High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Markarian (Mrk) 180 is a High frequency-peaked BL Lacertae object or HBL object, located at a redshift of 0.045 and a potential candidate for high-energy cosmic ray acceleration. In this work, we have done a temporal and spectral study using Fermi Large Area Telescope (Fermi-LAT) $\gamma$-ray data, collected over 12.8 years. In the case of the temporal study, the 12.8 years long, 30-day binned, Fermi-LAT $\gamma$-ray light curve does not show any significant enhancement of the flux. To understand the underlying physical mechanism, we focused our study on multi-wavelength spectral analysis. We constructed multi-wavelength spectral energy distribution (MWSED) using Swift X-ray, ultraviolet & optical, and X-ray Multi-Mirror Mission (XMM-Newton) data, which have been analysed thoroughly. The SED has been modelled with three different models: (i) pure leptonic scenario and lepto-hadronic scenario where we considered two types of lepto-hadronic interactions (ii) line-of-sight interactions of ultrahigh-energy cosmic rays (UHECR; $E\gtrsim 10^{17}$ eV) with the cosmic background radiation and (iii) interaction between relativistic protons with the cold proton within the blazar jet. In this literature, we have done a detailed comparative study between all these three models. In an earlier study, Mrk 180 was associated with the Telescope Array (TA) hotspot of UHECRs at $E>57$ EeV which motivates us to check whether Mrk 180 can be a source of UHECRs, contributing to the TA hotspot. From our study, we find, for conservative strengths of the extragalactic magnetic field, Mrk 180 is unlikely to be a source of UHECR events.

Abstract: The number of muons in an air shower is a strong indicator of the mass of the primary particle and increases with a small power of the cosmic ray mass by the $\beta$-exponent, $N_{\mu} \sim A^{(1-\beta)}$. This behaviour can be explained in terms of the Heitler-Matthews model of hadronic air showers. In this paper, we present a method for calculating $\beta$ from the Heitler-Matthews model. The method has been successfully verified with a series of simulated events observed by the Pierre Auger Observatory at $10^{19}$ eV. To follow real measurements of the mass composition at this energy, the generated sample consists of a certain fraction of events produced with p, He, N and Fe primary energies. Since hadronic interactions at the highest energies can differ from those observed at energies reached by terrestrial accelerators, we generate a mock data set with $\beta =0.92$ (the canonical value) and $\beta =0.96$ (a more exotic scenario). The method can be applied to measured events to determine the muon signal for each primary particle as well as the muon scaling factor and the $\beta$-exponent. Determining the $\beta$-exponent can effectively constrain the parameters that govern hadronic interactions and help solve the so-called muon problem, where hadronic interaction models predict too few muons relative to observed events. In this paper, we lay the foundation for the future analysis of measured data from the Pierre Auger Observatory with a simulation study.

Abstract: Geminga is an enigmatic radio-quiet gamma-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy gamma-ray emission around the pulsar has been detected by multiple water Cherenkov detector based instruments. However, the detection of extended TeV gamma-ray emission around the Geminga pulsar has proven challenging for IACTs due to the angular scale exceeding the typical field-of-view. By detailed studies of background estimation techniques and characterising systematic effects, a detection of highly extended TeV gamma-ray emission could be confirmed by the H.E.S.S. IACT array. Building on the previously announced detection, in this contribution we further characterise the emission and apply an electron diffusion model to the combined gamma-ray data from the H.E.S.S. and HAWC experiments, as well as X-ray data from XMM-Newton.

Abstract: HESS J1702-420 is a multi-TeV gamma-ray source with an unusual energy-dependent morphology. The recent H.E.S.S. observations suggest that the emission is well described by a combination of point-like HESS J1702-420A (dominating at highest energies, $\gtrsim$ 30 TeV ) and diffuse ($\sim$ 0.3$^\circ$) HESS J1702-420B (dominating below $\lesssim$ 5TeV) sources with very hard (${\Gamma} \sim 1.5$) and soft (${\Gamma}$ ~2.6) power-law spectra, respectively. Here we propose a model which postulates that the proton accelerator is located at the position of HESS J1702-420A and is embedded into a dense molecular cloud that coincides with HESS J1702-420B. In the proposed model, the VHE radiation of HESS J1702-420 is explained by the pion-decay emission from the continuously injected relativistic protons propagating through a dense cloud. The energy-dependent morphology is defined by the diffusive nature of the low-energy protons propagation, transiting sharply to (quasi) ballistic propagation at higher energies. Adopting strong energy dependence of the diffusion coefficient, $D \propto E^\beta$ with $\beta \geq 1$, we argue that HESS J1702-420 as the system of two gamma-ray sources is the result of the propagation effect. Protons injected by a single accelerator at the rate $Q_0 \simeq 10^{38} \, (n_0/100 \, \rm cm^{-3})^{-1}\, (d/ \, 0.25\,kpc)^{-1} \rm erg/s$ can reasonably reproduce the morphology and fluxes of two gamma-ray components.

