Astrophysics

The precise measurement of the positron spectrum by AMS-02 reveals a sharp dropoff above ??84 GeV. However, the AMS-02 electron spectrum shows a power-law from ??0 GeV to ?? TeV without significant features. We propose that the difference of spectral features between electrons and positrons indicates the existence of a nearby source component of primary electrons. Taking into account the observational results of the positron excess, the spectral softenings of cosmic ray protons and helium nuclei, and the energy-dependences of the large-scale anisotropies of cosmic rays, we suggest that the scenario of supernova remnant origin of cosmic rays with non-negligible contribution from a nearby source can successfully account for most of the observations of cosmic rays below PeV energies. The Geminga pulsar and its past supernova remnant might be a proper candidate of the nearby source.

Magnetars are highly magnetized neutron stars that are characterized by recurrent emission of short-duration bursts in soft gamma-rays/hard X-rays. Recently, FRB 200428 were found to be associated with an X-ray burst from a Galactic magnetar. Two fast radio bursts (FRBs) show mysterious periodic activity. However, whether magnetar X-ray bursts are periodic phenomena is unclear. In this paper, we investigate the period of SGR 1806-20 activity. More than 3000 short bursts observed by different telescopes are collected, including the observation of RXTE, HETE-2, ICE and Konus. We consider the observation windows and divide the data into two sub-samples to alleviate the effect of unevenly sample. The epoch folding and Lomb-Scargle methods are used to derive the period of short bursts. We find a possible period about 398.20±25.45 days. While other peaks exist in the periodograms. If the period is real, the connection between short bursts of magnetars and FRBs should be extensively investigated.

As a powerful source of gravitational waves (GW), a supermassive black hole (SMBH) merger may be accompanied by a relativistic jet that leads to detectable electromagnetic (EM) emission. We model the propagation of post-merger jets inside a pre-merger circumnuclear environment formed by disk winds, and calculate multi-wavelength EM spectra from the forward shock region. We show that the non-thermal EM signals from SMBH mergers are detectable up to the detection horizon of future GW facilities such as the Laser Interferometer Space Antenna (LISA). Calculations based on our model predict slowly fading transients with time delays from days to months after the coalescence, leading to implications for EM follow-up observations after the GW detection.

The discovery of 3C 273 in 1963, and the emergence of the Kerr solution shortly thereafter, precipitated the current era in astrophysics focused on using black holes to explain active galactic nuclei (AGN). But while partial success was achieved in separately explaining the bright nuclei of some AGN via thin disks, as well as powerful jets with thick disks, the combination of both powerful jets in an AGN with a bright nucleus, such as in 3C 273, remained elusive. Although numerical simulations have taken center stage in the last 25 years, they have struggled to produce the conditions that explain them. This is because radiatively efficient disks have proved a challenge to simulate. Radio quasars have thus been the least understood objects in high energy astrophysics. But recent simulations have begun to change this. We explore this milestone in light of scale-invariance and show that transitory jets, possibly related to the jets seen in these recent simulations, as some have proposed, cannot explain radio quasars. We then provide a road map for a resolution.

Short gamma-ray bursts that are followed by long-duration X-ray plateaus may be powered by the birth, and hydrodynamic evolution, of magnetars from compact binary coalescence events. If the rotation and magnetic axes of the system are not orthogonal to each other, the star will undergo free precession, leading to fluctuations in the luminosity of the source. In some cases, precession-induced modulations in the spin-down power may be discernible in the X-ray flux of the plateau. In this work, 25 X-ray light curves associated with bursts exhibiting a plateau are fitted to luminosity profiles appropriate for precessing, oblique rotators. Based on the Akaike Information Criterion, 16 (64 per cent) of the magnetars within the sample display either moderate or strong evidence for precession. Additionally, since the precession period of the star is directly tied to its quadrupolar ellipticity, the fits allow for an independent measure of the extent to which the star is deformed by internal stresses. Assuming these deformations arise due to a mixed poloidal-toroidal magnetic field, we find that the distribution of magnetic-energy ratios is bimodal, with data points clustering around energetically equal and toroidally dominated partitions. Implications of this result for gravitational-wave emission and dynamo activity in newborn magnetars are discussed.

Numerical experiments are the primary method of studying the evolution of circumbinary disks due to the strong nonlinearities involved. Many circumbinary simulations also require the use of numerical mass sinks: source terms which prevent gas from unphysically accumulating around the simulated point masses by removing gas at a given rate. However, special care must be taken when drawing physical conclusions from such simulations to ensure that results are not biased by numerical artifacts. We demonstrate how the use of improved sink methods reduces some of these potential biases in vertically-integrated simulations of aspect ratio 0.1 accretion disks around binaries with mass ratios between 0.1 and 1. Specifically, we show that sink terms that do not reduce the angular momentum of gas relative to the accreting object: 1) reduce the dependence on the sink rate of physical quantities such as the torque on the binary, distribution of accretion between binary components, and evolution of the binary semi-major axis; 2) reduce the degree to which the sink rate affects the structure of the accretion disks around each binary component; 3) alter the inferred variability of accretion onto the binary, making it more regular in time. We also investigate other potential sources of systematic error, such as the precise from of gravitational softening and previously employed simplifications to the viscous stress tensor. Because of the strong dependence of the orbital evolution of the binary on both the torque and distribution of mass between binary components, the sink methods employed can have a significant effect on the inferred orbital evolution of the binary.

