Abstracts of Interest
Selected by:
Rami Alsulami
Abstract: 1911.05364
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Multi-messenger signals from short gamma ray bursts
(Submitted on 13 Nov 2019)
Abstract: We present the results of simulations done with the code HARM-COOL developed in the CTP PAS Warsaw research group over the years 2017-2019. It is based in the original GR MHD scheme proposed by Gammie et al. (2003) for the simulation of Active Galactic Nucleus, but now it has been suited for the engine of a short Gamma Ray Burst event.
We compute time-dependent evolution of a black hole accretion disk, in two-dimensional, axisymmetric scheme. The code includes neutrino cooling and accounts for nuclear structure of dense, degenerate matter. Free protons, neutrons, and electron-positron pairs form a neutron-rich, magnetically driven outflow that provides site for subsequent r-process nucleosynthesis.
Here the heavy elements up to the Uranium and Gold are synthesized and may contribute to the chemical enrichment of the circum-burst medium. Their radio-active decay will give signal in lower energies in a timescale of weeks-months after the GRB prompt phase.
In addition, the magnetic fields are responsible for the launching of ultra-relativistic jets along the rotation axis of the central black hole, according to the well-known Blandford-Znajek mechanism. These jets are sites of variable high energy emission in gamma rays. We find that the magnetic field and the black hole spin account for the observed variability timescales and jet energetics.
Abstract: 1911.04761
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:H.E.S.S. detection of very-high-energy gamma-ray emission from the quasar PKS 0736+017
(Submitted on 12 Nov 2019)
Abstract: Flat-spectrum radio-quasars (FSRQs) are rarely detected at very-high-energies (VHE; E>100 GeV) due to their low-frequency-peaked SEDs. At present, only 6 FSRQs are known to emit VHE photons, representing only 7% of the VHE extragalactic catalog. Following the detection of MeV-GeV gamma-ray flaring activity from the FSRQ PKS 0736+017 (z=0.189) with Fermi, the H.E.S.S. array of Cherenkov telescopes triggered ToO observations on February 18, 2015, with the goal of studying the gamma-ray emission in the VHE band. H.E.S.S. ToO observations were carried out during the nights of February 18, 19, 21, and 24, 2015. Together with Fermi-LAT, the multi-wavelength coverage of the flare includes Swift observations in soft-X-rays and optical/UV, and optical monitoring (photometry and spectro-polarimetry) by the Steward Observatory, the ATOM, the KAIT and the ASAS-SN telescope. VHE emission from PKS 0736+017 was detected with H.E.S.S. during the night of February 19, 2015, only. Fermi data indicate the presence of a gamma-ray flare, peaking at the time of the H.E.S.S. detection, with a flux doubling time-scale of around six hours. The gamma-ray flare was accompanied by at least a 1 mag brightening of the non-thermal optical continuum. No simultaneous observations at longer wavelengths are available for the night of the H.E.S.S. detection. The gamma-ray observations with H.E.S.S. and Fermi are used to put constraints on the location of the gamma-ray emitting region during the flare: it is constrained to be just outside the radius of the broad-line-region with a bulk Lorentz factor $\simeq 20$, or at the level of the radius of the dusty torus with Gamma > 60. PKS 0736+017 is the seventh FSRQ known to emit VHE photons and, at z=0.189, is the nearest so far. The location of the gamma-ray emitting region during the flare can be tightly constrained thanks to opacity, variability, and collimation arguments.
Abstract: 1911.05079
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Supernovae within Pre-existing Wind-Blown Bubbles: Dust Injection vs. Ambient Dust Destruction
(Submitted on 12 Nov 2019)
Abstract: By means of 3-D hydrodynamical simulations, here we evaluate the impact that supernova explosions occurring within wind-driven bubbles have on the survival or destruction of dust grains. We consider both, the dust generated within the ejecta and the dust initially present in the ambient gas and later locked-up in the surrounding wind-driven shell. The collision of the supernova blast wave with the wind-driven shell leads to a transmitted shock that moves into the shell and a reflected shock into the ejecta. The transmitted shock is capable of destroying large amounts of the dust locked in the shell, but only if the mass of the wind-driven shell is small, less than a few tens the ejected mass. Conversely, massive wind-driven shells, with several times the ejected mass, lead upon the interaction to strong radiative cooling, which inhibits the Sedov-Taylor phase and weakens the transmitted shock, making it unable to traverse the wind-driven shell. In such a case, the destruction/disruption of the ambient dust is largely inhibited. On the other hand, the SNRs grow rapidly in the very tenuous region excavated by the stellar winds, and thus a large fraction of the dust generated within the ejecta is not efficiently destroyed by the supernova reverse shock, nor by the reflected shock. Our calculations favor a scenario in which core-collapse supernovae within sufficiently massive wind-driven shells supply more dust to the ISM than what they are able to destroy.
