HESS J1745$-$290 spectrum explained by a transition in the diffusion regime of PeV cosmic rays in the Sgr A* accretion flow (original) (raw)
Related papers
Diffusive cosmic-ray acceleration at the Galactic Centre
Monthly Notices of the Royal Astronomical Society: Letters, 2011
The diffuse TeV emission detected from the inner ∼ 2 • of the Galaxy appears to be strongly correlated with the distribution of molecular gas along the Galactic ridge.
High-Energy Activity in the Unusually Soft TeV Source HESS J1804-216 toward the Galactic Center
arXiv (Cornell University), 2006
In recent years, apparent anisotropies in the EeV cosmic ray (CR) flux arriving at Earth from the general direction of the galactic center have been reported from the analysis of AGASA and SUGAR data. The more recently commissioned Auger Observatory has not confirmed these results. HESS has now detected an unusually soft TeV source roughly coincident with the location of the previously claimed CR anisotropy. In this paper, we develop a model for the TeV emission from this object, consistent with observations at other wavelengths, and examine the circumstances under which it might have contributed to the ∼ EeV cosmic ray spectrum. We find that the supernova remnant G8.7-0.1 can plausibly account for all the known radiative characteristics of HESS J1804-216, but that it can accelerate cosmic rays only up to an energy ∼ 10 5 GeV. On the other hand, the pulsar (PSR J1803-2137) embedded within this remnant can in principle inject EeV protons into the surrounding medium, but it cannot account for the broadband spectrum of HESS J1804-216. We therefore conclude that although G8.7-0.1 is probably the source of TeV photons originating from this direction, there is no compelling theoretical motivation for expecting a cosmic ray anisotropy at this location. However, if G8.7-0.1 is indeed correctly identified with HESS J1804-216, it should also produce a ∼ GeV flux detectable in a one-year all sky survey by GLAST.
Diffuse TeV emission at the Galactic Centre
Monthly Notices of the Royal Astronomical Society, 2008
The High-Energy Stereoscopic System (HESS) has detected intense diffuse TeV emission correlated with the distribution of molecular gas along the galactic ridge at the centre of our Galaxy. Earlier HESS observations of this region had already revealed the presence of several point sources at these energies, one of them (HESS J1745-290) coincident with the supermassive black hole Sagittarius A*. It is still not entirely clear what the origin of the TeV emission is, nor even whether it is due to hadronic or leptonic interactions. It is reasonable to suppose, however, that at least for the diffuse emission, the tight correlation of the intensity distribution with the molecular gas indicates a pionic-decay process involving relativistic protons. In this paper, we explore the possible source(s) of energetic hadrons at the galactic centre, and their propagation through a turbulent medium. We conclude that though Sagittarius A* itself may be the source of cosmic rays producing the emission in HESS J1745-290, it cannot be responsible for the diffuse emission farther out. A distribution of point sources, such as pulsar wind nebulae dispersed along the galactic plane, similarly do not produce a TeV emission profile consistent with the HESS map. We conclude that only a relativistic proton distribution accelerated throughout the intercloud medium can account for the TeV emission profile measured with HESS.
