Core-collapse (CC) supernova remnants (SNRs) are the nebular leftovers of defunct massive stars that died during a supernova explosion, mostly while undergoing the red supergiant phase of their ...evolution. The morphology and emission properties of those remnants are a function of the distribution of circumstellar material at the moment of the supernova, as well as the intrinsic properties of the explosion and those of the ambient medium. By means of 2.5-dimensional (2.5D) numerical magneto-hydrodynamic (MHD) simulations, we modelled the long-term evolution of SNRs generated by runaway rotating massive stars moving into a magnetised interstellar medium (ISM). Radiative transfer calculations reveal that the projected non-thermal emission of SNRs decreases over time, namely: older remnants are fainter than younger ones. Older ($80\ kyr$) SNRs whose progenitors were moving with a space velocity corresponding to a Mach number of $M=1$ ($v_ km\ $) in the Galactic plane of the interstellar medium ($n_ ISM $) are brighter in synchrotron than when moving with a Mach number of $M=2$ ($v_ km\ $). We show that runaway red supergiant progenitors first induce an asymmetric non-thermal $1.4\ GHz$ barrel-like synchrotron SNRs (at the age of about $8\ kyr$), before further evolving to adopt a Cygnus-loop-like shape (at about $80\ kyr$). It is conjectured that a significative fraction of SNRs are currently in this bilateral-to-Cygnus loop evolutionary sequence. Therefore, this population should be taken into account with repect to interpreting the data as part of the forthcoming Cherenkov Telescope Array (CTA) observatory.
Recurrent novae are repeating thermonuclear explosions in the outer layers of white dwarfs, due to the accretion of fresh material from a binary companion. The shock generated when ejected material ...slams into the companion star's wind can accelerate particles. We report very-high-energy (VHE; Formula: see text) gamma rays from the recurrent nova RS Ophiuchi, up to 1 month after its 2021 outburst, observed using the High Energy Stereoscopic System (H.E.S.S.). The temporal profile of VHE emission is similar to that of lower-energy giga-electron volt emission, indicating a common origin, with a 2-day delay in peak flux. These observations constrain models of time-dependent particle energization, favoring a hadronic emission scenario over the leptonic alternative. Shocks in dense winds provide favorable environments for efficient acceleration of cosmic rays to very high energies.
Abstract
We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2.°2 away from the best-fit position of the IceCube neutrino event IceCube-211208A ...detected on 2021 December 8. The source was in a high-flux state in the optical, ultraviolet, X-ray, and GeV
γ
-ray bands around the time of the neutrino event, exhibiting daily variability in the soft X-ray flux. The X-ray data from Swift-XRT and NuSTAR characterize the transition between the low-energy and high-energy components of the broadband spectral energy distribution (SED), and the
γ
-ray data from Fermi-LAT, VERITAS, and H.E.S.S. require a spectral cutoff near 100 GeV. Both the X-ray and
γ
-ray measurements provide strong constraints on the leptonic and hadronic models. We analytically explore a synchrotron self-Compton model, an external Compton model, and a lepto-hadronic model. Models that are entirely based on internal photon fields face serious difficulties in matching the observed SED. The existence of an external photon field in the source would instead explain the observed
γ
-ray spectral cutoff in both the leptonic and lepto-hadronic models and allow a proton jet power that marginally agrees with the Eddington limit in the lepto-hadronic model. We show a numerical lepto-hadronic model with external target photons that reproduces the observed SED and is reasonably consistent with the neutrino event despite requiring a high jet power.
Abstract
Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the ...universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron−positron pairs, which can subsequently initiate electromagnetic cascades. The gamma-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of
B
> 7.1 × 10
−16
G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of 10
4
(10
7
) yr, IGMF strengths below 1.8 × 10
−14
G (3.9 × 10
−14
G) are excluded, which rules out specific models for IGMF generation in the early universe.
Abstract
GRB 221009A is the brightest gamma-ray burst (GRB) ever detected. To probe the very-high-energy (VHE; >100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 ...hr after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of
Φ
UL
95
%
=
9.7
×
10
−
12
erg
cm
−
2
s
−
1
above
E
thr
= 650 GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the spectral energy distribution occurring above the X-ray band. Compared to the VHE-bright GRB 190829A, the upper limits for GRB 221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB 221009A, effectively ruling out an IC-dominated scenario.
Abstract
In 2021 July,
PKS 1510−089
exhibited a significant flux drop in the high-energy
γ
-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. ...Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy
γ
-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy
γ
-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy
γ
-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line of sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images.
