Context. The supermassive black hole named Sgr A* is located at the dynamical center of the Milky Way. This closest supermassive black hole is known to have a luminosity several orders of magnitude ...lower than the Eddington luminosity. Flares coming from the Sgr A* environment can be observed in infrared, X-ray, and submillimeter wavelengths, but their origins are still debated. Interestingly, the close passage of the Dusty S-cluster Object (DSO)/G2 near Sgr A* may increase the black hole flaring activity and could therefore help us to better constrain the radiation mechanisms from Sgr A*. Aims. Our aim is to study the X-ray, infrared, and radio flaring activity of Sgr A* close to the time of the DSO/G2 pericenter passage in order to constrain the physical properties and origin of the flares. Methods. Simultaneous observations were made with XMM-Newton and WFC3 onboard HST during the period Feb.–Apr. 2014, in addition to coordinated observations with SINFONI at ESO’s VLT, VLA in its A-configuration, and CARMA. Results. We detected two X-ray flares on 2014 Mar. 10 and Apr. 2 with XMM-Newton, three near-infrared (NIR) flares with HST on 2014 Mar. 10 and Apr. 2, and two NIR flares on 2014 Apr. 3 and 4 with VLT. The X-ray flare on 2014 Mar. 10 is characterized by a long rise (~7700 s) and a rapid decay (~844 s). Its total duration is one of the longest detected so far in X-rays. Its NIR counterpart peaked well before (4320 s) the X-ray maximum, implying a dramatic change in the X-ray-to-NIR flux ratio during this event. This NIR/X-ray flare is interpreted as either a single flare where variation in the X-ray-to-NIR flux ratio is explained by the adiabatic compression of a plasmon, or two distinct flaring components separated by 1.2 h with simultaneous peaks in X-rays and NIR. We identified an increase in the rising radio flux density at 13.37 GHz on 2014 Mar. 10 with the VLA that could be the delayed radio emission from a NIR/X-ray flare that occurred before the start of our observation. The X-ray flare on 2014 Apr. 2 occurred for HST during the occultation of Sgr A* by the Earth, therefore we only observed the start of its NIR counterpart. With NIR synchrotron emission from accelerated electrons and assuming X-rays from synchrotron self-Compton emission, the region of this NIR/X-ray flare has a size of 0.03−7 times the Schwarzschild radius and an electron density of 108.5–1010.2 cm-3, assuming a synchrotron spectral index of 0.3−1.5. When Sgr A* reappeared to the HST view, we observed the decay phase of a distinct bright NIR flare with no detectable counterpart in X-rays. On 2014 Apr. 3, two 3.2-mm flares were observed with CARMA, where the first may be the delayed (4.4 h) emission of a NIR flare observed with VLT. Conclusions. We observed a total of seven NIR flares, with three having a detected X-ray counterpart. The physical parameters of the flaring region are less constrained for the NIR flare without a detected X-ray counterpart, but none of the possible radiative processes (synchrotron, synchrotron self-Compton, or inverse Compton) can be ruled out for the production of the X-ray flares. The three X-ray flares were observed during the XMM-Newton total effective exposure of ~256 ks. This flaring rate is statistically consistent with those observed during the 2012 Chandra XVP campaign, implying that no increase in the flaring activity was triggered close to the pericenter passage of the DSO/G2. Moreover, higher flaring rates had already been observed with Chandra and XMM-Newton without any increase in the quiescent level, showing that there is no direct link between an increase in the flaring rate in X-rays and the change in the accretion rate.
We describe the results of a radio continuum survey of the central image with the 100 m Green Bank Telescope (GBT) at wavelengths of 3.5, 6, 20, and 90 cm. The 3.5 and 6 cm surveys are the most ...sensitive and highest resolution single-dish surveys made of the central degrees of our Galaxy. We present catalogs of compact and extended sources in the central 4 degree of our Galaxy, including detailed spectral index studies of all sources. The analysis covers star-forming regions such as Sgr B and Sgr C, where we find evidence of a mixture of thermal and nonthermal emission. The analysis quantifies the relative contribution of thermal and nonthermal processes to the radio continuum flux density toward the Galactic center (GC) region. In the central image of the GC, the thermal and nonthermal flux fractions for all compact and diffuse sources are 28%/72% at 3.5 cm and 19%/81% at 6 cm. The total flux densities from these sources are image and image Jy at 3.5 and 6 cm, respectively, excluding the contribution of Galactic synchrotron emission.
