The center of the Galaxy in the recent past - A view from GRANAT

Sunyaev, R.; Markevitch, M.; Pavlinsky, M.

doi:10.1086/172542

Cited by 234 publications

(240 citation statements)

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“…At first glance, this seems plausible because shock waves in this active high-mass star-forming region should be able to maintain some molecular gas at temperatures of 500-700 K, or because the region contains luminous heating sources of X-rays and cosmic rays. Moreover, strong X-ray emission in iron fluorescence at 6.4 keV has been taken as evidence that the envelope of Sagittarius B2 was recently illuminated by an X-ray flash, attributable to a flare from the region around the central black hole that is associated with the Sagittarius A * radio continuum source (Sunyaev et al 1993;Koyama et al 1996). The Sagittarius B2 X-ray flux is now fading (Terrier et al 2010), with a characteristic decay time of eight years, comparable to the light-travel time across the central part of the molecular cloud.…”

Section: Discussionmentioning

confidence: 99%

Widespread Rotationally Hot Hydronium Ion in the Galactic Interstellar Medium

Lis

Schilke

Bergin

et al. 2014

ApJ

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We present new Herschel observations of the (6,6) and (9,9) inversion transitions of the hydronium ion toward Sagittarius B2(N) and W31C. Sensitive observations toward Sagittarius B2(N) show that the high, ∼500 K, rotational temperatures characterizing the population of the highly excited metastable H 3 O + rotational levels are present over a wide range of velocities corresponding to the Sagittarius B2 envelope, as well as the foreground gas clouds between the Sun and the source. Observations of the same lines toward W31C, a line of sight that does not intersect the Central Molecular Zone but instead traces quiescent gas in the Galactic disk, also imply a high rotational temperature of ∼380 K, well in excess of the kinetic temperature of the diffuse Galactic interstellar medium. While it is plausible that some fraction of the molecular gas may be heated to such high temperatures in the active environment of the Galactic center, characterized by high X-ray and cosmic-ray fluxes, shocks, and high degree of turbulence, this is unlikely in the largely quiescent environment of the Galactic disk clouds. We suggest instead that the highly excited states of the hydronium ion are populated mainly by exoergic chemical formation processes and the temperature describing the rotational level population does not represent the physical temperature of the medium. The same arguments may be applicable to other symmetric top rotors, such as ammonia. This offers a simple explanation of the long-standing puzzle of the presence of a pervasive, hot molecular gas component in the central region of the Milky Way. Moreover, our observations suggest that this is a universal process not limited to the active environments associated with galactic nuclei.

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Section: Discussionmentioning

confidence: 99%

Widespread Rotationally Hot Hydronium Ion in the Galactic Interstellar Medium

Lis

Schilke

Bergin

et al. 2014

ApJ

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“…In this case the emission is a combination of the emission from electrons injected during the strong flare, which occurred ∼300 yrs ago (see e.g. Sunyaev et al 1993;Koyama et al 1996Koyama et al , 2008 and during much weaker, recent activity. Electrons propagate through a medium with the density of about 10 3 cm −3 , through the soft photon field with density ∼5 × 10 4 eV/cm −3 at ∼0.3−3 eV (Davidson et al 1992;Mezger et al 1996;Hinton & Aharonian 2007) and magnetic field of order 10−1000 µG (Ferrière 2009;Eatough et al 2013).…”

Section: Leptonic Modelmentioning

confidence: 99%

Leptonic origin of the 100 MeVγ-ray emission from the Galactic centre

Malyshev

Chernyakova

Neronov

et al. 2015

A&A

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Context. The Galactic centre is a bright γ-ray source with the GeV−TeV band spectrum composed of two distinct components in the 1−10 GeV and 1−10 TeV energy ranges. The nature of these two components is not clearly understood. Aims. We investigate the γ-ray properties of the Galactic centre to clarify the origin of the observed emission. Methods. We report imaging, spectral, and timing analysis of data from 74 months of observations of the Galactic centre by Fermi/LAT γ-ray telescope complemented by sub-MeV data from approximately ten years of INTEGRAL/PICsIT observations. Results. We find that the Galactic centre is spatially consistent with the point source in the GeV band. The tightest 3σ upper limit on its radius is 0.13 • in the 10−300 GeV energy band. The spectrum of the source in the 100 MeV energy range does not have a characteristic turnover that would point to the pion decay origin of the signal. Instead, the source spectrum is consistent with a model of inverse Compton scattering by high-energy electrons. In this a model, the GeV bump in the spectrum originates from an episode of injection of high-energy particles, which happened ∼300 years ago. This injection episode coincides with the known activity episode of the Galactic centre region, previously identified using X-ray observations. The hadronic model of source activity could be still compatible with the data if bremsstrahlung emission from high-energy electrons was present in addition to pion decay emission.

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“…Being illuminated by powerful hard X-ray radiation from am outburst of the supermassive black hole at the Galactic Centre the neutral matter of molecular clouds can reflect and reprocess the radiation into the observed hard continuum and fluorescent 6.4 keV line emission (Sunyaev et al 1993, Sunyaev andChurazov 1998). Indeed, Revnivtsev et al (2004) associated the hard X-ray source IGR J17475-2822 discovered with the International Gamma-Ray Astrophysics Laboratory, INTEGRAL with the giant molecular cloud Sgr B2 in the Galactic Center vicinity.…”

Section: Galactic Center Clustersmentioning

confidence: 99%

Nonthermal particles and photons in starburst regions and superbubbles

Bykov

2014

Astron Astrophys Rev

118

102

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Starforming factories in galaxies produce compact clusters and loose associations of young massive stars. Fast radiation-driven winds and supernovae input their huge kinetic power into the interstellar medium in the form of highly supersonic and superalfvenic outflows. Apart from gas heating, collisionless relaxation of fast plasma outflows results in fluctuating magnetic fields and energetic particles. The energetic particles comprise a long-lived component which may contain a sizeable fraction of the kinetic energy released by the winds and supernova ejecta and thus modify the magnetohydrodynamic flows in the systems. We present a concise review of observational data and models of nonthermal emission from starburst galaxies, superbubbles, and compact clusters of massive stars. Efficient mechanisms of particle acceleration and amplification of fluctuating magnetic fields with a wide dynamical range in starburst regions are discussed. Sources of cosmic rays, neutrinos and multi-wavelength nonthermal emission associated with starburst regions including potential galactic "PeVatrons" are reviewed in the global galactic ecology context.

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The center of the Galaxy in the recent past - A view from GRANAT

Cited by 234 publications

References 0 publications

Widespread Rotationally Hot Hydronium Ion in the Galactic Interstellar Medium

Widespread Rotationally Hot Hydronium Ion in the Galactic Interstellar Medium

Leptonic origin of the 100 MeVγ-ray emission from the Galactic centre

Nonthermal particles and photons in starburst regions and superbubbles

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