The distribution of the very-high-energy diffuse emission in the inner 200 pc measured by H.E.S.S. indicates the existence of a pronounced cosmic-ray (CR) gradient peaking on the Galactic center ...(GC). Previous studies have shown that these data are consistent with a scenario in which the CRs are diffused away from a stationary source at the GC. We previously showed that, taking the 3D gas distribution and a realistic distribution of supernova explosions into account, CRs accelerated in supernova remnants should account for a large fraction of the GC CRs observed by H.E.S.S.; but the model did not fully reproduce the apparent over-density in the inner 30 pc. In this work, we study the time-energy dependent cosmic rays escape from the remnant that is expected to occur when the shock wave decelerates in the surrounding medium. We show that the resulting CR distribution follows the quasi-stationary profile observed by H.E.S.S. more closely. The main signature is that the energy-dependent escape creates a strong dependency of the morphology of the
γ
-ray emission with the energy. The existence of this energy dependency should be observable by the Cherenkov Telescope Array.
The analysis and combination of data from different gamma-ray instruments involves the use of collaboration proprietary software and case-by-case methods. The effort of defining a common data format ...for high-level data, namely event lists and instrument response functions (IRFs), has recently started for very-high-energy gamma-ray instruments, driven by the upcoming Cherenkov Telescope Array (CTA). In this work we implemented this prototypical data format for a small set of MAGIC, VERITAS, FACT, and H.E.S.S. Crab nebula observations, and we analyzed them with the open-source gammapy software package. By combining data from Fermi-LAT, and from four of the currently operating imaging atmospheric Cherenkov telescopes, we produced a joint maximum likelihood fit of the Crab nebula spectrum. Aspects of the statistical errors and the evaluation of systematic uncertainty are also commented upon, along with the release format of spectral measurements. The results presented in this work are obtained using open-access on-line assets that allow for a long-term reproducibility of the results.
The center of our Galaxy hosts a Super-Massive Black Hole (SMBH) of about 4 × 106 M⊙. Since it has been argued that the SMBH might accelerate particles up to very high energies, its current and past ...activity could contribute to the population of Galactic cosmic-rays (CRs). Additionally, the condition in the Galactic Center (GC) are often compared with the one of a starburst system. The high supernovae (SN) rate associated with the strong massive star formation in the region must create a sustained CR injection in the GC via the shocks produced at the time of their explosion. The presence of an excess of very high energy (VHE) cosmic rays in the inner 100 pc of the Galaxy in close correlation with the massive gas complex known as the central molecular zone (CMZ) has been revealed in 2006 by the H.E.S.S. collaboration. Recently, by analysing 10 years of H.E.S.S. data, the H.E.S.S. collaboration confirmed the presence of this extended VHE diffuse emission and deduced a CR density peaked toward the GC. The origin of the CR over-abundance in the GC still remains mysterious: Is it due to a single accelerator at the center or to multiple accelerators filling the region? In order to investigate the presence of these multiple CR accelerators, and in particular the impact of their spatial distribution on the VHE emission morphology, we build a 3D model of CR injection and diffusive propagation with a realistic 3D gas distribution. We discuss the CR injection in the region by a spectral and morphological comparison with H.E.S.S. data. We show that a peaked γ-ray profile towards the GC center is obtained using a realistic SN spatial distribution taking into account the central massive star clusters. The contribution of theses sources cannot be neglected in particular at high longitudes. In order to fit the very central excess observed with H.E.S.S., another central VHE component is probably necessary.
Abstract
The High Energy Stereoscopic System (H.E.S.S.) collaboration revealed the presence of a very high energy (VHE) diffuse emission in the inner 100 pc of the Galaxy in close correlation with ...the central molecular zone (CMZ). Recently, they deduced from this emission a cosmic ray (CR) overdensity in the region with a local peak towards the Galactic Centre (GC) and proposed a diffusive scenario with a stationary source at the GC to explain it. However, the high supernovae (SN) rate in the GC must also create a sustained CR injection in the region via the shocks produced at the time of their explosion. Considering a typical diffusion coefficient close to the interstellar medium (ISM) value yields a diffuse escape time much lower than the recurrence time between each SN explosion, showing that a steady-state model will fail to reproduce the data. This work aims to study the impact of the spatial and temporal distribution of SNs in the CMZ on the VHE emission morphology and spectrum: we build a 3D model of VHE CR injection and diffusive propagation with a realistic gas distribution. We show that a peaked γ-ray profile towards the GC can be obtained using realistic SN spatial distribution taking into account the central massive star cluster. We conclude that the contribution of SNs cannot be neglected, in particular at large longitudes; however, an additional CR injection at the GC is required to reproduce the very central excess.