Aims. The radiative counterpart of the supermassive black hole at the Galactic center (GC), Sgr A⋆, is subject to frequent flares that are visible simultaneously in X-rays and the near-infrared ...(NIR). Often, enhanced radio variability from centimeter to sub-millimeter wavelengths is observed to follow these X-ray/NIR eruptions. We present here a multi-wavelength campaign carried out in April 2009, with the aim of characterizing this broadband flaring activity. Methods. Concurrent data from the XMM-Newton/EPIC (2–10 keV), VLT/NACO (2.1 μm, 3.8 μm), APEX/LABOCA (870 μm), and Fermi/LAT (0.1–200 GeV) instruments are employed to derive light curves and spectral energy distributions of new flares from Sgr A⋆. Results. We detected two relatively bright NIR flares, both associated with weak X-ray activity, one of which was followed by a strong sub-mm outburst ~200 min later. Photometric spectral information on a NIR flare was obtained for the first time with NACO, giving a power-law photon index α = −0.4 ± 0.3 (Fν ∝ ν α). The first attempt to detect flaring activity from the Fermi GC source 1FGL J1745.6–2900 is also reported. We model NIR, X-ray, and sub-mm flares in the context of non-thermal emission processes. We find that the simplest scenario involving a single expanding plasmoid releasing synchrotron NIR/sub-mm and synchrotron self-Compton X-ray radiation is inadequate to reproduce the data, but we offer suggestions to reconcile the basic elements of the theory and the observations.
ALMA observations of the Galactic center with a spatial resolution of 2".61 x 0".97 resulted in the detection of 11 SiO (5-4) clumps of molecular gas within 0.6 pc (15") of Sgr A*, interior to the 2 ...pc circumnuclear molecular ring. The three SiO (5-4) clumps closest to Sgr A* show the largest central velocities, ~150 km s super(-1), and the broadest asymmetric line widths with full width zero intensity (FWZI) ~110-147 km s super(-1). The remaining clumps, distributed mainly to the NE of the ionized mini-spiral, have narrow FWZI (~18-56 km s super(-1)). Using CARMA SiO (2-1) data, Large Velocity Gradient modeling of the SiO line ratios for the broad velocity clumps constrains the column density N(SiO) ~ 10 super(14) cm super(-2), and the H sub(2) gas density n sub(H2) = (3-9) x 10 super(5) cm super(-3) for an assumed kinetic temperature 100-200 K. The SiO clumps are interpreted as highly embedded protostellar outflows, signifying an early stage of massive star formation near Sgr A* in the last 10 super(4)-10 super(5) yr. Support for this interpretation is provided by the SiO (5-4) line luminosities and velocity widths which lie in the range measured for protostellar outflows in star-forming regions in the Galaxy. Furthermore, spectral energy distribution modeling of stellar sources shows two young stellar object candidates near SiO clumps, supporting in situ star formation near Sgr A*. We discuss the nature of star formation where the gravitational potential of the black hole dominates. In particular, we suggest that external radiative pressure exerted on self-shielded molecular clouds enhances the gas density, before the gas cloud becomes gravitationally unstable near Sgr A*. Alternatively, collisions between clumps in the ring may trigger gravitational collapse.
Understanding the processes occurring in the nuclear disk of our Galaxy is interesting in its own right, as part of the Milky Way, but also because it is the closest galactic nucleus. It has been ...more than two decades since the general phenomenon of higher gas temperature in the inner few hundred parsecs by comparison with local clouds in the disk of the Galaxy was recognized. This is one of the least understood characteristics of giant molecular clouds having a much higher gas temperature than dust temperature in the inner few degrees of the Galactic center. We propose that an enhanced flux of cosmic-ray electrons, as evidenced recently by a number of studies, is responsible for directly heating the gas clouds in the nuclear disk, elevating the temperature of molecular gas ( similar to 75 K) above the dust temperature ( similar to 20 K). In addition, we report the detection of nonthermal radio emission from Sgr B2 F based on low-frequency GMRT and VLA observations. The higher ionization fraction and thermal energy due to the impact of nonthermal electrons in star-forming sites have important implications in slowing down star formation in the nuclear disk of our Galaxy and nuclei of galaxies.
