High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Here, we present the results of our multi-wavelength campaign aimed at classifying \textit{Swift} J170800$-$402551.8 as part of the \textit{Swift} Deep Galactic Plane Survey (DGPS). We utilized Target of Opportunity (ToO) observations with \textit{Swift}, \textit{NICER}, \textit{XMM-Newton}, \textit{NuSTAR}, and the Southern African Large Telescope (SALT), as well as multi-wavelength archival observations from \textit{Gaia}, VPHAS, and VVV. The source displays a periodicity of 784 s in our \textit{XMM-Newton} observation. The X-ray spectrum (\textit{XMM-Newton} and \textit{NuSTAR}) can be described by thermal bremsstrahlung radiation with a temperature of $kT$\,$\approx$\,$30$ keV. The phase-folded X-ray lightcurve displays a double-peaked, energy-dependent pulse-profile. We used \textit{Chandra} to precisely localize the source, allowing us to identify and study the multi-wavelength counterpart. Spectroscopy with SALT identified a Balmer H$\alpha$ line, and potential HeI lines, from the optical counterpart. The faintness of the counterpart ($r$\,$\approx$\,$21$ AB mag) favors a low-mass donor star. Based on these criteria, we classify \textit{Swift} J170800$-$402551.8 as a candidate intermediate polar cataclysmic variable, where the spin period of the white dwarf is 784 s.

Abstract: The IceCube Neutrino Observatory is equipped with the unique possibility to measure cosmic ray induced air showers simultaneously by their particle footprint on the surface with the IceTop detector and by the high-energy muonic shower component at a depth of more than 1.5 km. Since 2019 additionally two Imaging Air Cherenkov Telescopes, called IceAct, measure the electromagnetic component of air showers in the atmosphere above the IceCube detector. This opens the possibility to measure air shower parameters in three independent detectors and allows to improve mass composition studies with the IceCube data. One IceAct camera consists of 61 SiPM pixels in a hexagonal grid. Each pixel has a field of view of 1.5 degree resulting in an approximately 12-degree field of view per camera. A single telescope tube has a diameter of 50 cm, is built robust enough to withstand the harsh Antarctic conditions, and is able to detect cosmic ray particles with energies above approximately 10 TeV. A Graph Neural Network (GNN) is trained to determine the air shower properties from IceAct data. The composition analysis is then performed using Random Forest Regression (RF). Since all three detectors have a different energy threshold, we train several RFs with different inputs, combining the different detectors and taking advantage of the lower energy threshold of the IceAct telescopes. This will result in composition measurements for different detector combinations and enables cross-checks of the results in overlapping energy bands. We present the method, parameters for data selection, and the status of this analysis.

Abstract: IceAct is an array of compact Imaging Air Cherenkov Telescopes at the ice surface as part of the IceCube Neutrino Observatory. The telescopes, featuring a camera of 61 silicon photomultipliers and fresnel-lens-based optics, are optimized to be operated in harsh environmental conditions, such as at the South Pole. Since 2019, the first two telescopes have been operating in a stereoscopic configuration in the center of IceCube's surface detector IceTop. With an energy threshold of about 10 TeV and a wide field-of-view, the IceAct telescopes show promising capabilities of improving current cosmic-ray composition studies: measuring the Cherenkov light emissions in the atmosphere adds new information about the shower development not accessible with the current detectors. First simulations indicate that the added information of a single telescope leads, e.g., to an improved discrimination between flux contributions from different primary particle species in the sensitive energy range. We review the performance and detector operations of the telescopes during the past 3 years (2020-2022) and give an outlook on the future of IceAct.

Abstract: The IceCube-Gen2 facility will extend the energy range of IceCube to ultra-high energies. The key component to detect neutrinos with energies above 10 PeV is a large array of in-ice radio detectors. In previous work, direction reconstruction algorithms using the forward-folding technique have been developed for both shallow ($\lesssim 20$ m) and deep in-ice detectors, and have also been successfully used to reconstruct cosmic rays with ARIANNA. Here, we focus on the reconstruction algorithm for the deep in-ice detector, which was recently introduced in the context of the Radio Neutrino Observatory in Greenland (RNO-G). We discuss the performance-critical aspects of the algorithm, as well as recent and future improvements, and apply it to study the performance of a station of the IceCube-Gen2 in-ice radio array. We obtain the angular resolution, which turns out to be strongly asymmetric, and use this to optimize the configuration of a single station.

Abstract: Understanding the spectral evolution along the `Z'-shaped track in the hardness-intensity diagram of Z-sources, which are a class of luminous neutron star low-mass X-ray binaries, is crucial to probe accretion processes close to the neutron star. Here, we study the horizontal branch (HB) and the normal branch (NB) of the Z source GX 340+0 using $AstroSat$ data. We find that the HB and the NB appear as two different types of X-ray intensity dips, which can appear in any sequence and with various depths. Our $0.8-25$~keV spectra of dips and the hard apex can be modeled by the emissions from an accretion disk, a Comptonizing corona covering the inner disk, and the neutron star surface. We find, as the source moves onto the HB the corona is replenished and energized by the disk and a reduced amount of disk matter reaches the neutron star surface. We also conclude that quasi-periodic oscillations during HB/NB are strongly associated with the corona, and explain the evolution of strength and hard-lag of this timing feature using the estimated coronal optical depth evolution.