Abstract: 1911.05077
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:The survival of star clusters with black hole subsystems
(Submitted on 12 Nov 2019)
Abstract: Recent observations have detected top-heavy IMFs in dense star forming regions like the Arches cluster. Whether such IMFs also exist in old dense stellar systems like globular clusters is difficult to constrain, because massive stars already became black holes (BHs) and neutron stars (NSs). However, studies of stellar dynamics find that BHs/NSs influence the long-term evolution of star clusters. Following Breen & Heggie (2013) and by carrying out two-component $N$-body simulations, we demonstrate how this dynamical impact connects with the shape of IMFs. By investigating the energy balance between the BH subsystem and the global, we find that to properly describe the evolution of clusters, a corrected two-body relaxation time, $T_{rh,p} = T_{rh}/\psi$, is necessary. Because $\psi$ depends on the total mass fraction of BHs, $M_2 / M$, and the mass ratio, $m_2 / m_1$, the cluster dissolution time is sensitive to the property of BHs or IMFs. Especially, the escape rate of BHs via ejections from few-body encounters is linked to mass segregation. In strong tidal fields, top-heavy IMFs easily lead to the fast dissolution of star clusters and the formation of BH-dominant dark clusters, which suggests that the observed massive GCs with dense cores are unlikely to have extreme top-heavy IMFs. With the future observations of gravitational waves providing unique information of BHs/NSs, it is possible to combine the multi-message observations to have better constrains on the IMFs of old star clusters.
Abstract: 1911.06134
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Cherenkov Telescope Array potential in the search for Galactic PeVatrons
(Submitted on 14 Nov 2019)
Abstract: One of the major scientific objectives of the future Cherenkov Telescope Array (CTA) Observatory is the search for PeVatrons. PeVatrons are cosmic-ray factories able to accelerate nuclei at least up to the knee feature seen in the spectrum of cosmic rays measured near the Earth. CTA will perform a survey of the full Galactic plane at TeV energies and beyond with unprecedented sensitivity. The determination of efficient criteria to identify PeVatron candidates during the survey is essential in order to trigger further dedicated observations. Here, we present results from a study based on simulations to determine these criteria. The outcome of the study is a PeVatron figure of merit, defined as a metric that provides relations between spectral parameters and spectral cutoff energy lower limits. In addition, simulations of the PeVatron candidate HESS J1641$-$463 and its parental particle spectrum are presented and discussed. Eventually, our work is applied to simulated population of Galactic PeVatrons, with the aim to determine the sensitivity of CTA.
Abstract: 1911.06097
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Multi-messenger astronomy with very-high-energy gamma-ray observations
(Submitted on 14 Nov 2019)
Abstract: After decades of development, multi-messenger astronomy, the combination of information on cosmic sources from photons, neutrinos, charged particles and gravitational waves, is now an established reality. Within this emerging discipline we argue that very-high-energy (VHE) gamma-ray observations play a special role. We discuss the recent progress on explosive transients, the connections between neutrino and gamma-ray astronomy and the search for search for dark matter. Finally, the experimental prospects for the next decade in the VHE gamma-ray field are summarised.
Abstract: 1911.06680
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:New hard-TeV extreme blazars detected with the MAGIC telescopes
(Submitted on 15 Nov 2019)
Abstract: Extreme high-frequency peaked BL Lac objects (EHBLs) are blazars which exhibit extremely energetic synchrotron emission. They also feature non-thermal gamma-ray emission whose peak lies in the very high-energy (VHE, E > 100 GeV) range, and in some sources exceeds 1TeV: this is the case of hard-TeV EHBLs such as 1ES 0229+200. With the aim of increasing the EHBL population, ten targets were observed with the MAGIC telescopes from 2010 to 2017, for a total of 262 h of good quality data. The data were complemented by coordinated Swift observations. The X-ray data analysis confirms that all the sources but two are EHBLs. The sources show only a modest variability and a harder-when-brighter behavior, typical for this class of objects. At VHE gamma rays, three new sources were detected and a hint of signal was found for another new source. In each case the intrinsic spectrum is compatible with the hypothesis of a hard-TeV nature of these EHBLs. The broadband spectral energy distributions (SEDs) of all sources are built and modeled in the framework of a single-zone purely leptonic model. The VHE gamma-ray detected sources were also interpreted with a spine-layer and a proton synchrotron models. The three models provide a good description of the SEDs. However, the resulting parameters differ substantially in the three scenarios, in particular the magnetization parameter. This work presents a first mini-catalog of VHE gamma-ray and multi-wavelength observations of EHBLs.
