G2 can Illuminate the Black Hole Population near the Galactic Center. (arXiv:1302.3220v1 [astro-ph.GA])

G2 can Illuminate the Black Hole Population near the Galactic Center. (arXiv:1302.3220v1 [astro-ph.GA]):
Galactic nuclei are expected to be densely populated with stellar and
intermediate mass black holes. Exploring this population will have important
consequences for the observation prospects of gravitational waves as well as
understanding galactic evolution. The gas cloud G2 currently approaching Sgr A*
provides an unprecedented opportunity to probe the black hole and neutron star
population of the Galactic nucleus. We examine the possibility of a G2-black
hole encounter and its detectability with current X-ray satellites, such as
Chandra and NuSTAR. We find that multiple encounters are likely to occur close
to the pericenter, which may be detectable upon favorable circumstances. This
opportunity provides an additional, important science case for leading X-ray
observatories to closely follow G2 on its way to the nucleus.

Constraining the Accretion Flow in Sgr A* by General Relativistic Dynamical and Polarized Radiative Modeling. (arXiv:1212.4149v1 [astro-ph.HE])

Constraining the Accretion Flow in Sgr A* by General Relativistic Dynamical and Polarized Radiative Modeling. (arXiv:1212.4149v1 [astro-ph.HE]):
We briefly summarize the method of simulating Sgr A* polarized sub-mm spectra
from the accretion flow and fitting the observed spectrum. The dynamical flow
model is based on three-dimensional general relativistic magneto hydrodynamic
simulations. Fully self-consistent radiative transfer of polarized
cyclo-synchrotron emission is performed. We compile a mean sub-mm spectrum of
Sgr A* and fit it with the mean simulated spectra. We estimate the ranges of
inclination angle theta=42-75deg, mass accretion rate
Mdot=(1.4-7.0)*10^{-8}Msun/yr, and electron temperature Te=(3-4)*10^{10}K at
6M. We discuss multiple caveats in dynamical modeling, which must be resolved
to make further progress.

X-ray Echo from the Sagittarius C Complex and 500-year Activity History of Sagittarius A*. (arXiv:1211.4529v1 [astro-ph.GA])

X-ray Echo from the Sagittarius C Complex and 500-year Activity History of Sagittarius A*. (arXiv:1211.4529v1 [astro-ph.GA]):
This paper presents the Suzaku results obtained for the Sagittarius (Sgr) C
region using the concept of X-ray reflection nebulae (XRNe) as the echo of past
flares from the super massive black hole, Sgr A*. The Sgr C complex is composed
of several molecular clouds proximately located in projected distance. The
X-ray spectra of Sgr C were analyzed on the basis of a view that XRNe are
located inside the Galactic center plasma X-ray emission with an oval
distribution around Sgr A*. We found that the XRNe are largely separated in the
line-of-sight position, and are associated with molecular clouds in different
velocity ranges detected by radio observations. We also applied the same
analysis to the Sgr B XRNe and completed a long-term light curve for Sgr A*
occurring in the past. As a new finding, we determined that Sgr A* was
experiencing periods of high luminosity already 500 years ago, which is longer
than the previously reported value. Our results are consistent with a scenario
that Sgr A* was continuously active with sporadic flux variabilities of Lx =
1-3 x 10^39 erg s^-1 in the past 50 to 500 years. The average past luminosity
was approximately 4-6 orders of magnitude higher than that presently observed.
In addition, two short-term flares of 5-10 years are found. Thus, the past
X-ray flare should not be a single short-term flare, but can be interpreted as
multiple flares superposed on a long-term high state.

The X-ray flaring properties of Sgr A* during six years of monitoring with Swift. (arXiv:1210.7237v1 [astro-ph.HE])

The X-ray flaring properties of Sgr A* during six years of monitoring with Swift. (arXiv:1210.7237v1 [astro-ph.HE]):
Starting in 2006, Swift has been targeting a region of \sim 21'X21' around
Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short,
quasi-daily observations offer an unique view of the long-term X-ray behavior
of the supermassive black hole. We report on the data obtained between 2006
February and 2011 October, which encompasses 715 observations with a total
accumulated exposure time of \sim 0.8 Ms. A total of six confirmed X-ray flares
were detected with Swift, which all had an average 2-10 keV luminosity of Lx
(1-4)E35 erg/s (assuming a distance of 8 kpc). This more than doubles the
number of such bright X-ray flares observed from Sgr A*. The most luminous
X-ray flare seen with Swift may have reached a 2-10 keV peak intensity of Lx
6E35 erg/s, which would make it the brightest X-ray flare detected so far. One
of the Swift-detected flares was considerably softer than the other five,
indicating that flares of similar intensity can have different spectral
properties. An additional ten candidate X-ray flares were detected with an
estimated average intensity of Lx (0.7-1)E35 erg/s (2-10 keV). The Swift
campaign allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s)
X-ray flares to be ~0.2-0.5 per day, which is consistent with previous
estimates. This analysis of the occurrence rate and properties of the X-ray
flares seen with Swift offers an important calibration point to asses whether
the flaring behavior of Sgr A* changes as a result of its interaction with the
gas cloud that is projected to make a close passage in 2013.