ABSTRACT
We report here on the first multiwavelength (MWL) campaign on the blazar TXS 1515–273, undertaken in 2019 and extending from radio to very-high-energy gamma-rays (VHE). Up until now, this ...blazar had not been the subject of any detailed MWL observations. It has a rather hard photon index at GeV energies and was considered a candidate extreme high-synchrotron-peaked source. MAGIC observations resulted in the first-time detection of the source in VHE with a statistical significance of 7.6σ. The average integral VHE flux of the source is 6 ± 1 per cent of the Crab nebula flux above 400 GeV. X-ray coverage was provided by Swift-XRT, XMM–Newton, and NuSTAR. The long continuous X-ray observations were separated by ∼9 h, both showing clear hour scale flares. In the XMM–Newton data, both the rise and decay time-scales are longer in the soft X-ray than in the hard X-ray band, indicating the presence of a particle cooling regime. The X-ray variability time-scales were used to constrain the size of the emission region and the strength of the magnetic field. The data allowed us to determine the synchrotron peak frequency and classify the source as a flaring high, but not extreme synchrotron-peaked object. Considering the constraints and variability patterns from the X-ray data, we model the broad-band spectral energy distribution. We applied a simple one-zone model, which could not reproduce the radio emission and the shape of the optical emission, and a two-component leptonic model with two interacting components, enabling us to reproduce the emission from radio to VHE band.
ABSTRACT
PKS 1830–211 is a known macrolensed quasar located at a redshift of z = 2.5. Its high-energy gamma-ray emission has been detected with the Fermi-Large Area Telescope (LAT) instrument and ...evidence for lensing was obtained by several authors from its high-energy data. Observations of PKS 1830–211 were taken with the High Energy Stereoscopic System (H.E.S.S.) array of Imaging Atmospheric Cherenkov Telescopes in 2014 August, following a flare alert by the Fermi-LAT Collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20–27 d from the alert flare, as expected from observations at other wavelengths. More than 12 h of good-quality data were taken with an analysis threshold of ∼67 GeV. The significance of a potential signal is computed as a function of the date and the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT. No photon excess or significant signal is detected. An upper limit on PKS 1830–211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.
Using the High Energy Spectroscopic System (H.E.S.S.) telescopes we have discovered a steady and extended very high-energy (VHE) γ-ray source towards the luminous blue variable candidate LBV 1806−20, ...massive stellar cluster Cl* 1806−20, and magnetar SGR 1806−20. The new VHE source, HESS J1808−204, was detected at a statistical significance of >6σ (post-trial) with a photon flux normalisation (2.9 ± 0.4stat ± 0.5sys) × 10−13 ph cm−2 s−1 TeV−1 at 1 TeV and a power-law photon index of 2.3 ± 0.2stat ± 0.3sys. The luminosity of this source (0.2 to 10 TeV; scaled to distance d = 8.7 kpc) is LVHE ~ 1.6 × 1034(d/8.7 kpc)2 erg s−1. The VHE γ-ray emission is extended and is well fit by a single Gaussian with statistical standard deviation of 0.095° ± 0.015°. This extension is similar to that of the synchrotron radio nebula G10.0−0.3, which is thought to be powered by LBV 1806−20. The VHE γ-ray luminosity could be provided by the stellar wind luminosity of LBV 1806−20 by itself and/or the massive star members of Cl* 1806−20. Alternatively, magnetic dissipation (e.g. via reconnection) from SGR 1806−20 can potentially account for the VHE luminosity. The origin and hadronic and/or leptonic nature of the accelerated particles responsible for HESS J1808−204 is not yet clear. If associated with SGR 1806−20, the potentially young age of the magnetar (650 yr) can be used to infer the transport limits of these particles to match the VHE source size. This discovery provides new interest in the potential for high-energy particle acceleration from magnetars, massive stars, and/or stellar clusters.
The distribution of the very-high-energy diffuse emission in the inner 200 pc measured by HE.S.S. indicates the existence of a pronounced cosmic-ray (CR) gradient peaking on the Galactic center (GC). ...Previous studies have shown that these data are consistent with a scenario in which the CRs are diffused away from a stationary source at the GC. We previously showed that, taking the 3D gas distribution and a realistic distribution of supernova explosions into account, CRs accelerated in supernova remnants (SNR) should account for a large fraction of the GC CRs observed by H.E.S.S.; but the model did not fully reproduce the apparent over-density in the inner 30 pc. In this work, we study the time-energy dependent cosmic rays escape from the remnant that is expected to occur when the shock wave decelerates in the surrounding medium. We show that the resulting CR distribution follows the quasi-stationary profile observed by H.E.S.S. more closely. The main signature is that the energy-dependent escape creates a strong dependency of the morphology of the gamma-ray emission with the energy. The existence of this energy dependency should be observable by the Cherenkov Telescope Array.
The H.E.S.S. collaboration revealed the presence of a very high energy (VHE) diffuse emission in the inner 100pc of the Galaxy in close correlation with the Central Molecular Zone (CMZ). Recently, ...they deduced from this emission a cosmic-ray (CR) over-density in the region with a local peak toward the Galactic Center (GC) and proposed a diffusive scenario with a stationary source at the GC to explain it. However, the high supernovae (SN) rate in the GC must also create a sustained CR injection in the region via the shocks produced at the time of their explosion. Considering typical diffusion coefficient close the interstellar medium (ISM) value yields to a diffuse escape time much lower than the recurrence time between each SN explosion, showing that a steady state model will fail to reproduce the data. This work aims to study the impact of the spatial and temporal distribution of SNs in the CMZ on the VHE emission morphology and spectrum: we build a 3D model of VHE CR injection and diffusive propagation with a realistic gas distribution. We show that a peaked gamma-ray profile towards the GC can be obtained using realistic SN spatial distribution taking into account the central massive star cluster. We conclude that the contribution of SNs can not be neglected, in particular at large longitudes, however an additional CR injection at the GC is required to reproduce the very central excess.
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