Abstract: 1911.06776
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:36 GHz methanol lines from nearby galaxies: maser or quasi-thermal emission?
(Submitted on 15 Nov 2019)
Abstract: Methanol (CH3OH) is one of the most abundant interstellar molecules, offering a vast number of transitions to be studied, including many maser lines. While the strongest Galactic CH3OH lines, the so-called class II masers, show no indications for the presence of superluminous counterparts in external galaxies, the less luminous Galactic class I sources appear to be different. Here we report class I 36GHz CH3OH 4(-1) - 3(0) E line emission from the nearby galaxies Maffei2 and IC342, measured with the 100-m telescope at Effelsberg at three different epochs within a time span of about five weeks. The 36GHz methanol line of Maffei2 is the second most luminous among the sources detected with certainty outside the Local Group of galaxies. This is not matched by the moderate infrared luminosity of Maffei2. Higher resolution data are required to check whether this is related to its prominent bar and associated shocks. Upper limits for M82, NGC4388, NGC5728 and Arp220 are also presented. The previously reported detection of 36GHz maser emission in Arp220 is not confirmed. Non-detections are reported from the related class I 44GHz methanol transition towards Maffei2 and IC342, indicating that this line is not stronger than its 36GHz counterpart. In contrast to the previously detected 36GHz CH3OH emission in NGC253 and NGC4945, our 36GHz profiles towards Maffei2 and IC342 are similar to those of previously detected non-masing lines from other molecular species. However, by analogy to our Galactic center region, it may well be possible that the 36GHz methanol lines in Maffei~2 and IC~342 are composed of a large number of faint and narrow maser features that remain spatially unresolved. In view of this, a search for a weak broad 36GHz line component would also be desirable in NGC253 and NGC4945.
Abstract: 1911.06330
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Photometric properties of reionization-epoch galaxies in the Simba simulations
(Submitted on 14 Nov 2019)
Abstract: We study the photometric properties and sizes of the reionization-epoch galaxies in high-resolution Simba cosmological hydrodynamical simulations with box sizes of $[25,50]\ h^{-1}$Mpc. Assuming various attenuation laws, we compute photometry by extincting each star particle's spectrum using the line-of-sight gas metal column density. The predicted ultraviolet luminosity function (UVLF) generally agrees with observations at $z=6$, owing to a partial cancellation between the high metallicities of the simulated galaxies and lower dust-to-metal ratios. The simulated $z=8$ UVLF is low compared to observations, likely owing to excessive dust extinction. Simba predicts UV continuum slopes ($\beta$) in agreement with the $z=6$ observations, with the best agreement obtained using a Calzetti extinction law. Interestingly, the gas-phase mass-metallicity relation in Simba is higher at $z\sim 6$ than at $z\sim 2$, suggesting that rapid early enrichment (and dust growth) is necessary to match the observed $\beta$. We find that $\beta$ is much more sensitive to the dust extinction law than the UVLF. By generating mock James Webb Space Telescope (JWST) images and analysing in a manner similar to observations, we show that Simba's galaxy size-luminosity relation well reproduces the current $z=6$ Hubble observations. Unlike observations at lower redshifts, Simba predicts similar rest-UV and rest-optical sizes of $z=6$ galaxies, owing to weak age gradients and dust extinction in star-forming regions counteract each other to weaken the color gradients within galaxies. These predictions will be testable with JWST.