Abstract: The evidence for multi-messenger photon and neutrino emission from the blazar TXS 0506+056 has demonstrated the importance of realtime follow-up of neutrino events by various ground- and space-based facilities. The effort of H.E.S.S. and other experiments in coordinating observations to obtain quasi-simultaneous multiwavelength flux and spectrum measurements has been critical in measuring the chance coincidence with the high-energy neutrino event IC-170922A and constraining theoretical models. For about a decade, the H.E.S.S. transient program has included a search for gamma-ray emission associated with high-energy neutrino alerts, looking for gamma-ray activity from known sources and newly detected emitters consistent with the neutrino location. In this contribution, we present an overview of follow-up activities for realtime neutrino alerts with H.E.S.S. in 2021 and 2022. Our analysis includes both public IceCube neutrino alerts and alerts exchanged as part of a joint H.E.S.S.-IceCube program. We focus on interesting coincidences observed with gamma-ray sources, particularly highlighting the significant detection of PKS 0625-35, an AGN previously detected by H.E.S.S., and three IceCube neutrinos.

Abstract: HESS J1813-178 is one of the brightest sources detected during the first HESS Galactic Plane survey. The compact source, also detected by MAGIC, is believed to be a pulsar wind nebula powered by one of the most powerful pulsars known in the Galaxy, PSR J1813-1749 with a spin-down luminosity of $\dot{\mathrm{E}} = 5.6 \cdot 10^{37}\,\mathrm{erg}\,\mathrm{s}^{-1}$. With its extreme physical properties, as well as the pulsar's young age of 5.6 kyrs, the $\gamma$-rays detected in this region allow us to study the evolution of a highly atypical system. Previous studies of the region in the GeV energy range show emission extended beyond the size of the compact H.E.S.S. source. Using the archival H.E.S.S. data with improved background methods, we perform a detailed morphological and spectral analysis of the region. Additionally to the compact, bright emission component, we find significantly extended emission, whose position is coincident with HESS J1813-178. We reanalyse the region in GeV and derive a joint-model in order to find a continuous description of the emission in the region from GeV to TeV. Using the results derived in this analysis, as well as X-ray and radio data of the region, we perform multi-wavelength spectral modeling. Possible hadronic or leptonic origins of the $\gamma$-ray emission are investigated, and the diffusion parameters necessary to explain the extended emission are examined.

Abstract: Recent polarimetric mm-observations of the galactic centre by Wielgus et al. (2022a) showed sinusoidal loops in the Q-U plane with a duration of one hour. The loops coincide with a quasi-simultaneous X-ray flare. A promising mechanism to explain the flaring events are magnetic flux eruptions in magnetically arrested accretion flows (MAD). In our previous work (Porth et al. 2021), we studied the accretion flow dynamics during flux eruptions. Here, we extend our previous study by investigating whether polarization loops can be a signature produced by magnetic flux eruptions. We find that loops in the Q-U plane are robustly produced in MAD models as they lead to enhanced emissivity of compressed disk material due to orbiting flux bundles. A timing analysis of the synthetic polarized lightcurves demonstrate a polarized excess variability at timescales of ~ 1 hr. The polarization loops are also clearly imprinted on the cross-correlation of the Stokes parameters which allows to extract a typical periodicity of 30 min to 1 hr with some evidence for a spin dependence. These results are intrinsic to the MAD state and should thus hold for a wide range of astrophysical objects. A subset of GRMHD simulations without saturated magnetic flux (single temperature SANE models) also produces Q-U loops. However, in disagreement with the findings of Wielgus et al. (2022a), loops in these simulations are quasi-continuous with a low polarization excess

Abstract: Weakly interacting massive particles, as a major candidate of dark matter (DM), may directly annihilate or decay into high-energy photons, producing monochromatic spectral lines in the gamma-ray band. These spectral lines, if detected, are smoking-gun signatures for the existence of new physics. Using the 5 years of DAMPE and 13 years of Fermi-LAT data, we search for line-like signals in the energy range of 3 GeV to 1 TeV from the Galactic halo. Different regions of interest are considered to accommodate different DM density profiles. We do not find any significant line structure, and the previously reported line-like feature at $\sim$133 GeV is also not detected in our analysis. Adopting a local DM density of $\rho_{\rm local}=0.4\,{\rm GeV\,cm^{-3}}$, we derive 95% confidence level constraints on the velocity-averaged cross-section of $\langle{\sigma v}\rangle_{\gamma\gamma} \lesssim 4 \times 10^{-28}\,{\rm cm^{3}\,s^{-1}}$ and the decay lifetime of $\tau_{\gamma\nu} \gtrsim 5 \times 10^{29}\,{\rm s}$ at 100 GeV, achieving the strongest constraints to date for the line energies of 6-660 GeV. The improvement stems from the longer Fermi-LAT data set used and the inclusion of DAMPE data in the analysis. The simultaneous use of two independent data sets could also reduce the systematic uncertainty of the search.

