Rates and delay times of Type Ia supernovae in the helium-enriched main-sequence donor scenario [SSA]


The nature of the progenitors of Type Ia supernovae (SNe Ia) remains a mystery. Comparing theoretical rates and delay-time distributions of SNe Ia with those inferred observationally can constrain their progenitor models. In this work, taking thermohaline mixing into account in the helium-enriched main-sequence (HEMS) donor scenario, we address rates and delay times of SNe Ia in this channel by combining the results of self-consistent binary evolution calculations with population synthesis models. We find that the Galactic SN Ia rate from the HEMS donor scenario is around 0.6-1.2 per century, which is about 30% of the observed rate. Delay times of SNe Ia in this scenario cover a wide range of 0.1-1.0 Gyr. We also present the pre-explosion properties of companion stars in the HEMS donor scenario, which will be helpful for placing constraints on SN Ia progenitors through analyzing their pre-explosion images.

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Z. Liu and R. Stancliffe
Fri, 19 Jan 18

Comments: 11 pages, 8 figures, Accepted for publication in MNRAS

The Dynamics of Truncated Black Hole Accretion Disks II: Magnetohydrodynamic Case [HEAP]


We study a truncated accretion disk using a well-resolved, semi-global magnetohydrodynamic simulation that is evolved for many dynamical times (6096 inner disk orbits). The spectral properties of hard state black hole binary systems and low-luminosity active galactic nuclei are regularly attributed to truncated accretion disks, but a detailed understanding of the flow dynamics is lacking. In these systems the truncation is expected to arise through thermal instability driven by sharp changes in the radiative efficiency. We emulate this behavior using a simple bistable cooling function with efficient and inefficient branches. The accretion flow takes on an arrangement where a “transition zone” exists in between hot gas in the inner most regions and a cold, Shakura $\&$ Sunyaev thin disk at larger radii. The thin disk is embedded in an atmosphere of hot gas that is fed by a gentle outflow originating from the transition zone. Despite the presence of hot gas in the inner disk, accretion is efficient. Our analysis focuses on the details of the angular momentum transport, energetics, and magnetic field properties. We find that the magnetic dynamo is suppressed in the hot, truncated inner region of the disk which lowers the effective $\alpha$-parameter by $65\%$.

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J. Hogg and C. Reynolds
Fri, 19 Jan 18

Comments: 18 pgs, 16 figures, accepted for publication in ApJ

Detection of the Characteristic Pion-Decay Signature in the Molecular Clouds [HEAP]


The $\gamma$-ray emission from molecular clouds is widely believed to have a hadronic origin, while unequivocal evidence is still lacking. In this work, we analyze the Fermi-LAT Pass 8 publicly available data accumulated from 2008 August 4 to 2017 June 29 and report the significant detection of a characteristic $\pi^0$-decay feature in the $\gamma$-ray spectra of the molecular clouds Orion A and Orion B. This detection provides a direct evidence for the hadronic origin of their $\gamma$-ray emission.

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Z. Shen, Y. Liang, K. Duan, et. al.
Fri, 19 Jan 18

Comments: 11 pages, 5 figures and 1 table

Evidence for a decreasing X-ray afterglow emission of GW170817A and GRB 170817A in XMM-Newton [HEAP]


We report our observation of the short GRB 170817A, associated to the binary neutron star merger event GW 170817, perfomed in the X-ray band with XMM-Newton 135 d after the event (on the 29th December 2017). We find that the X-ray light curve started to decrease with respect to the previous steadily brightening trend. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the decrease in flux is due to the passage of the cooling frequency: its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models: i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and the X-ray one, making hard to distinguish between them at the present stage. Polarimetric measures and the rate of short GRB-GW association in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios.

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P. DAvanzo, S. Campana, G. Ghisellini, et. al.
Fri, 19 Jan 18

Comments: Letter submitted to Astronomy & Astrophysics. Other authors will be added to the present author list as soon as they have permission from the respective collaborations

Another look at AM Herculis $-$ radio-astrometric campaign with the e-EVN at 6 cm [SSA]


We conducted radio-interferometric observations of the well known binary cataclysmic system AM Herculis. This particular system is formed by the magnetic white dwarf (primary) and the red dwarf (secondary), and is the prototype of so-called polars. Our observations were conducted with the European VLBI Network (EVN) in the e-EVN mode at 5 GHz. We obtained six astrometric measurements spanning one year, which make it possible to update the annual parallax for this system with the best precision to date ($\pi=11.29\pm0.08$ mas), equivalent to the distance of $88.6\pm0.6$ pc. The system was observed mostly in the quiescent phase (visual magnitude $m_v\sim15.3$), when the radio emission was at the level of about $300~\mu$Jy. Our analysis suggests that the radio flux of AM Herculis is modulated with the orbital motion. Such specific properties of the radiation can be explained using the emission mechanism similar to the scenario proposed for V471 Tauri and, in general, for RS CVn type stars. In this scenario the radio emission arises near the surface of the red dwarf, where the global magnetic field strength may reach a few kG. We argue that the quiescent radio emission distinguishes AM Herculis together with AR Ursae Majoris (the second known persistent radio polar) from other polars, as the systems with a magnetized secondary star.

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M. Gawronski, K. Gozdziewski, K. Katarzynski, et. al.
Fri, 19 Jan 18

Comments: 12 pages, 9 figures, Accepted for publication in MNRAS

Imprints of the redshift evolution of double neutron star merger rate on the signal to noise ratio distribution [CL]


Proposed third generation gravitational wave (GW) interferometers such as Einstein Telescope will have the sensitivity to observe double neutron star (DNS) mergers up to a redshift of $\sim 2$ with good signal to noise ratios. We argue that the measurement of {\it redshifted signal to noise ratio} defined by $\sigma=\rho (1+z)^{1/6}$, where $\rho$ is the signal to noise ratio (SNR) of a detected GW event and $z$ is its redshift can be used to study the distribution of DNS mergers. We show that if the DNS binaries are distributed uniformly within the co-moving volume, the distribution of redshifted SNR, $\sigma$, will be inversely proportional to the fourth power of $\sigma$, $p(\sigma)\propto \frac{1}{\sigma^4}$. We argue that the redshift evolution of DNS mergers will leave imprints on the distribution of $\sigma$ and hence this may provide a method to probe their redshift evolution. Using various parametric models for evolution of co-moving merger rate density as a function of redshift and assuming the sensitivity of Einstein Telescope, we discuss the distinguishability of the $\sigma$ distributions of these models with that of constant co-moving number density of the mergers.

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S. Kastha and K. Arun
Fri, 19 Jan 18

Comments: 7 pages, 5 figures

Investigating the region of 3C 397 in High Energy Gamma rays [HEAP]


We investigate the supernova remnant (SNR) 3C 397 and its neighboring pulsar PSR J1906+0722 in high energy gamma rays by using nearly six years of archival data of {\it Large Area Telescope} on board {\it Fermi Gamma Ray Space Telescope} (Fermi-LAT). The off-pulse analysis of gamma-ray flux from the location of PSR J1906+0722 reveals an excess emission which is found to be very close to the radio location of 3C 397. Here, we present the preliminary results of this gamma-ray analysis of 3C 397 and PSR J1906+0722.

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P. Bhattacharjee, P. Majumdar, T. Ergin, et. al.
Fri, 19 Jan 18

Comments: 4 pages, 3 figures, Proceeding of IAU Symposium No. 331, 2017