High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: The IceCube-Gen2 Neutrino Observatory is proposed to extend the all-flavour energy range of IceCube beyond PeV energies. It will comprise two key components: I) An enlarged 8$\,$km$^3$ in-ice optical Cherenkov array to measure the continuation of the IceCube astrophysical neutrino flux and improve IceCube's point source sensitivity above $\sim\,$100$\,$TeV; and II) A very large in-ice radio array with a surface area of about 500$\,$km$^2$. Radio waves propagate through ice with a kilometer-long attenuation length, hence a sparse radio array allows us to instrument a huge volume of ice to achieve a sufficient sensitivity to detect neutrinos with energies above tens of PeV. The different signal topologies for neutrino-induced events measured by the optical and in-ice radio detector - the radio detector is mostly sensitive to the cascades produced in the neutrino interaction, while the optical detector can detect long-ranging muon and tau leptons with high accuracy - yield highly complementary information. When detected in coincidence, these signals will allow us to reconstruct the neutrino energy and arrival direction with high fidelity. Furthermore, if events are detected in coincidence with a sufficient rate, they resemble the unique opportunity to study systematic uncertainties and to cross-calibrate both detector components. We present the expected rate of coincidence events for 10 years of operation. Furthermore, we analyzed possible detector optimizations to increase the coincidence rate.

Abstract: Observations from past missions such as RXTE and Beppo-SAX suggested the presence of inverse Compton (IC) scattering at hard X-ray energies within the intracluster medium of some massive galaxy clusters. In subsequent years, observations by, e.g., Suzaku, and now NuSTAR, have not been able to confirm these detections. We report on NuSTAR hard X-ray searches for IC emission in two massive galaxy clusters, Abell 665 and Abell 2146. To constrain the global IC flux in these two clusters, we fit global NuSTAR spectra with three models: single (1T) and two-temperature (2T) models, and a 1T plus power law component (T$+$IC). The temperature components are meant to characterize the thermal ICM emission, while the power law represents the IC emission. We find that the 3-30 keV Abell 665 and 3-20 keV Abell 2146 spectra are best described by thermal emission alone, with average global temperatures of $kT = (9.15\pm 0.1)$ keV for Abell 665 and $kT = (8.29\pm 0.1)$ keV for Abell 2146. We constrain the IC flux to $F_{\rm NT} < 0.60 \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$ and $F_{\rm NT} < 0.85 \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$ (20-80 keV) for Abell 665 and Abell 2146, respectively both at the 90% confidence level. When we couple the IC flux limits with 1.4 GHz diffuse radio data from the VLA, we set lower limits on the average magnetic field strengths of $>$0.14 $\mu$G and $>$0.011 $\mu$G for Abell 665 and Abell 2146, respectively.

Abstract: The explosion processes of supernovae (SNe) are imprinted in their explosion geometries. Here, we study the intrinsic polarization of 15 hydrogen-rich core-collapse SNe and explore the relation with the photometric and spectroscopic properties. Our sample shows diverse properties of the continuum polarization. The polarization of most SNe has a low degree at early phases but shows a sudden rise to $\sim 1$ \% degree at certain points during the photospheric phase as well as a slow decline during the tail phase, with a constant polarization angle. The variation in the timing of peak polarisation values implies diversity in the explosion geometry: some SNe have aspherical structures only in their helium cores, while in other SNe these reach out to a significant part of the outer hydrogen envelope with a common axis from the helium core to the hydrogen envelope. Other SNe show high polarization from early phases and a change of the polarization angle around the middle of the photospheric phase. This implies that the ejecta are significantly aspherical to the outermost layer and have multi-directional aspherical structures. Exceptionally, the Type~IIL SN~2017ahn shows low polarization at both the photospheric and tail phases. Our results show that the timing of the polarization rise in Type~IIP SNe is likely correlated with their brightness, velocity and the amount of radioactive Ni produced: brighter SNe with faster ejecta velocity and a larger $^{56}$Ni mass have more extended-aspherical explosion geometries. In particular, there is a clear correlation between the timing of the polarization rise and the explosion energy, that is, the explosion asphericity is proportional to the explosion energy. This implies that the development of a global aspherical structure, e.g., a jet, might be the key to realising an energetic SN in the mechanism of SN explosions.