Abstract: 1911.06447
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Radio/X-ray Correlation in the Mini-Outburts of Black Hole X-ray Transient GRS 1739--278
(Submitted on 15 Nov 2019)
Abstract: We present quasi-simultaneous radio and X-ray observations of the black hole X-ray binary GRS 1739--278 of its 2015-2016 mini-outbursts, i.e. between 2015 June 10 and 2016 October 31, with the X-ray-to-radio time interval being less than one day. The monitor champaign was run by \swift\, in the X-rays and by JVLA in the radio (at both 5 GHz and 8 GHz). We find the brightest radio emission is actually achieved during the soft sate, where radio spectrum is unexpectedly flat with a spectral index $\alpha\approx -0.2$ (flux $F_\nu\propto \nu^\alpha$). For the radio emission in hard state, we also find a large diversity in the spectral index, i.e. a majority of radio spectra are optically thick with $-0.5 < \alpha < 0.5$, while a few are optically thin with $\alpha < -1$. We also investigate the correlation between the luminosities in radio (monochromatic at 5 GHz, $L_R$) and 1-10 keV X-rays ($L_X$) during the hard state. We find this source does not follow the standard correlation whose $p\approx 0.6$ (in the form of $L_R\propto L_X^p$), but instead belongs to the "outlier" category that may follow a hybrid correlation. For more than orders of magnitude variation in the X-ray luminosity this source exhibits a flat correlation with $p\approx 0.16$. Both the slope and the corresponding luminosity range agree well with those in H1743--322, the prototype of the hybrid correlation.
Abstract: 1911.06328
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Sound-Wave Instabilities in Dilute Plasmas with Cosmic Rays: Implications for Cosmic-Ray Confinement and the Perseus X-ray Ripples
(Submitted on 14 Nov 2019)
Abstract: Weakly collisional, magnetised plasmas characterised by anisotropic viscosity and conduction are ubiquitous in galaxies, halos and the intracluster medium (ICM). Cosmic rays (CRs) play an important role in these environments as well, by providing additional pressure and heating to the thermal plasma. We carry out a linear stability analysis of weakly collisional plasmas with cosmic rays using Braginskii MHD for the thermal gas. We assume that the CRs stream at the Alfvén speed, which in a weakly collisional plasma depends on the pressure anisotropy ($\Delta p$) of the thermal plasma. We find that this $\Delta p$-dependence introduces a phase shift between the CR-pressure and gas-density fluctuations. This drives a fast-growing acoustic instability: CRs offset the damping of acoustic waves by anisotropic viscosity and give rise to wave growth when the ratio of CR pressure to gas pressure is $\gtrsim \alpha \beta^{-1/2}$, where $\beta$ is the ratio of thermal to magnetic pressure, and $\alpha$, typically $\lesssim 1$, depends on other dimensionless parameters. In high-$\beta$ environments like the ICM, this condition is satisfied for small CR pressures. We speculate that the instability studied here may contribute to the scattering of high-energy CRs and to the excitation of sound waves in galaxy-halo, group and cluster plasmas, including the long-wavelength X-ray fluctuations in \textit{Chandra} observations of the Perseus cluster. It may also be important in the vicinity of shocks in dilute plasmas (e.g., cluster virial shocks or galactic wind termination shocks), where the CR pressure is locally enhanced.
Abstract: 1911.06029
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Optical intensity interferometry observations using the MAGIC imaging atmospheric Cherenkov telescopes
(Submitted on 14 Nov 2019)
Abstract: Imaging Atmospheric Cherenkov Telescopes (IACTs) currently in operation feature large mirrors and order of 1 ns time response to signals of a few photo-electrons produced by optical photons. This means that they are ideally suited for optical interferometry observations. Thanks to their sensitivity to visible wavelengths and long baselines optical intensity interferometry with IACTs allows reaching angular resolutions of tens to microarcsec. We have installed a simple optical setup on top of the cameras of the two 17 m diameter MAGIC IACTs and observed coherent fluctuations in the photon intensity measured at the two telescopes for three different stars. The sensitivity is roughly 10 times better than that achieved in the 1970s with the Narrabri interferometer.
Abstract: 1911.04171
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:UHE Cosmic Rays and AGN Jets
(Submitted on 11 Nov 2019)
Abstract: Active Galactic Nuclei (AGN) and their relativistic jets are believed to be potential sites of ultra-high-energy (UHE) cosmic ray acceleration. This paper reviews basic observational findings as well as requirements on source energetics, and then discusses the relevance of different acceleration sites and mechanisms, such as black hole gap, shock in back-flows or jet shear acceleration. When put in context, the result suggests that Fermi-type particle acceleration at trans-relativistic shocks and/or in shearing, relativistic flows offers the most promising framework for UHECR production in AGN. Truly deciphering the astrophysical sources of UHECRs, however, still needs improved statistical information on arrival directions and source correlations.
This page created: Tue Nov 19 10:09:25 CST 2019 by Rami Alsulami
For a printable title listing click here
For details on generating this page see the instructions. If there are problems with this page (and I expect there will be from time to time) contact Jose.
For previous lists of abstracts of interest click Previous abstracts of interest