Galactic microquasars are binaries with a neutron star or stellar-mass black hole accreting gas from a companion star. They can accelerate particles to relativistic energies and emit radio flares, but the mechanism of jet energization is not known. A survey of the Cygnus region between mid-2007 and mid-2009 has detected four major γ-ray flaring episodes with energies over 100 MeV, each lasting 1 or 2 days, emanating from the microquasar Cygnus X-3. There is a clear pattern of temporal correlation between the γ-ray flares and transitional spectral states of radio and X-ray emission. Particle acceleration occurred a few days before radio jet ejections for two of the flares, suggesting that jet formation involves the production of highly energetic particles. Super-massive black holes in active galaxies can accelerate particles to relativistic energies, producing jets with associated γ-ray emission. Galactic 'microquasars' also produce relativistic jets; however, apart from an isolated event detected in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar. Here, a report of four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 illuminates this important problem. Super-massive black holes in active galaxies can accelerate particles to relativistic energies1, producing jets with associated γ-ray emission. Galactic ‘microquasars’, which are binary systems consisting of a neutron star or stellar-mass black hole accreting gas from a companion star, also produce relativistic jets, generally together with radio flares2. Apart from an isolated event detected3 in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar, with the consequence that we are as yet unsure about the mechanism of jet energization. Here we report four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 (an exceptional X-ray binary4,5,6 that sporadically produces radio jets7,8,9). There is a clear pattern of temporal correlations between the γ-ray flares and transitional spectral states of the radio-frequency and X-ray emission. Particle acceleration occurred a few days before radio-jet ejections for two of the four flares, meaning that the process of jet formation implies the production of very energetic particles. In Cygnus X-3, particle energies during the flares can be thousands of times higher than during quiescent states.

Extreme particle acceleration in the microquasar Cygnus X-3

Galactic bars are often suspected to be channels of gas inflow to the galactic center and to trigger central star formation and active galactic nucleus (AGN) activity. However, the current status on this issue based on empirical studies is unsettling, especially regarding AGNs. We investigate this question based on the Sloan Digital Sky Survey Data Release 7. From the nearby (0.01 < z < 0.05) bright (M r < –19) database, we have constructed a sample of 6658 relatively face-on late-type galaxies through visual inspection. We found 36% of them to have a bar. Bars are found to be more common in galaxies with earlier morphology. This makes sample selection critical. Parameter-based selections would miss a large fraction of barred galaxies of early morphology. Bar effects on star formation or AGNs are difficult to understand properly because multiple factors (bar frequency, stellar mass, black hole mass, gas contents, etc.) seem to contribute to them in intricate manners. In the hope of breaking these degeneracies, we inspect bar effects for fixed galaxy properties. Bar effects on central star formation seem higher in redder galaxies. Bar effects on AGNs on the other hand are higher in bluer and less massive galaxies. These effects seem more pronounced with increasing bar length. We discuss possible implications in terms of gas contents, bar strength, bar evolution, fueling timescale, and the dynamical role of supermassive black hole.

http://iopscience.iop.org/article/10.1088/0067-0049/198/1/4/pdf

Bar effects on central star formation and active galactic nucleus activity

Three-dimensional hydrodynamical simulations are used to investigate the structural stability of cooling flows that are episodically heated by jets from a central AGN. The radial profile of energy deposition is controlled by (a) the power of the jets, and (b) the pre-outburst density profile. A delay in the ignition of the jets causes more powerful jets to impact on a more centrally concentrated medium. The net effect is a sufficient increase in the central concentration of energy deposition to cause the post-outburst density profile to be less centrally concentrated than that of an identical cluster in which the outburst happened earlier and was weaker. These results suggest that the density profiles of cooling flows oscillate around an attracting profile, thus explaining why cooling flows are observed to have similar density profiles. The possibility is raised that powerful FR II systems are ones in which this feedback mechanism has broken down and a runaway growth of the source parameters has occurred.