Chandra-HETGS Observations of the Brightest Flare Seen from Sgr A*. (arXiv:1209.6354v1 [astro-ph.HE])

Chandra-HETGS Observations of the Brightest Flare Seen from Sgr A*. (arXiv:1209.6354v1 [astro-ph.HE]):
Starting in 2012, we began an unprecedented observational program focused on
the supermassive black hole in the center of our Galaxy, Sgr A*, utilizing the
High Energy Transmission Gratings Spectrometer (HETGS) instrument on the
Chandra X-ray Observatory. These observations will allow us to measure the
quiescent X-ray spectra of Sgr A* for the first time at both high spatial and
spectral resolution. The X-ray emission of Sgr A*, however, is known to flare
roughly daily by factors of a few to ten times over quiescent emission levels,
with rarer flares extending to factors of greater than 100 times quiescence.
Here were report an observation performed on 2012 February 9 wherein we
detected what is the highest peak flux and fluence flare ever observed from Sgr
A*. The flare, which lasted for 5.6 ks and had a decidedly asymmetric profile
with a faster decline than rise, achieved a mean absorbed 2-8 keV flux of
(8.5+/-0.9)X10^{-12} erg cm^{-2} s^{-1}. The peak flux was 2.5 times higher,
and the total 2-10 keV emission of the event was approximately 10^{39} erg.
Only one other flare of comparable magnitude, but shorter duration, has been
observed in Sgr A* by XMM-Newton in 2002 October. We perform spectral fits of
this Chandra observed flare, and compare our results to the two brightest
flares ever observed with XMM-Newton. We find good agreement among the fitted
spectral slopes (Gamma~2) and X-ray absorbing columns (N_H~15X10^{22} cm^{-2})
for all three of these events, resolving prior differences (which are most
likely due to the combined effects of pileup and spectral modeling) among
Chandra and XMM-Newton observations of Sgr A* flares. We also discuss fits to
the quiescent spectra of Sgr A*.

Self-consistent spectra from radiative GRMHD simulations of accretion onto Sgr A*. (arXiv:1209.4599v1 [astro-ph.HE])

Self-consistent spectra from radiative GRMHD simulations of accretion onto Sgr A*. (arXiv:1209.4599v1 [astro-ph.HE]):
We present the first spectral energy distributions produced self-consistently
by 2.5D general relativistic magneto-hydrodynamical (GRMHD) numerical
simulations, where radiative cooling is included in the dynamical calculation.
As a case study, we focus on the accretion flow around the supermassive black
hole in the Galactic Centre, Sagittarius A* (Sgr A*), which has the best
constrained physical parameters. We compare the simulated spectra to the
observational data of Sgr A* and explore the parameter space of our model to
determine the effect of changing the initial magnetic field configuration, ion
to electron temperature ratio T_i/T_e and the target accretion rate. We find
the best description of the data for a mass accretion rate of ~ 1e-9 Msun/yr,
and rapid spin (0.7 < a_* < 0.9). The submillimeter peak flux seems largely
independent of initial conditions, while the higher energies can be very
sensitive to the initial magnetic field configuration. Finally, we also discuss
flaring features observed in some simulations, that may be due to artifacts of
the 2D configuration.

Constraining the nature of the Galactic center black hole Sgr A* with present and future observations. (arXiv:1209.0251v1 [astro-ph.HE])

Constraining the nature of the Galactic center black hole Sgr A* with present and future observations. (arXiv:1209.0251v1 [astro-ph.HE]):
The Galactic center is an ideal laboratory to study strong-field general
relativistic phenomena, as the supermassive black hole Sgr A* has the biggest
angular Schwarzschild radius among all black holes. This article presents three
different ways of using the immediate surroundings of Sgr A* as a probe in
order to either constrain its spin, or even test the very nature of this
compact object.

