Astrophysics

A black hole X-ray binary produces hard X-ray radiation from its corona and disk when the accreting matter heats up. During an outburst, the disk and corona co-evolves with each other. However, such an evolution is still unclear in both its geometry and dynamics. Here we report the unusual decrease of the reflection fraction in MAXI J1820+070, which is the ratio of the coronal intensity illuminating the disk to the coronal intensity reaching the observer, as the corona is observed to contrast during the decay phase. We postulate a jet-like corona model, in which the corona can be understood as a standing shock where the material flowing through. In this dynamical scenario, the decrease of the reflection fraction is a signature of the corona's bulk velocity. Our findings suggest that as the corona is observed to get closer to the black hole, the coronal material might be outflowing faster.

The recent measurement of the spectra of intermediate mass nuclei and iron nuclei carried out with the AMS-02 experiment provided us with the most complete set of data on cosmic ray fluxes to date, and allowed us to test the standard model for the transport of these particles through the Galaxy to the finest details. We show that the parameters derived from lighter primary and secondary elements in the cosmic radiation also lead to a good description of the data on heavier nuclei, with no need to invoke different injection spectra for such nuclei, provided the whole chain of fragmentation is properly accounted for. The only exception to this finding is represented by iron nuclei, which show a very unusual trend at rigidity ??00 GV. This trend reflects in a Fe/O ratio that is at odds with the results of the standard model of cosmic ray transport, and is in contradiction with data collected by HEAO, ACE-CRIS and Voyager at lower energy. We speculate on possible origins of such findings.

Gamma-ray bursts (GRBs) are powered by relativistic jets that exhibit intermittency over a broad range of timescales - from ??ms to seconds. Previous numerical studies have shown that hydrodynamic (i.e., unmagnetized) jets that are expelled from a variable engine are subject to strong mixing of jet and cocoon material, which strongly inhibits the GRB emission. In this paper we conduct 3D RMHD simulations of mildly magnetized jets with power modulation over durations of 0.1 s and 1 s, and a steady magnetic field at injection. We find that when the jet magnetization at the launching site is ???.1 , the initial magnetization is amplified by shocks formed in the flow to the point where it strongly suppresses baryon loading. We estimate that a significant contamination can be avoided if the magnetic energy at injection constitutes at least a few percent of the jet energy. The variability timescales of the jet after it breaks out of the star are then governed by the injection cycles rather than by the mixing process, suggesting that in practice jet injection should fluctuate on timescales as short as ??0 ms in order to account for the observed light curves. Better stability is found for jets with shorter modulations. We conclude that for sufficiently hot jets, the Lorentz factor near the photosphere can be high enough to allow efficient photospheric emission. Our results imply that jets with 10 ?? <?<1 injected by a variable engine with ??0 ms duty cycle are plausible sources of long GRBs.

It has been reported that the extinction law for Type Ia Supernovae (SNe Ia) may be different from the one in the Milky Way, but the intrinsic color of SNe Ia and the dust extinction are observationally mixed. In this study, we examine photometric properties of SNe Ia in the nearby universe ( z??.04 ) to investigate the SN Ia intrinsic color and the dust extinction. We focus on the Branch spectroscopic classification of 34 SNe Ia and morphological types of host galaxies. We carefully study their distribution of peak colors on the B?�V , V?�R color-color diagram, as well as the color excess and absolute magnitude deviation from the stretch-color relation of the bluest SNe Ia. We find that SNe Ia which show the reddest color occur in early-type spirals and the trend holds when divided into Branch sub-types. The dust extinction becomes close to the Milky-Way like extinction if we exclude some peculiar red Broad Line (BL) sub-type SNe Ia. Furthermore, two of these red BLs occur in elliptical galaxies, less-dusty environment, suggesting intrinsic color diversity in BL sub-type SNe Ia.

We present a formulation for a local cooling function to be employed in the diffuse, hot corona region of 3D GRMHD simulations of accreting black holes. This new cooling function calculates the cooling rate due to inverse Compton scattering by considering the relevant microphysics in each cell in the corona and approximating the radiation energy density and Compton temperature there by integrating over the thermal seed photon flux from the disk surface. The method either assumes ion and electron temperatures are equal (1T), or calculates them separately (2T) using an instantaneous equilibrium approach predicated on the actual relevant rate equations (Coulomb and Compton). The method is shown to be consistent with a more detailed ray-tracing calculation where the bulk of the cooling occurs, but is substantially less costly to perform. As an example, we apply these methods to a \textsc{harm3d} simulation of a 10 M ⊙ , non-spinning black hole, accreting at nominally 1\% the Eddington value. Both 1T and 2T approaches lead to increased radiative efficiency and a larger fraction of total cooling in the corona as compared to the original target-temperature cooling function used by \textsc{harm3d}, especially in the 1T case. Time-averaged post-processing reveals that the continuum spectral observations predicted from these simulations are qualitatively similar to actual X-ray binary data, especially so for the 1T approach which yields a harder power-law component ( Γ=2.25 ) compared to the 2T version ( Γ=2.53 )

