rate. Neither strong natal BH kicks nor dynamical interactions are required to recover GW170104. LIGO’s observations do not provide evidence for dynamical formation over classical isolated binary formation and it is not yet possible to establish the evolutionary channel through which GW170104 was formed.

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K. Belczynski, J. Klencki, G. Meynet, et. al.
Fri, 23 Jun 17

Comments: to be submitted to A&A (comments welcome)

Gravitational effects of condensed dark matter on strange stars [CL]

In the present work we study the gravitational effects of condensed dark matter on strange stars. We consider self-interacting dark matter particles with properties consistent with current observational constraints, and dark matter inside the star is modelled as a Bose-Einstein condensate. We integrate numerically the Tolman-Oppenheimer-Volkoff equations in the two-fluid formalism assuming that strange stars are made of up to 4 per cent of dark matter. It is shown that for a mass of the dark matter particles in the range $50 MeV-160 MeV$ strange stars are characterized by a maximum mass and radius similar to the ones found for neutron stars.

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G. Panotopoulos and I. Lopes
Fri, 23 Jun 17

Comments: Two-column REVTEX, 5 pages, 3 figures

The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440 [HEAP]

We used a combination of deep optical and Halpha images of the Galactic globular cluster NGC 6440, acquired with the Hubble Space Telescope, to identify the optical counterpart to the accreting millisecond X-ray pulsar SAX J1748.9-2021during quiescence. A strong Halpha emission has been detected from a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15″ from the nominal position of the X-ray source. The position of the star also agrees with the optical counterpart found by Verbunt et al. (2000) during an outburst. We propose this star as the most likely optical counterpart to the binary system. By direct comparison with isochrones, we estimated that COM-SAX J1748.9-2021 has a mass of 0.70 Msun – 0.83 Msun, a radius of 0.88 pm 0.02 Rsun and a superficial temperature of 5250pm80 K. These parameters combined with the orbital characteristics of the binary suggest that the system is observed at a very low inclination angle (~8 deg -14 deg) and that the star is filling or even overflowing its Roche Lobe. This, together with the equivalent width of the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The possibile presence of such a on-going mass transfer during a quiescence state also suggests that the radio pulsar is not active yet and thus this system, despite its similarity with the class of redback millisecond pulsars, is not a transitional millisecond pulsar.

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M. Cadelano, C. Pallanca, F. Ferraro, et. al.
Fri, 23 Jun 17

Comments: 8 pages, 6 figures. Accepted for publication in ApJ

Are eikonal quasinormal modes linked to the unstable circular null geodesics? [CL]

In Phys. Rev. D 79, 064016 (2009) it was claimed that quasinormal modes which any stationary, spherically symmetric and asymptotically flat black hole emits in the eikonal regime are determined by the parameters of the circular null geodesic: the real and imaginary parts of the quasinormal mode are multiples of the frequency and instability timescale of the circular null geodesics respectively. We shall consider asymptotically flat black hole in the Einstein-Lovelock theory, find analytical expressions for gravitational quasinormal modes in the eikonal regime and analyze the null geodesics. Comparison of the both phenomena shows that the expected link between the null geodesics and quasinormal modes is violated in the Einstein-Lovelock theory. Nevertheless, the correspondence exists for a number of other cases and here we formulate its actual limits.

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R. Konoplya and Z. Stuchlik
Fri, 23 Jun 17

Comments: 8 pages, revtex, refereed version, accepted for publication in Physics Letters B

Possible correlations between the emission properties of short GRBs and their offsets from the host galaxies [HEAP]

Short Gamma-Ray Bursts(SGRBs) are widely believed to be from mergers of binary compact objects involving at least one neutron star and hence have a broad range of spatial offsets from their host galaxies. In this work we search for possible correlations between the emission properties of 18 SGRBs and their offsets from the host galaxies. The SGRBs with and without extended emission do not show significant difference between their offset distribution, in agreement with some previous works. There are however possible correlations between the optical and X-ray afterglow emission and the offsets. The underlying physical origins are examined.

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S. Zhang, Z. Jin, F. Zhang, et. al.
Fri, 23 Jun 17

Comments: 5 pages, 2 figures, Accepted for publication in ApJ

Prospects for the detection of high-energy (E>25 GeV) Fermi pulsars with the Cherenkov Telescope Array [HEAP]

Around 160 gamma-ray pulsars were discovered by the Fermi Large Area Telescope (LAT) since 2008. The most energetic of them, 12 objects with emission above 25 GeV, are suitable candidates for the detection with the current and future Imaging Atmospheric Cherenkov Telescopes above few tens of GeV. We perform an analysis of the Fermi-LAT data of these high-energy pulsars in order to determine if such objects can be detected with the Cherenkov Telescope Array (CTA). Our goal is to forecast the significance of their point source detection with CTA. We analyze 5 years of the Fermi-LAT data fitting the spectra of each pulsar at energies E>10 GeV with a power-law function. Assuming no spectral cut-off, we extrapolate the resulting spectra to the very high energy range (VHE, E>0.1 TeV) and simulate CTA observations of all 12 pulsars with the ctools software package. Using different analysis tools, individual CTA sensitivity curves are independently calculated for each pulsar and cross-checked with the ctools results. Our simulations result in significant CTA detections of up to 8 pulsars in 50 h. Observations of the most energetic Fermi pulsars with CTA will shed light on the nature of the high-energy emission of pulsars, clarifying whether the VHE emission detected in the Crab pulsar spectrum is present also in other gamma-ray pulsars.

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A. Burtovoi, T. Saito, L. Zampieri, et. al.
Fri, 23 Jun 17

Comments: 17 pages, 7 figures, 7 tables

Modification of magnetohydrodynamic waves by the relativistic Hall effect [CL]

This study shows that a relativistic Hall effect significantly changes the properties of wave propagation by deriving a linear dispersion relation for relativistic Hall magnetohydrodynamics (HMHD). Whereas, in non- relativistic HMHD, the phase and group velocities of fast magnetosonic wave become anisotropic with an increasing Hall effect, the relativistic Hall effect brings upper bounds to the anisotropies. The Alfve\'{e}n wave group velocity with strong Hall effect also becomes less anisotropic than non-relativistic case. Moreover, the group velocity surfaces of Alfv\'{e}n and fast waves coalesce into a single surface in the direction other than near perpendicular to the ambient magnetic field. It is also remarkable that a characteristic scale length of the relativistic HMHD depends on ion temperature, magnetic field strength, and density while the non-relativistic HMHD scale length, i.e., ion skin depth, depends only on density. The modified characteristic scale length increases as the ion temperature increases and decreases as the magnetic field strength increases.

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Y. Kawazura
Fri, 23 Jun 17

Comments: N/A