We study the onset of vector instabilities in a post-inflationary epoch of the Universe as a mechanism for primordial magnetic fields amplification between the end of inflation and the electroweak (EW) transition. We assume the presence of a charged spectator scalar field arbitrarily coupled to gravity. This field is in its vacuum state during inflation, but becomes highly excited after the transition to the radiation dominance due to the gravitational particle creation. At the beginning of radiation era the ensuing state admits a hydrodynamic description. In particular since its high temperature, the fluid may be regarded as a conformal one. The large quantum fluctuations induced during reheating now become statistical fluctuations whereby an excess charge and anisotropic pressure will be observed in any finite domain. Under these conditions a Weibel instability could be triggered thus opposing the dilution of a primordial magnetic field because of the expansion of the Universe. The magnitude of the effect is determined by the size of the domain, the coupling to curvature of the field and the relaxation time of the fluid. We find that for scales of the order or smaller than the particle horizon at the EW phase transition, the Weibel instability can overcome the cosmic expansion provided that the reheating temperature of the Universe and the coupling of the scalar field to gravity are small enough.

We introduce a new method to estimate the probability that an extragalactic transient source is associated with a candidate host galaxy. This approach relies solely on simple observables: sky coordinates and their uncertainties, galaxy fluxes and angular sizes. The formalism invokes Bayes' rule to calculate the posterior probability P(O_i|x) from the galaxy prior P(O), observables x, and an assumed model for the true distribution of transients in/around their host galaxies. Using simulated transients placed in the well-studied COSMOS field, we consider several agnostic and physically motivated priors and offset distributions to explore the method sensitivity. We then apply the methodology to the set of 13~fast radio bursts (FRBs) localized with an uncertainty of several arcseconds. Our methodology finds nine of these are securely associated to a single host galaxy, P(O_i|x)>0.95. We examine the observed and intrinsic properties of these secure FRB hosts, recovering similar distributions as previous works. Furthermore, we find a strong correlation between the apparent magnitude of the securely identified host galaxies and the estimated cosmic dispersion measures of the corresponding FRBs, which results from the Macquart relation. Future work with FRBs will leverage this relation and other measures from the secure hosts as priors for future associations. The methodology is generic to transient type, localization error, and image quality. We encourage its application to other transients where host galaxy associations are critical to the science, e.g. gravitational wave events, gamma-ray bursts, and supernovae. We have encoded the technique in Python on GitHub: this https URL.

Axion-like particles (ALPs) are predicted in some well-motivated theories beyond the Standard Model. The TeV gamma-rays from active galactic nuclei (AGN) suffers attenuation by the pair production interactions with the cosmic infrared background light (EBL/CMB) during its travel to the earth. The attenuation can be circumvented through photon-ALP conversions in the AGN and Galaxy magnetic-field, and a flux enhancement is expected to arise in the observed spectrum. In this work, we study the potential of the AGN gamma-ray spectrum for energy up to above 100 TeV to probe ALP-parameter space at around μ eV, where the coupling g aγ is so far relatively weak constrained. We find the nearby and bright sources, Mrk 501, IC 310 and M 87, are suitable for our objective. Assuming an intrinsic spectrum exponential cutoff energy at E c =100 TeV, we extrapolate the observed spectra of these sources up to above 100 TeV by the models with/without ALPs. For g aγ ??? 10 ??1 Ge V ?? with m a ??μ eV, the flux at around 100 TeV predicted by the ALP model can be enhanced more than an order of magnitude than that from the standard absorption, and could be detected by LHAASO. Our result is subject to the uncertainty from the intrinsic spectrum above tens of TeV, which require further observations on these sources by the forthcoming CTA, LHAASO, SWGO and so on.

The discovery of GW170817 and GRB 170817A in tandem with AT 2017gfo cemented the connection between neutron star mergers, short gamma-ray bursts (GRBs), and kilonovae. To investigate short GRB observations in the context of diverse kilonova behavior, we present a comprehensive optical and near-infrared (NIR) catalog of 85 bursts discovered over 2005-2020 on timescales of ??2 days. The sample includes previously unpublished observations of 23 bursts, and encompasses both detections and deep upper limits. We identify 11.8% and 15.3% of short GRBs in our catalog with upper limits that probe luminosities lower than those of AT 2017gfo and a fiducial NSBH kilonovae model (for pole-on orientations), respectively. We quantify the ejecta masses allowed by the deepest limits in our catalog, constraining blue and `extremely blue' kilonova components of 14.1% of bursts to M ej ??.01??.1 M ??. The sample of short GRBs is not particularly constraining for red kilonova components. Motivated by the large catalog as well as model predictions of diverse kilonova behavior, we investigate modified search strategies for future follow-up to short GRBs. We find that ground-based optical and NIR observations on timescales of ?? days can play a significant role in constraining more diverse outcomes. We expect future short GRB follow up efforts, such as from the {\it James Webb Space Telescope}, to expand the reach of kilonova detectability to redshifts of z?? .