The unidentified TeV source MGRO J1908+06, with emission extending from hundreds of GeV to beyond 100TeV, is one of the most intriguing sources in the Galactic plane. MGRO J1908+06 spatially associates with an IceCube hotspot of neutrino emission. Although the hotspot is not significant yet, this suggests a possible hadronic origin of the observed gamma-ray radiation. Here we describe a multiwavelength analysis on MGRO J1908+06 to determine its nature. We identify, for the first time, an extended GeV source as the counterpart of MGRO J1908+06, discovering possibly associated molecular clouds (MCs). The GeV spectrum shows two well-differentiated components: a soft spectral component below ??0 GeV, and a hard one ( ???.6 ) above these energies. The lower-energy part is likely associated with the dense MCs surrounding the supernova remnant SNR G40.5 ??0.5, whereas the higher-energy component, which connects smoothly with the spectrum observed in TeV range, resembles the inverse Compton emission observed in relic pulsar wind nebulae. This simple scenario seems to describe the data satisfactorily, but raises questions about the interpretation of the emission at hundreds of TeV. In this scenario, no detectable neutrino flux would be expected.

We report on the creation and application of a novel decay network that uses the latest data from experiment and evaluation. We use the network to simulate the late-time phase of the rapid neutron capture (r) process. In this epoch, the bulk of nuclear reactions, such as radiative capture, have ceased and nuclear decays are the dominant transmutation channels. We find that the decay from short-lived to long-lived species naturally leads to an isochronic evolution in which nuclei with similar half-lives are populated at the same time. We consider random perturbations along each isobaric chain to initial solar-like r-process compositions to demonstrate the isochronic nature of the late-time phase of the r-process. Our analysis shows that detailed knowledge of the final isotopic composition allows for the prediction of late-time evolution with a high degree of confidence despite uncertainties that exist in astrophysical conditions and the nuclear physics properties of the most neutron-rich nuclei. We provide the time-dependent nuclear composition in the Appendix as supplemental material.

It is generally thought that FRII Radio Galaxies host thin optically thick disks, while FRIs are powered by Advected Dominated Accretion Flows. The sources with an efficient engine are optically classified as High Excitation Radio Galaxies (HERGs) and those with an inefficient motor as Low Excitation Radio Galaxies (LERGs). Recently, the study of Radio Galaxies down to mJy fluxes has cast serious doubts on the LERG-FRI and HERG-FRII correspondence, revealing that many LERGs show FRII radio morphologies. The FR catalogs recently compiled by Capetti et al. (2017a,b) and Baldi et al. (2018) have allowed us to explore this issue in the local ( z??.15 ) mJy Universe. Our statistical study shows that the majority of nearby mJy objects are in a late stage of their life. FRII-LERGs appear more similar to the old FRI-LERGs than to the young FRII-HERGs. FRII-LERGs may be aged HERGs that, exhausted the cold fuel, have changed their accretion regime or a separate LERG class particularly efficient in launching jets. Exploiting the empirical relations which convert L [OIII] and L 1.4 GHz into accretion power and jet kinetic power, respectively, we observed that LERGs with similar masses and accretion rates seem to expel jets of different power. We speculate that intrinsic differences related to the black hole properties (spin and magnetic field at its horizon) can determine the observed spread in jet luminosity. In this view, FRII-LERGs should have the fastest spinning black holes and/or the most intense magnetic fluxes. On the contrary, compact LERGs (i.e. FR0s) should host extremely slow black holes and/or weak magnetic fields.

With its large effective area at hard X-rays, high time resolution and having co-aligned other instruments, AstroSat/LAXPC was designed to usher in a new era in rapid variability studies and wide spectral band measurements of the X-ray binaries. Over the last five years, the instrument has successfully achieved to a significant extent these Science goals. In the coming years, it is poised to make more important discoveries. This paper highlights the primary achievements of AstroSat/LAXPC in unraveling the behavior of black hole and neutron star systems and discusses the exciting possibility of the instrument's contribution to future science.

We present the long-term X-ray spectral and temporal analysis of a 'bare-type AGN' Ark 120. We consider the observations from XMM-Newton, Suzaku, Swift, and NuSTAR from 2003 to 2018. The spectral properties of this source are studied using various phenomenological and physical models present in the literature. We report (a) the variations of several physical parameters, such as the temperature and optical depth of the electron cloud, the size of the Compton cloud, and accretion rate for the last fifteen years. The spectral variations are explained from the change in the accretion dynamics; (b) the X-ray time delay between 0.2-2 keV and 3-10 keV light-curves exhibited zero-delay in 2003, positive delay of 4.71 \pm 2.1 ks in 2013, and negative delay of 4.15 \pm 1.5 ks in 2014. The delays are explained considering Comptonization, reflection, and light-crossing time; (c) the long term intrinsic luminosities obtained using nthcomp, of the soft-excess and the primary continuum show a correlation with a Pearson Correlation Coefficient of 0.922. This indicates that the soft-excess and the primary continuum are originated from the same physical process. From a physical model fitting, we infer that the soft excess for Ark 120 could be due to a small number of scatterings in the Compton cloud. Using Monte-Carlo simulations, we show that indeed the spectra corresponding to fewer scatterings could provide a steeper soft-excess power-law in the 0.2-3 keV range. Simulated luminosities are found to be in agreement with the observed values.