Abstract: Powerful relativistic jets are one of the ubiquitous features of accreting black holes in all scales. GRS 1915+105 is a well-known fast-spinning black-hole X-ray binary with a relativistic jet, termed as a ``microquasar'', as indicated by its superluminal motion of radio emission. It exhibits persistent x-ray activity over the last 30 years, with quasi-periodic oscillations of $\sim 1-10$ Hz and 34 and 67 Hz in the x-ray band. These oscillations likely originate in the inner accretion disk, but other origins have been considered. Radio observations found variable light curves with quasi-periodic flares or oscillations with periods of $\sim 20-50$ minutes. Here we report two instances of $\sim$5 Hz transient periodic oscillation features from the source detected in the 1.05-1.45 GHz radio band that occurred in January 2021 and June 2022, respectively. Circular polarization was also observed during the oscillation phase.

Abstract: Sources of astrophysical neutrinos can potentially be discovered through the detection of neutrinos in coincidence with electromagnetic counterparts. Real-time alerts generated by IceCube play an important role in this search, acting as triggers for follow-up observations with instruments sensitive to electromagnetic signals in various wavelengths. In previous studies, we investigated the treatment of the systematic uncertainties on the reconstruction method currently used in IceCube's real-time program, concluding that a new approach, more robust against systematic variations, is needed. Here we present the state-of-the-art of these analyses, and discuss a modification to an already-existing and reliable reconstruction method that results in an improved solution under many metrics. The proposed reconstruction method is faster, more precise, and significantly less influenced by systematic uncertainties, than the current one. This system provides a more robust estimate of angular uncertainties than the previous algorithm, making it a solid benchmark for real-time event analyses.

Abstract: Recent LHAASO observations of the prompt emission phase of the brightest-of-all-time GRB 221009A imposes a stringent limit on the flux ratio between the TeV and MeV emissions, $F_{\rm TeV}/F_{\rm MeV}\le 2\times10^{-5}$. Within the framework of internal shocks, we study the internal $\gamma\gamma$ absorption in GRB 221009A by generating a set of synthetic bursts in a simulation that reproduces the observed feature of GRB 221009A. We find that the $\gamma\gamma$ absorption does not lead to an exponential cutoff, but rather a power-law spectrum, consistent with previous works. We further find that the attenuation due to $\gamma\gamma$ absorption alone cannot explain the flux limit ratio of GRB 221009A, suggesting a low ratio of synchrotron self-Compton (SSC) to synchrotron emission outputs. This requires that the magnetic field energy density is much larger than the synchrotron photon energy density so that the SSC flux is greatly suppressed. This indicates that the jet composition of GRB 221009A is likely Poynting-flux-dominated.

Abstract: Changing-look active galactic nuclei (CL-AGN) have been observed to change optical spectral type. Mrk 1018 is unique: first classified as a type 1.9 Seyfert galaxy, it transitioned to a type 1 before returning to its initial classification after approximately 30 years. We present a high-cadence monitoring programme that caught a major outburst in 2020. Due to sunblock, only the decline could be observed. We studied X-ray, UV, optical, and IR before and after the outburst to investigate the responses of the AGN structures. We derived a u'-band light curve of the AGN contribution alone. The flux increased by a factor of the order of 13. We confirmed this in other optical bands and determined the shape and speed of the decline in each waveband. The shapes of H beta and H alpha were analysed before and after the event. Two XMM-Newton observations from before and after the outburst were also exploited. The outburst is asymmetric, with a swifter rise than decline. The decline is best fit by a linear function, ruling out a tidal disruption event. The optical spectrum shows no change approximately 8 months before and 17 months after. The UV flux increased slightly after the outburst but the X-ray primary flux is unchanged. However, the 6.4 keV Iron line has doubled in strength. IR data taken 13 days after the observed optical peak show an increased emission level. Calculating the distance of the broad-line region and inner edge of the torus from the supermassive black hole can explain the multi-wavelength response to the outburst, in particular: i) the unchanged H beta and H alpha lines, ii) the unchanged primary X-ray spectral components, iii) the rapid and extended infrared response, as well as iv) the enhanced emission of the reflected 6.4 keV line. The outburst was due to a dramatic and short-lasting change in the intrinsic accretion rate. We discuss different models as potential causes.

Abstract: We study breaking properties of a solid neutron star crust. We consider the case in which the crust at any fixed density consists of two types ions, forming a strongly ordered Coulomb crystal. It is shown that the breaking stress of a such matter noticeably depends on ionic composition, and it is typically larger than for a one-component crystal. The difference may reach a factor of several.

Abstract: The Large High Altitude Air Shower Observatory~(LHAASO) reported observation of photons with energies above 10~TeV from gamma ray burst GRB221009A. A suggestion was proposed that this result may contradict with our knowledge of special relativity~(SR) and the standard model~(SM), according to which photons of about 10~TeV from such a distant object should be severely suppressed because of the absorption by extragalactic background light. As a result, a number of mechanisms have been proposed to solve this potential puzzle, including Lorentz invariance violation~(LIV). In this work, we perform a detailed numerical calculation and show the feasibility to constrain LIV of photons from the LHAASO observation of GRB221009A quantitatively.

Abstract: The population of radio pulsars is observed to demonstrate certain polarization properties not explained by the conventional picture of pulsar polarization, namely frequency evolution of polarization, deviations of the linear polarization angle from a curve of geometric origins and the presence of features in the circular polarization. We present the partial-coherence model as a way to explain the co-occurrence of these features and to provide an origin for circular polarization in radio pulsar profiles. We describe the mathematics of the model and demonstrate how it can explain these observed features, both on a population level and for the idiosyncrasies of individual pulsars. The partial coherence model can account for complex polarization behaviour, enabling improved access to information about pulsar geometries. We discuss the scientific implications of this for our understanding of pulsar radio emission and propagation.