Astrophysics

Ultralight dark photons predicted in several Standard Model extensions can trigger the superradiant instability around rotating black holes if their Compton wavelength is comparable to the Blackhole radius. Consequently, the angular momentum of the black hole is reduced to a value which depends upon the mass and spin of the black hole as well as the mass of the dark photon. We use the mass and spin measurements of the primary black holes in two recently observed binary black hole systems: GW190517 and GW190426_152155 to constrain dark photon mass in the ranges 1.7? 10 ??4 eV< m A ??<7.6? 10 ??3 eV and 1.3? 10 ??3 eV< m A ??<4.2? 10 ??2 eV respectively, assuming a timescale of a few million years from the time of formation of the binary black hole system to the time of their merger. We also discuss an interesting X-ray binary system, MAXI J1820_070, albeit with a relatively small value of the spin parameter.

SNR G0.9+0.1 is a well known source in the direction of the Galactic Center composed by a Supernova Remnant (SNR) and a Pulsar Wind Nebula (PWN) in the core. We investigate the potential of the future Cherenkov Telescope Array (CTA), simulating observations of SNR G0.9+0.1. We studied the spatial and spectral properties of this source and estimated the systematic errors of these measurements. The source will be resolved if the VHE emission region is bigger than ∼ 0.65 ′ . It will also be possible to distinguish between different spectral models and calculate the cut-off energy. The systematic errors are dominated by the IRF instrumental uncertainties, especially at low energies. We computed the evolution of a young PWN inside a SNR using a one-zone time-dependent leptonic model. We applied the model to the simulated CTA data and found that it will be possible to accurately measure the cut-off energy of the γ -ray spectrum. Fitting of the multiwavelength spectrum will allow us to constrain also the magnetization of the PWN. Conversely, a pure power law spectrum would rule out this model. Finally, we checked the impact of the spectral shape and the energy density of the Inter-Stellar Radiation Fields (ISRFs) on the estimate of the parameters of the PWN, finding that they are not significantly affected.

Mergers of black hole (BH) and neutron star (NS) binaries are of interest since the emission of gravitational waves (GWs) can be followed by an electromagnetic (EM) counterpart, which could power short gamma-ray bursts. Until now, LIGO/Virgo has only observed a candidate BH-NS event, GW190426\_152155, which was not followed by any EM counterpart. We discuss how the presence (absence) of a remnant disk, which powers the EM counterpart, can be used along with spin measurements by LIGO/Virgo to derive a lower (upper) limit on the radius of the NS. For the case of GW190426\_152155, large measurement errors on the spin and mass ratio prevent from placing an upper limit on the NS radius. Our proposed method works best when the aligned component of the BH spin (with respect to the orbital angular momentum) is the largest, and can be used to complement the information that can be extracted from the GW signal to derive valuable information on the NS equation of state.

The Parkes, ASKAP, CHIME and UTMOST telescopes, which have all detected FRBs, each works at a different frequency and has a different detection criteria. Using simulations, we have combined the constraints from all four telescopes to identify an allowed range of model parameters $(\alpha, \E33)$ for the FRB source population. Here α is the spectral index and $\E33$ is the mean FRB energy in units of 10 33 J across a 2128??848MHz band in the FRB rest frame. We have considered several different FRB energy distributions, and also different scenarios for the scattering pulse broadening, the event rate density variation with z and the host dispersion measure. We find that in all cases, the common allowed region includes the range ??.9?�α≤??.3 and $0.42\leq\E33\leq1$. In all case, large values α>4 and $\E33 >60$ are ruled out. Considering the allowed $(\alpha, \E33)$ parameter range, we predict that CHIME is unlikely to detect an FRB with extra-galactic dispersion measure (D M Ex ) exceeding 3700 pccm ?? . A substantially larger D M Ex in the large FRB sample anticipated from CHIME would falsify the assumptions of the present analysis. Our analysis is expected to yield tighter parameter constraints with the advent of more FRB data.

Using a density dependent quark mass (QMDD) model for strange quark matter, we investigate the effects of non-Newtonian gravity on the properties of strange stars and constrain the parameters of the QMDD model by employing the mass of PSR J0740+6620 and the tidal deformability of GW170817. We find that for QMDD model these mass and tidal deformability observations would rule out the existenceof str ange stars if non-Newtonian gravity effects are ignored. For the current quark masses of m u0 =2.16 MeV, m d0 =4.67 MeV, and m s0 =93 MeV, we find that a strange star can exist for values of the non-Newtonian gravity parameter g 2 / μ 2 in the range of 4.58 GeV ?? ??g 2 / μ 2 ??9.32 GeV ?? , and that the parameters D and C of the QMDD modelare restricted to 158.3 MeV ??D 1/2 ??181.2 MeV and ??.65?�C?��?0.12 . It is found that the largest possible maximum mass of a strange star obtained with the QMDD model is 2.42 M ??, and that the secondary componentof GW190814 with a mass of 2.59_{-0.09}^{+0.08}\, M_{\dot} could not be a static strange star. We also find that forthe mass and radius of PSR J0030+0451 given by Riley et al. through the analysis of observational data of NICER, there exists a very tiny allowed parameter space for which strange stars computed for the QMDD model agree with the observations of PSR J0740+6620, GW17 0817 and PSR J0030+0451 simultaneously. However, for the mass and radius given by Miller et al., no such parameter space exist.