Abstract: Cygnus X-3 is a high-mass X-ray binary with a compact object accreting matter from a Wolf-Rayet donor star. Recently, it has been revealed by the Imaging X-ray Polarimetry Explorer (IXPE) as a hidden Galactic ultra-luminous X-ray (ULX) source with a luminosity above the Eddington limit along the direction of a narrow (opening angle <~32 degree) funnel. In between the IXPE observations, we observed Cyg X-3 with the European VLBI (very long baseline interferometry) Network at 22 GHz and the NICER X-ray instrument. To probe possible relations between the X-ray funnel and the potential radio jet from the ULX, we analyzed the simultaneous multi-wavelength data. Our high-resolution VLBI image reveals an elongated structure with a position angle of -3.2+/-0.4 degree, accurately perpendicular to the direction of the linear X-ray polarization. Because Cyg X-3 was in the radio quiescent state on 2022 November 10, we identify the mas-scale structure as the innermost radio jet. The finding indicates that the radio jet propagates along and within the funnel. Moreover, the jet is marginally resolved in the transverse direction. This possibly results from the strong stellar winds and the rapid orbital motion of the binary system.

Abstract: The IceCube realtime alert system has been operating since 2016. It provides prompt alerts on high-energy neutrino events to the astroparticle physics community. The localization regions for the incoming direction of neutrinos are published through NASA's Gamma-ray Coordinate Network (GCN). The IceCube realtime system consists of infrastructure dedicated to the selection of alert events, the reconstruction of their topology and arrival direction, the calculation of directional uncertainty contours and the distribution of the event information through public alert networks. Using a message-based workflow management system, a dedicated software (SkyDriver) provides a representational state transfer (REST) interface to parallelized reconstruction algorithms. In this contribution, we outline the improvements of the internal infrastructure of the IceCube realtime system that aims to streamline the internal handling of neutrino events, their distribution to the SkyDriver interface, the collection of the reconstruction results as well as their conversion into human- and machine-readable alerts to be publicly distributed through different alert networks. An approach for the long-term storage and cataloging of alert events according to findability, accessibility, interoperability and reusability (FAIR) principles is outlined.

Abstract: A prime motivation for compiling catalogues of any celestial X-ray source is to increase our numbers of rare sub-classes. In this work we take a recent multi-mission catalogue of ultraluminous X-ray sources (ULXs) and look for hitherto poorly-studied ULX candidates that are luminous ($L_{\rm X} \geq 10^{40} \rm ~erg~s^{-1}$), bright ($f_{\rm X} \geq 5 \times 10^{-13} \rm ~erg~cm~s^{-1}$) and have archival XMM-Newton data. We speculate that this luminosity regime may be ideal for identifying new pulsating ULXs (PULXs), given that the majority of known PULXs reach similar high luminosities. We find three sources that match our criteria, and study them using archival data. We find 4XMM J165251.5-591503 to possess a bright and variable Galactic optical/IR counterpart, and so conclude it is very likely to be a foreground interloper. 4XMM J091948.8-121429 does appear an excellent ULX candidate associated with the dwarf irregular galaxy PGC 26378, but has only one detection to date with low data quality. The best dataset belongs to 4XMM J112054.3+531040 which we find to be a moderately variable, spectrally hard ($\Gamma \approx 1.4$) X-ray source located in a spiral arm of NGC 3631. Its spectral hardness is similar to known PULXs, but no pulsations are detected by accelerated pulsation searches in the available data. We discuss whether other missions provide objects for similar studies, and compare this method to others suggested for identifying good PULX candidates.

Abstract: The sources of the astrophysical neutrino flux discovered by IceCube are for the most part unresolved. Extragalactic core-collapse supernovae (CCSNe) have been suggested as candidate multi-messenger sources. In interaction-powered supernovae, a shock propagates in a dense circumstellar medium (CSM), producing a bright optical emission and potentially accelerating particles to relativistic energies. Shock interaction is believed to be the main energy source for Type IIn supernovae (identified by narrow lines in the spectrum), hydrogen-rich superluminous supernovae and a subset of hydrogen-poor superluminous supernovae. Production of high-energy neutrinos is expected in collisions between the accelerated protons in the shocks and the cold CSM particles. We select a catalog of interaction-powered supernovae from the Bright Transient Survey of the Zwicky Transient Facility. We exploit a novel modeling effort that connects the time evolution of the optical emission to the properties of the ejecta and the CSM, allowing us to set predictions of the neutrino flux for each source. In this contribution, we describe a stacking search for high-energy neutrinos from this population of CCSNe with the IceCube Neutrino Observatory.