Sgr A* is embedded within the nuclear cluster, which consists of a mixture of evolved and young populations of stars dominating the light over a wide range of angular scales. Here we present Hubble ...Space Telescope/NICMOS data to study the surface brightness distribution of stellar light within the inner 10'' of Sgr A* at 1.45 Delta *mm, 1.7 Delta *mm, and 1.9 Delta *mm. We use these data to independently examine the surface brightness distribution that had been measured previously with NICMOS and to determine whether there is a drop in the surface density of stars very near Sgr A*. Our analysis confirms that a previously reported drop in the surface brightness within 08 of Sgr A* is an artifact of bright and massive stars near that radius. We also show that the surface brightness profile within 5'' or ~0.2 pc of Sgr A* can be fitted with broken power laws. The power laws are consistent with previous measurements, in that the profile becomes shallower at small radii. For radii >07, the slope is Delta *b = --0.34 ? 0.04, where Delta *S is r Delta *b and becomes flatter at smaller radii with Delta *b = --0.13 ? 0.04. Modeling of the surface brightness profile gives a stellar density that increases roughly as r --1 within the inner 1'' of Sgr A*. This slope confirms earlier measurements in that it is not consistent with that expected from an old, dynamically relaxed stellar cluster with a central supermassive black hole. Assuming that the diffuse emission is not contaminated by a faint population of young stars down to the 17.1 mag limit of our imaging data at 1.70 Delta *mm, the shallow cusp profile is not consistent with a decline in stellar density in the inner arcsecond. In addition, converting our measured diffuse light profile to a stellar mass profile, with the assumption that the light is dominated by K0 dwarfs, the enclosed stellar mass within radius r 0.1 pc of Sgr A* is 3.2 X 104 M (r/0.1 pc)2.1.
This paper reports measurements of Sgr A* made with NACO in L' band (3.80 is a subset of m), Ks band (2.12 is a subset of m), and H band (1.66 is a subset of m), and with VISIR in N band (11.88 is a ...subset of m) at the ESO VLT, as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 2007 April 4, a very bright flare was observed from Sgr A* simultaneously at L' band and X-ray wavelengths. No emission was detected using VISIR. The resulting spectral energy distribution has a blue slope ( beta >0 for Delta L Delta Delta beta , consistent with Delta L Delta Delta 0.4) between 12 is a subset of m and 3.8 is a subset of m. For the first time, our high-quality data allow a detailed comparison of infrared (IR) and X-ray light curves with a resolution of a few minutes. The IR and X-ray flares are simultaneous to within 3 minutes. However, the IR flare lasts significantly longer than the X-ray flare (both before and after the X-ray peak), and prominent substructures in the 3.8 is a subset of m light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L'-band light curve, we find that the flaring region must be no more than 1.2RS in size. The high X-ray to IR flux ratio, blue Delta L Delta slope MIR to L' band, and the soft Delta L Delta spectral index of the X-ray flare together place strong constraints on possible flare emission mechanisms. We find that it is quantitatively difficult to explain this bright X-ray flare with inverse Compton processes. A synchrotron emission scenario from an electron distribution with a cooling break is a more viable scenario.
We present an observation with the Chandra X-Ray Observatory of the unusual radio source G359.23-0.82 ("the Mouse"), along with updated radio timing data from the Parkes radio telescope on the ...coincident young pulsar J1747-2958. We find that G359.23-0.82 is a very luminous X-ray source L sub(X)(0.5-8.0 keV) = 5 x 10 super(34) ergs s super(-1) for a distance of 5 kpc whose morphology consists of a bright head coincident with PSR J1747-2958 plus a 45" long narrow tail whose power-law spectrum steepens with distance from the pulsar. We thus confirm that G359.23-0.82 is a bow shock pulsar wind nebula powered by PSR J1747-2958; the nebular standoff distance implies that the pulsar is moving with a Mach number of similar to 60, suggesting a space velocity approximately 600 km s super(-1) through gas of density approximately 0.3 cm super(-3). We combine the theory of ion-dominated pulsar winds with hydrodynamic simulations of pulsar bow shocks to show that a bright elongated X-ray and radio feature extending 10" behind the pulsar represents the surface of the wind termination shock. The X-ray and radio "trails" seen in other pulsar bow shocks may similarly represent the surface of the termination shock, rather than particles in the postshock flow as is usually argued. The tail of the Mouse contains two components: a relatively broad region seen only at radio wavelengths, and a narrow region seen in both radio and X-rays. We propose that the former represents material flowing from the wind shock ahead of the pulsar's motion, while the latter corresponds to more weakly magnetized material streaming from the backward termination shock. This study represents the first consistent attempt to apply our understanding of "Crab-like" nebulae to the growing group of bow shocks around high-velocity pulsars.
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