Binary neutron star mergers are thought to be one of the dominant sites of production for rapid neutron capture elements, including platinum and gold. Since the discovery of the binary neutron star merger GW170817, and its associated kilonova AT2017gfo, numerous works have attempted to determine the composition of its outflowing material, but they have been hampered by the lack of complete atomic data. Here, we demonstrate how inclusion of new atomic data in synthetic spectra calculations can provide insights and constraints on the production of the heaviest elements. We employ theoretical atomic data (obtained using GRASP 0 ) for neutral, singly- and doubly-ionised platinum and gold, to generate photospheric and simple nebular phase model spectra for kilonova-like ejecta properties. We make predictions for the locations of strong transitions, which could feasibly appear in the spectra of kilonovae that are rich in these species. We identify low-lying electric quadrupole and magnetic dipole transitions that may give rise to forbidden lines when the ejecta becomes optically thin. The strongest lines lie beyond 8000? , motivating high quality near-infrared spectroscopic follow-up of kilonova candidates. We compare our model spectra to the observed spectra of AT2017gfo, and conclude that no platinum or gold signatures are prominent in the ejecta. From our nebular phase modelling, we place tentative upper limits on the platinum and gold mass of ??a few 10 ?? M ??, and ??10 ?? M ??, respectively. This work demonstrates how new atomic data of heavy elements can be included in radiative transfer calculations, and motivates future searches for elemental signatures.

The formation mechanism of the W50/SS433 complex has long been a mystery. We propose a new scenario in which the SS433 jets themselves form the W50/SS433 system. We carry out magnetohydrodynamics simulations of two-side jet propagation using the public code CANS+. As found in previous jet studies, when the propagating jet is lighter than the surrounding medium, the shocked plasma flows back from the jet tip to the core. We find that the morphology of light jets is spheroidal at early times, and afterward, the shell and wings are developed by the broadening spherical cocoon. The morphology strongly depends on the density ratio of the injected jet to the surrounding medium. Meanwhile, the ratio of the lengths of the two-side jets depends only on the density profile of the surrounding medium. We also find that most of the jet kinetic energy is dissipated at the oblique shock formed by the interaction between the backflow and beam flow, rather than at the jet terminal shock. The position of the oblique shock is spatially consistent with the X-ray and TeV gamma-ray hotspots of W50.

Gamma-ray bursts (GRBs) are believed to be powered by ultrarelativistic jets. If these jets encounter and accelerate excess electrons and positrons produced by particle dark matter (DM) annihilation, the observed electromagnetic radiation would be enhanced. In this paper, we study GRB afterglow emission with the presence of abundant DM under the weakly interacting massive particle annihilation conditions. We calculate the light curves and spectra of the GRB afterglows with different parameters, i.e., DM density, particle DM mass, annihilation channel, and electron density of the interstellar medium. We find that the effect of DM may become noticeable in the afterglow spectra if the circumburst has a low electron number density ( n≲0.1 c m −3 ) and if the DM has a high number density ( ρ χ ≳ 10 3 GeV c m −3 ). According to the standard galaxy DM density profile, GRB afterglows with DM contribution might occur at distances of several to tens of parsecs from the centers of massive galaxies.

Blazars are known to show flux variations over a range of energies from low energy radio to high energy gamma-rays. Cross-correlation analysis of the optical and γ -ray light curves in blazars shows that flux variations are generally correlated in both bands, however, there are exceptions. We explored this optical-GeV connection in four flat spectrum radio quasars (FSRQs) by a systematic investigation of their long term optical and γ -ray light curves. On analysis of the four sources, namely 3C 273, 3C 279, PKS 1510 − 089 and CTA 102 we noticed different behaviours between the optical and GeV flux variations. We found instances when (i) the optical and GeV flux variations are closely correlated (ii) there are optical flares without γ -ray counterparts and (iii) γ -ray flares without optical counterparts. To understand these diverse behaviours, we carried out broad band spectral energy distribution (SED) modelling of the sources at different epochs using a one-zone leptonic emission model. The optical-UV emission is found to be dominated by emission from the accretion disk in the sources PKS 1510 − 089, CTA 102 and 3C 273, while in 3C 279, the synchrotron radiation from the jet dominates the optical-UV emission. Our SED analysis indicates that (i) correlated optical and γ -ray flux variations are caused by changes in the bulk Lorentz factor ( Γ ), (ii) γ -ray flares without optical counterparts are due to increase in Γ and/or the electron energy density and (iii) an optical flare without γ -ray counterpart is due to increase in the magnetic field strength.

By fitting high-quality and simultaneous multi-wavelength (MWL) spectral energy distributions (SEDs) at multiple epochs with a one-zone leptonic jet model, we study jet properties of the three famous blazars Mrk 421, 3C 454.3 and 3C 279. In the jet model, the emitting electron energy distributions (EEDs) are calculated by solving the kinetic equation of electron injection, escape, adiabatic and radiative energy losses. To explore multi-dimensional parameter space systematically, we employ a Markov chain Monte Carlo (MCMC) fitting technique. The properties of emission regions we derived here are consistent with those in previous studies, e.g., the particle-dominated and low-magnetization jet. The new finding is that there is a tight correlation between γ -ray luminosity and electron injection power and an anti-correlation between γ -ray luminosity and jet magnetization parameter. The results suggest that same energy-dissipative mechanism (like a shock) could be operating in the jets of different types of blazars, and the origin of γ -ray flares is associated with the particle acceleration process.