Abstract: Neutron stars (NSs) which could contain exotic degrees of freedom in the core and the self-bound quark stars (QSs) made purely of absolutely stable deconfined quark matter are still two main candidates for the compact objects observed in pulsars and gravitational wave (GW) events in binary star mergers. We perform a Bayesian model-agnostic inference of the properties of NSs and QSs by combining multi-messenger data of GW170817, GW190425, PSR J0030+0451, PSR J0740+6620, PSR J1614-2230, PSR J0348+0432 as well as ab initio calculations from perturbative quantum chromodynamics and chiral effective field theory. We find the NS scenario is strongly favored against the QS scenario with a Bayes factor of NS over QS $\mathcal{B}^\text{NS}_\text{QS} = 11.5$. In addition, the peak of the squared sound velocity $c_s^2 \sim 0.5c^2$ around $3.5$ times nuclear saturation density $n_0$ observed in the NS case disappears in the QS case which suggests that the $c_s^2$ first increases and then saturates at $c_s^2 \sim 0.5c^2$ above $\sim 4n_0$. The sound velocity and trace anomaly are found to approach the conformal limit in the core of heavy NSs with mass $M \gtrsim 2M_{\odot}$, but not in the core of QSs.

Abstract: In kilonovae, freshly-synthesized $r$-process elements imprint features on optical spectra, as observed in AT2017gfo, the counterpart to the GW170817 binary neutron star merger. However, measuring the $r$-process compositions of the merger ejecta is computationally challenging. Vieira et al. (2023) introduced Spectroscopic $r$-Process Abundance Retrieval for Kilonovae (SPARK), a software tool to infer elemental abundance patterns of the ejecta, and associate spectral features with particular species. Previously, we applied SPARK to the 1.4 day spectrum of AT2017gfo and inferred its abundance pattern for the first time, characterized by electron fraction $Y_e=0.31$, a substantial abundance of strontium, and a dearth of lanthanides and heavier elements. This ejecta is consistent with wind from a remnant hypermassive neutron star and/or accretion disk. We now extend our inference to spectra at 2.4 and 3.4 days, and test the need for multi-component ejecta, where we stratify the ejecta in composition. The ejecta at 1.4 and 2.4 days is described by the same single blue component. At 3.4 days, a new redder component with lower $Y_e=0.16$ and a significant abundance of lanthanides emerges. This new redder component is consistent with dynamical ejecta and/or neutron-rich ejecta from a magnetized accretion disk. As expected from photometric modelling, this component emerges as the ejecta expands, the photosphere recedes, and the earlier bluer component dims. At 3.4 days, we find an ensemble of lanthanides, with the presence of cerium most concrete. This presence of lanthanides has important implications for the contribution of kilonovae to the $r$-process abundances observed in the Universe.

Abstract: The MMGPS-L is the most sensitive pulsar survey in the Southern Hemisphere. We present a follow-up study of one of these new discoveries, PSR J1208-5936, a 28.71-ms recycled pulsar in a double neutron star system with an orbital period of Pb=0.632 days and an eccentricity of e=0.348. Through timing of almost one year of observations, we detected the relativistic advance of periastron (0.918(1) deg/yr), resulting in a total system mass of Mt=2.586(5) Mo. We also achieved low-significance constraints on the amplitude of the Einstein delay and Shapiro delay, in turn yielding constraints on the pulsar mass (Mp=1.26(+0.13/-0.25) Mo), the companion mass (Mc=1.32(+0.25/-0.13) Mo, and the inclination angle (i=57(2) degrees). This system is highly eccentric compared to other Galactic field double neutron stars with similar periods, possibly hinting at a larger-than-usual supernova kick during the formation of the second-born neutron star. The binary will merge within 7.2(2) Gyr due to the emission of gravitational waves. With the improved sensitivity of the MMGPS-L, we updated the Milky Way neutron star merger rate to be 25(+19/-9) Myr$^{-1}$ within 90% credible intervals, which is lower than previous studies based on known Galactic binaries owing to the lack of further detections despite the highly sensitive nature of the survey. This implies a local cosmic neutron star merger rate of 293(+222/-103} Gpc/yr, consistent with LIGO and Virgo O3 observations. With this, we predict the observation of 10(+8/-4) neutron star merger events during the LIGO-Virgo-KAGRA O4 run. We predict the uncertainties on the component masses and the inclination angle will be reduced to 5x10$^{-3}$ Mo and 0.4 degrees after two decades of timing, and that in at least a decade from now the detection of the shift in Pb and the sky proper motion will serve to make an independent constraint of the distance to the system.