Core-collapse supernova remnants are the nebular leftover of defunct massive stars which have died during a supernova explosion, mostly while undergoing the red supergiant phase of their evolution. ...The morphology and emission properties of those remnants are a function of the distribution of circumstellar material at the moment of the supernova, the intrisic properties of the explosion, as well as those of the ambient medium. By means of 2.5 dimensional numerical magnetohydrodynamics simulations, we model the long term evolution of supernova remnants generated by runaway rotating massive stars moving into a magnetised interstellar medium. Radiative transfer calculations reveal that the projected non-thermal emission of the supernova remnants decreases with time, i.e. older remnants are fainter than younger ones. Older (80 kyr) supernova remnants whose progenitors were moving with space velocity corresponding to a Mach number M = 1 (v_star = 20 km/s ) in the Galactic plane of the ISM (nISM = 1/cm3 ) are brighter in synchrotron than when moving with a Mach number M = 2 (v_star = 40 km/s ). We show that runaway red supergiant progenitors first induce an asymmetric non thermal 1.4 GHz barrel like synchrotron supernova remnants (at the age of about 8 kyr), before further evolving to adopt a Cygnus loop like shape (at about 80 kyr). It is conjectured that a significative fraction of supernova remnants are currently in this bilateral-to-Cygnus-loop evolutionary sequence, and that this should be taken into account in the data interpretation of the forthcoming Cherenkov Telescope Array (CTA) observatory.
PSR B1259-63 is a gamma-ray binary system that hosts a pulsar in an eccentric orbit, with a 3.4 year period, around an O9.5Ve star. At orbital phases close to periastron passages, the system radiates ...bright and variable non-thermal emission. We report on an extensive VHE observation campaign conducted with the High Energy Stereoscopic System, comprised of ~100 hours of data taken from \(t_p-24\) days to \(t_p+127\) days around the system's 2021 periastron passage. We also present the timing and spectral analyses of the source. The VHE light curve in 2021 is consistent with the stacked light curve of all previous observations. Within the light curve, we report a VHE maximum at times coincident with the third X-ray peak first detected in the 2021 X-ray light curve. In the light curve -- although sparsely sampled in this time period -- we see no VHE enhancement during the second disc crossing. In addition, we see no correspondence to the 2021 GeV flare in the VHE light curve. The VHE spectrum obtained from the analysis of the 2021 dataset is best described by a power law of spectral index \(\Gamma = 2.65 \pm 0.04_{\text{stat}}\) \(\pm 0.04_{\text{sys}}\), a value consistent with the previous H.E.S.S. observations of the source. We report spectral variability with a difference of \(\Delta \Gamma = 0.56 ~\pm~ 0.18_{\text{stat}}\) \(~\pm~0.10_{\text{sys}}\) at 95% c.l., between sub-periods of the 2021 dataset. We also find a linear correlation between contemporaneous flux values of X-ray and TeV datasets, detected mainly after \(t_p+25\) days, suggesting a change in the available energy for non-thermal radiation processes. We detect no significant correlation between GeV and TeV flux points, within the uncertainties of the measurements, from \(\sim t_p-23\) days to \(\sim t_p+126\) days. This suggests that the GeV and TeV emission originate from different electron populations.
The radio galaxy M87 is a variable very-high energy (VHE) gamma-ray source, exhibiting three major flares reported in 2005, 2008, and 2010. Despite extensive studies, the origin of the VHE gamma-ray ...emission is yet to be understood. In this study, we investigate the VHE gamma-ray spectrum of M87 during states of high gamma-ray activity, utilizing 20.2\(\,\) hours the H.E.S.S. observations. Our findings indicate a preference for a curved spectrum, characterized by a log-parabola model with extra-galactic background light (EBL) model above 0.3\(\,\)TeV at the 4\(\sigma\) level, compared to a power-law spectrum with EBL. We investigate the degeneracy between the absorption feature and the EBL normalization and derive upper limits on EBL models mainly sensitive in the wavelength range 12.4$\,$$\mu\(m - 40\)\,$$\mu$m.
The Crab Nebula is a unique laboratory for studying the acceleration of electrons and positrons through their non-thermal radiation. Observations of very-high-energy \(\gamma\) rays from the Crab ...Nebula have provided important constraints for modelling its broadband emission. We present the first fully self-consistent analysis of the Crab Nebula's \(\gamma\)-ray emission between 1 GeV and \(\sim\)100 TeV, that is, over five orders of magnitude in energy. Using the open-source software package Gammapy, we combined 11.4 yr of data from the Fermi Large Area Telescope and 80 h of High Energy Stereoscopic System (H.E.S.S.) data at the event level and provide a measurement of the spatial extension of the nebula and its energy spectrum. We find evidence for a shrinking of the nebula with increasing \(\gamma\)-ray energy. Furthermore, we fitted several phenomenological models to the measured data, finding that none of them can fully describe the spatial extension and the spectral energy distribution at the same time. Especially the extension measured at TeV energies appears too large when compared to the X-ray emission. Our measurements probe the structure of the magnetic field between the pulsar wind termination shock and the dust torus, and we conclude that the magnetic field strength decreases with increasing distance from the pulsar. We complement our study with a careful assessment of systematic uncertainties.