The High-Energy, Arcminute-Scale Galactic Center Gamma-Ray Source
The Astrophysical Journal, 2011
Employing data collected during the first 25 months' observations by the Fermi -LAT, we describe and subsequently seek to model the very high energy (> 300 MeV) emission from the central few parsecs of our Galaxy. We analyse, in particular, the morphological, spectral and temporal characteristics of the central source, 1FGL J1745.6-2900. Remarkably, the data show a clear, statistically significant signal at energies above 10 GeV, where the Fermi -LAT has an excellent angular resolution comparable to the angular resolution of HESS at TeV energies. This not only reduces dramatically the contamination both from the diffuse background and the nearby gamma-ray sources, but also makes meaningful the joint analysis of the Fermi and HESS data. Our analysis does not show statistically significant variability of 1FGL J1745.6-2900. Using the combination of Fermi data on 1FGL J1745.6-2900 and HESS data on the coincident, TeV source HESS J1745-290, we show that the spectrum of the central γ-ray source is inflected with a relatively steep spectral region matching between the flatter spectrum found at both low and high energies. We seek to model the gamma-ray production in the inner 10 pc of the Galaxy and examine, in particular, cosmic ray (CR) proton propagation scenarios that reproduce the observed spectrum of the central source. We show that a model that instantiates a transition from diffusive propagation of the CR protons at low energy to almost rectilinear propagation at high energies (given a reasonable energy-dependence of the assumed diffusion coefficient) can well explain the spectral phenomenology. In general, however, we find considerable degeneracy between different parameter choices which will only be broken with the addition of morphological information that γ-ray telescopes cannot deliver given current angular resolution limits. We argue that a future analysis done in combination with higherresolution radio continuum data holds out the promise of breaking this degeneracy.
An Accretion-Induced X-ray Flare in Sgr A*
2001
The recent detection of a three-hour X-ray flare from Sgr A* by Chandra provides very strong evidence for a compact emitting region near this supermassive black hole at the Galactic center. Sgr A*'s mm/sub-mm spectrum and polarimetric properties, and its quiescent-state X-ray flux density, are consistent with a model in which low angular momentum gas captured at large radii circularizes to form a hot, magnetized Keplerian flow within tens of Schwarzschild radii of the black hole's event horizon. In Sgr A*'s quiescent state, the X-ray emission appears to be produced by self-Comptonization (SSC) of the mm/sub-mm synchrotron photons emitted in this region. In this paper, we show that the prominent X-ray flare seen in Sgr A* may be due to a sudden enhancement of accretion through the circularized flow. Depending on whether the associated response of the anomalous viscosity is to increase or decrease in tandem with this additional injection of mass, the X-ray photons during the outburst may be produced either via thermal bremsstrahlung (if the viscosity decreases), or via SSC (if the viscosity increases). However, the latter predicts a softer X-ray spectrum than was seen by Chandra, so it appears that a bremsstrahlung origin for the X-ray outburst is favored. A strong correlation is expected between the mm/sub-mm and X-ray fluxes when the flare X-rays are produced by SSC, while the correlated variability is strongest between the sub-mm/far-IR and X-rays when bremsstrahlung emission is dominant during the flare. In addition, we shows that future coordinated multi-wavelength observations planned for the 2002 and 2003 cycles may be able to distinguish between the accretion and jet scenarios.
The Astrophysical Journal, 2019
The cosmic-ray (CR) accelerator at the Galactic center (GC) is not yet established by current observations. Here we investigate the radiative-inefficient accretion flow (RIAF) of Sagittarius A * (SgrA *) as a CR accelerator assuming acceleration by turbulent magnetic reconnection, and derive possible emission fluxes of CRs interacting within the RIAF (the central ∼10 13 cm). The target environment of the RIAF is modeled with numerical, general relativistic magnetohydrodynamics together with leptonic radiative transfer simulations. The acceleration of the CRs is not computed here. Instead, we inject CRs constrained by the magnetic reconnection power of the accretion flow and compute the emission/absorption of γ-rays due to these CRs interacting with the RIAF, through Monte Carlo simulations employing the CRPropa 3 code. The resulting very-high-energy (VHE) fluxes are not expected to reproduce the point source HESS J1745-290 as the emission of this source is most likely produced at parsec scales. The emission profiles derived here intend to trace the VHE signatures of the RIAF as a CR accelerator and provide predictions for observations of the GC with improved angular resolution and differential flux sensitivity as those of the forthcoming Cerenkov Telescope Array (CTA). Within the scenario presented here, we find that for mass accretion rates 10 −7 M e yr −1 , the RIAF of SgrA * produces VHE fluxes that are consistent with the High Energy Stereoscopic System (H.E.S.S.) upper limits for the GC and potentially observable by the future CTA. The associated neutrino fluxes are negligible compared with the diffuse neutrino emission measured by the IceCube.