Galactic Center Research: Manifestations of the Central Black Hole. (arXiv:1207.6755v1 [astro-ph.GA])

Galactic Center Research: Manifestations of the Central Black Hole. (arXiv:1207.6755v1 [astro-ph.GA]):
This review summarizes a few of the frontiers of Galactic center research
that are currently the focus of considerable activity and attention. It is
aimed at providing a necessarily incomplete sketch of some of the timely work
being done on phenomena taking place in, or originating in, the central few
parsecs of the Galaxy, with particular attention to topics related to the
Galactic black hole (GBH). We have chosen to expand on the following exciting
topics: 1) the characterization and the implications for the variability of
emission from the GBH, 2) the strong evidence for a powerful X-ray flare in the
Galactic center within the past few hundred years, and the likelihood that the
GBH is implicated in that event, 3) the prospects for detecting the "shadow" of
the GBH, 4) an overview of the current state of research on the central S-star
cluster, and what has been learned from the stellar orbits within that cluster,
and 5) the current hypotheses for the origin of the G2 dust cloud that is
projected to make a close passage by the GBH in 2013.

Interacting Cosmic Rays with Molecular Clouds: A Bremsstrahlung Origin of Diffuse High Energy Emission from the Inner 2deg by 1deg of the Galactic Center. (arXiv:1206.6882v1 [astro-ph.HE])

Interacting Cosmic Rays with Molecular Clouds: A Bremsstrahlung Origin of Diffuse High Energy Emission from the Inner 2deg by 1deg of the Galactic Center. (arXiv:1206.6882v1 [astro-ph.HE]):
The high energy activity in the inner few degrees of the Galactic center is
traced by diffuse radio, X-ray and gamma-ray emission. The physical
relationship between different components of diffuse gas emitting at multiple
wavelengths is a focus of this work. We first present radio continuum
observations using Green Bank Telescope and model the nonthermal spectrum in
terms of a broken power-law distribution of GeV electrons emitting synchrotron
radiation. We show that the emission detected by Fermi is primarily due to
nonthermal bremsstrahlung produced by the population of synchrotron emitting
electrons in the GeV energy range interacting with neutral gas. The
extrapolation of the electron population measured from radio data to low and
high energies can also explain the origin of FeI 6.4 keV line and diffuse TeV
emission, as observed with Suzaku, XMM-Newton, Chandra and the H.E.S.S.
observatories. The inferred physical quantities from modeling multi-wavelength
emission in the context of bremsstrahlung emission from the inner 300x120
parsecs of the Galactic center are constrained to have the cosmic ray
ionization rate 1-10x10^{-15} s^-1, molecular gas heating rate elevating the
gas temperature to 75-200K, fractional ionization of molecular gas 10^{-6} to
10^{-5}, large scale magnetic field 10-20 micro Gauss, the density of diffuse
and dense molecular gas 100 and 10^3 cm^{-3} over 300pc and 50pc pathlengths,
and the variability of FeI Kalpha 6.4 keV line emission on yearly time scales.
Important implications of our study are that GeV electrons emitting in radio
can explain the GeV gamma-rays detected by Fermi and that the cosmic ray
irradiation model, like the model of the X-ray irradiation triggered by past
activity of Sgr A*, can also explain the origin of the variable 6.4 keV
emission from Galactic center molecular clouds.

The X-ray lightcurve of Sgr A* over the past 150 years inferred from Fe-Ka line reverberation in Galactic Centre molecular clouds. (arXiv:1207.1436v1 [astro-ph.HE])

The X-ray lightcurve of Sgr A* over the past 150 years inferred from Fe-Ka line reverberation in Galactic Centre molecular clouds. (arXiv:1207.1436v1 [astro-ph.HE]):
We examine the temporal and spectral properties of nine Fe-Ka bright
molecular clouds within about 30 pc of Sgr A*, in order to understand and
constrain the primary energising source of the Fe fluorescence. Significant
Fe-Ka variability was detected, with a spatial and temporal pattern consistent
with that reported in previous studies. The main breakthrough that sets our
paper apart from earlier contributions on this topic is the direct measurement
of the column density and the Fe abundance of the MCs in our sample. We used
the EW measurements to infer the average Fe abundance within the clouds to be
1.6$\pm$0.1 times solar. The cloud column densities derived from the spectral
analysis were typically of the order of 10$^{23}$ cm$^{-2}$, which is
significantly higher than previous estimates. This in turn has a significant
impact on the inferred geometry and time delays within the cloud system. Past
X-ray activity of Sgr A* is the most likely source of ionisation within the
molecular clouds in the innermost 30 pc of the Galaxy. In this scenario, the
X-ray luminosity required to excite these reflection nebulae is of the order of
10$^{37}-10^{38}$ erg s$^{-1}$, significantly lower than that estimated for the
Sgr B2 molecular cloud. Moreover, the inferred Sgr A* lightcurve over the past
150 years shows a long-term downwards trend punctuated by occasional
counter-trend brightening episodes of at least 5 years duration. Finally, we
found that contributions to the Fe fluorescence by X-ray transient binaries and
cosmic-ray bombardment are very likely, and suggest possible ways to study this
latter phenomenon in the near future.