Abstract: In the light of growing evidence that blazars are responsible for part of the astrophysical very-high-energy neutrino flux detected by IceCube, models for neutrino production through photo-pion interactions in blazar jets have been developed. Evidence is also mounting that photon fields originating external to the jet are strongly favored over the co-moving primary electron synchrotron photon field as target for photo-pion interactions. Even though those external photon fields appear highly anisotropic in the co-moving frame of the emission region, current models usually consider neutrino production to occur isotropically in the co-moving frame, resulting in a beaming pattern that is identical to intrinsically isotropic synchrotron and synchrotron self-Compton emission. In this paper, we derive the resulting beaming patterns of neutrinos produced by interactions wich external photon fields, taking into account all relevant anisotropy effects. It is shown that neutrino emission resulting from photo-pion production on a stationary and isotropic (in the AGN rest frame) external photon field is significantly more strongly beamed along the jet direction than intrinsically isotropic emission. For the most highly beamed sources, this implies that expected neutrino fluxes are grossly under-estimated or jet-power requirements for the production of a given neutrino flux grossly over-estimated when not accounting for the proper Doppler boosting and beaming characteristics.

Abstract: The IceCube Neutrino Observatory has been continuously taking data to search for O(0.5-10) s long neutrino bursts since 2007. Even if a Galactic core-collapse supernova is optically obscured or collapses to a black hole instead of exploding, it will be detectable via the O(10) MeV neutrino burst emitted during the collapse. We discuss a search for such events covering the time between April 17, 2008 and December 31, 2019. Considering the average data taking and analysis uptime of 91.7% after all selection cuts, this is equivalent to 10.735 years of continuous data taking. In order to test the most conservative neutrino production scenario, the selection cuts were optimized for a model based on a 8.8 solar mass progenitor collapsing to an O-Ne-Mg core. Conservative assumptions on the effects of neutrino oscillations in the exploding star were made. The final selection cut was set to ensure that the probability to detect such a supernova within the Milky Way exceeds 99%. No such neutrino burst was found in the data after performing a blind analysis. Hence, a 90% C.L. upper limit on the rate of core-collapse supernovae out to distances of ~ 25kpc was determined to be 0.23/yr. For the more distant Magellanic Clouds, only high neutrino luminosity supernovae will be detectable by IceCube, unless external information on the burst time is available. We determined a model-independent limit by parameterizing the dependence on the neutrino luminosity and the energy spectrum.

Abstract: Recent observations with the Imaging X-ray Polarimetry Explorer (IXPE) of two anomalous X-ray pulsars provided evidence that X-ray emission from magnetar sources is strongly polarized. Here we report on the joint IXPE and XMM-Newton observations of the soft {\gamma}-repeater SGR 1806-20. The spectral and timing properties of SGR 1806-20 derived from XMM-Newton data are in broad agreement with previous measurements; however, we found the source at an all-time-low persistent flux level. No significant polarization was measured apart from the 4-5 keV energy range, where a probable detection with PD=31.6\pm 10.5% and PA=-17.6\pm 15 deg was obtained. The resulting polarization signal, together with the upper limits we derive at lower and higher energies 2-4 and 5-8 keV, respectively) is compatible with a picture in which thermal radiation from the condensed star surface is reprocessed by resonant Compton scattering in the magnetosphere, similar to what proposed for the bright magnetar 4U 0142+61.

Abstract: The properties of selfinteracting boson stars with different scalar potentials going beyond the commonly used $\phi^4$ ansatz are studied. The scalar potential is extended to different values of the exponent $n$ of the form $V \propto \phi^n$. Two stability mechanism for boson stars are introduced, the first being a mass term and the second one a vacuum term. We present analytic scale-invariant expressions for these two classes of equations of state. The resulting properties of the boson star configurations differ considerably from previous calculations. We find three different categories of mass-radius relation: the first category resembles the mass-radius curve of selfbound stars, the second one those of neutron stars and the third one is the well known constant radius case from the standard $\phi^4$ potential. We demonstrate that the maximal compactness can reach extremely high values going to the limit of causality $C_\text{max} = 0.354$ asymptotically for $n\to\infty$. The maximal compactnesses exceed previously calculated values of $C_\text{max}=0.16$ for the standard $\phi^4$-theory and $C_\text{max}=0.21$ for vector-like interactions and is in line with previous results for solitonic boson stars. Hence, boson stars even described by a simple modified scalar potential in the form of $V \propto \phi^n$ can be ultra compact black hole mimickers where the photon ring is located outside the radius of the star.