Structural stability of cooling flows

We present a multi-wavelength study of the emission-line nebulae located ~38'' (3 kpc in projection) southeast of the nucleus of M87, the central dominant galaxy of the Virgo Cluster. We report the detection of far-infrared (FIR) [C II] line emission at 158 μm from the nebulae using observations made with the Herschel Photodetector Array Camera and Spectrometer (PACS). The infrared line emission is extended and co-spatial with optical Hα+ [N II], far-ultraviolet C IV lines, and soft X-ray emission. The filamentary nebulae evidently contain multi-phase material spanning a temperature range of at least five orders of magnitude, from ~100 K to ~107 K. This material has most likely been uplifted by the active galactic nucleus from the center of M87. The thermal pressure of the 104 K phase appears to be significantly lower than that of the surrounding hot intracluster medium (ICM), indicating the presence of additional turbulent and magnetic pressure in the filaments. If the turbulence in the filaments is subsonic then the magnetic field strength required to balance the pressure of the surrounding ICM is B ~ 30-70 μG. The spectral properties of the soft X-ray emission from the filaments indicate that it is due to thermal plasma with kT ~ 0.5-1 keV, which is cooling by mixing with the cold gas and/or radiatively. Charge exchange can be ruled out as a significant source of soft X-rays. Both cooling and mixing scenarios predict gas with a range of temperatures. This is at first glance inconsistent with the apparent lack of X-ray emitting gas with kT < 0.5 keV. However, we show that the missing very soft X-ray emission could be absorbed by the cold gas in the filaments with an integrated hydrogen column density of N H ~ 1.6 × 1021 cm–2, providing a natural explanation for the apparent temperature floor to the X-ray emission at kT ~ 0.5 keV. The FIR through ultraviolet line emission is most likely primarily powered by the ICM particles penetrating the cold gas following a shearing induced mixing process. An additional source of energy may, in principle, be provided by X-ray photoionization from cooling X-ray emitting plasma. The relatively small line ratio of [O I]/[C II] <7.2 indicates a large optical depth in the FIR lines. The large optical depth in the FIR lines and the intrinsic absorption inferred from the X-ray and optical data imply significant reservoirs of cold atomic and molecular gas distributed in filaments with small volume filling fraction, but large area covering factor.

http://iopscience.iop.org/article/10.1088/0004-637X/767/2/153/pdf

The nature of filamentary cold gas in the core of the virgo cluster

To take advantage of the astrophysical potential of Gamma-Ray Bursts (GRBs), Chinese and French astrophysicists have engaged the SVOM mission (Space-based multi-band astronomical Variable Objects Monitor) Major advances in GRB studies resulting from the synergy between space and ground observations, the SVOM mission implements space and ground instrumentation The scientific objectives of the mission put a special emphasis on two categories of GRBs: very distant GRBs at z$>$5 which constitute exceptional cosmological probes, and faint/soft nearby GRBs which allow probing the nature of the progenitors and the physics at work in the explosion These goals have a major impact on the design of the mission: the on-board hard X-ray imager is sensitive down to 4 keV and computes on line image and rate triggers, and the follow-up telescopes on the ground are sensitive in the NIR At the beginning of the next decade, SVOM will be the main provider of GRB positions and spectral parameters on very short time scale The SVOM instruments will operate simultaneously with a wide range of powerful astronomical devices This rare instrumental conjunction, combined with the relevance of the scientific topics connected with GRB studies, warrants a remarkable scientific return for SVOM In addition, the SVOM instrumentation, primarily designed for GRB studies, composes a unique multi-wavelength observatory with rapid slew capability that will find multiple applications for the whole astronomy community beyond the specific objectives linked to GRBs This report lists the scientific themes that will benefit from observations made with SVOM, whether they are specific GRB topics, or more generally all the issues that can take advantage of the multi-wavelength capabilities of SVOM

Peter Biermann

Papers

Active Galactic Nuclei