On the mechanism for breaks in the cosmic ray spectrum
Physics of Plasmas, 2012
The proof of cosmic ray (CR) origin in supernova remnants (SNR) must hinge on full consistency of the CR acceleration theory with the observations; direct proof is impossible because of the orbit stochasticity of CR particles. Recent observations of a number of galactic SNR strongly support the SNR-CR connection in general and the Fermi mechanism of CR acceleration, in particular. However, many SNR expand into weakly ionized dense gases, and so a significant revision of the mechanism is required to fit the data. We argue that strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by exactly one power. The spectral break is caused by a partial evanescence of Alfven waves that confine particles to the accelerator. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is also calculated. Using the recent Fermi spacecraft observation of the SNR W44 as an example, we demonstrate that the parent proton spectrum is a classical test particle power law / E À2 , steepening to E À3 at E br % 7
High-energy spectrum of the Galactic center black hole Sgr A
Advances in Space Research, 1997
The massive blackhole candidate Sgr A* may be accreting from an ambient Galactic center wind at a rate x 1O22 g s-l. Dissipational processes within the large-scale quasi-spherical infall, from 50 rg to lo5 rg (where rg = 3 x 1O1l cm is the Schwarzschild radius for M = 10" Ma) can account well for the observed radio spectrum and flux (due to cyclotron/synchrotron emission) and its Xray/Gamma-ray luminosity (apparently due to Bremsstrahlung emission). Small scale instabilities associated with the stagnation region of the flow produce fluctuations in the accretion rate with an amplitude of up to 30% and a time scale of several months to over a year. This may account for the long term variability of the high-energy (and radio) luminosity observed from Sgr A*. Although the average accreted angular momentum is approximately zero, these instabilities also induce fluctuations in the specific angular momentum of the accreted gas that lead to a circularized flow at distances less than about 20 rg. Optically thick emission from this "disk" (roughly the size of Mercury's orbit) is the origin of the IR flux recently detected from the location of Sgr A*, but is not itself a significant source of high-energy emission.
2011
Recent observations of the supernova remnant W44 by the Fermi spacecraft observatory strongly support the idea that the bulk of galactic cosmic rays is accelerated in such remnants by a Fermi mechanism, also known as diffusive shock acceleration. However, the W44 expands into weakly ionized dense gas, and so a significant revision of the mechanism is required. In this paper we provide the necessary modifications and demonstrate that strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by exactly one power. The spectral break is caused by Alfven wave evanescence leading to the fractional particle losses. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is also calculated and successfully fitted to the Fermi Observatory data. The parent proton spectrum is best represented by a classical test particle power law ∝ E −2 , steepening to E −3 at E br ≈ 7GeV due to deteriorated particle confinement.
Pevatron at the Galactic Center: multi-wavelength signatures from millisecond pulsars
Journal of Cosmology and Astroparticle Physics
Diffuse TeV emission has been observed by H.E.S.S. in the Galactic Center region, in addition to the GeV gamma rays observed by Fermi. We propose that a population of unresolved millisecond pulsars located around the Galactic Center, suggested as possible candidates for the diffuse Galactic Center excess observed by Fermi, accelerate cosmic rays up to very high energies, and are thus also responsible for the TeV excess. We model analytically the diffusion of these accelerated protons and their interaction with the molecular clouds, producing gamma rays. The spatial and spectral dependences of the gamma rays produced can reproduce the H.E.S.S. observations, for a population of ∼ 10 4 − 10 5 millisecond pulsars above the cosmic-ray luminosity 10 34 erg s −1 , with moderate acceleration efficiency. More precise measurements at the highest energies would allow us to constrain the properties of the pulsar population, such as the magnetic field or initial spin distributions.