3,801 research outputs found
Fe Ka line emission from the Arches cluster region - evidence for ongoing particle bombardment?
We present the results of eight years of XMM-Newton observations of the
region surrounding the Arches cluster in the Galactic Center. We study the
spatial distribution and temporal behaviour of the Fe-Ka line emission with the
objective of identifying the likely source of the excitation. We investigate
the variability of the 6.4-keV line emission of four clouds through spectral
fitting of the EPIC MOS data with the use of a modelled background, which
avoids many of the systematics inherent in local background subtraction. We
also employ spectral stacking of both EPIC PN and MOS data to search for
evidence of an Fe-K edge feature imprinted on the underlying X-ray continuum.
The lightcurves of the Fe-Ka line from three bright molecular knots close to
the Arches cluster are found to be constant over the 8-year observation window.
West of the cluster, however, we found a bright cloud exhibiting the fastest
Fe-Ka variability yet seen in a molecular cloud in the Galactic Center region.
The time-averaged spectra of the molecular clouds reveal no convincing evidence
of the 7.1-keV edge feature. The EW of the 6.4-keV line emitted by the clouds
near the cluster is found to be ~1.0 keV. The observed Fe-Ka line flux and the
high EW suggest the fluorescence has a photoionization origin, although
excitation by cosmic-ray particles is not specifically excluded. For the three
clouds nearest to the cluster, an identification of the source of
photo-ionizing photons with an earlier outburst of Sgr A* is however at best
tentative. The hardness of the nonthermal component associated with the 6.4-keV
line emission might be best explained in terms of bombardment by cosmic-ray
particles from the Arches cluster itself. The relatively short-timescale
variability seen in the 6.4-keV line emission from the cloud to the West of the
cluster is most likely the result of illumination by a nearby transient X-ray
source.Comment: 13 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
X-ray echoes of infrared flaring in Sgr A*
Sgr A* exhibits flaring in the infrared several times each day, occasionally
accompanied by flaring in X-rays. The infrared flares are believed to arise
through synchrotron emission from a transient population of accelerated
electrons. The X-ray flaring has been interpreted as self-synchrotron-compton,
inverse compton, or synchrotron emission associated with the transient
electrons. Here I consider the upscattering of infrared flare photons by
relativistic thermal electrons in the accretion flow around Sgr A*. Typical
profiles of electron density and temperature in the accretion flow are adopted
and the X-ray light curves produced by upscattering of infrared flare photons
by the accretion flow are computed. Peak X-ray luminosities between 1e33 and
1e34 erg/s are attained for a 10 mJy near-infrared flare, compatible with
observed coincident infrared/X-ray flares from Sgr A*. Even if this process is
not responsible for the observed flares it still presents a serious constraint
on accretion flow models, which must avoid over-producing X-rays and also
predicting observable time lags between flaring in infrared and in X-rays.
Future high-resolution infrared instrumentation will be able to place the
location of the infrared flare and in coordination with the X-ray would
severely constrain the disc geometry and the radial profiles of electron
density and temperature in the accretion flow.Comment: 6 pages, 2 figures, to appear in The Galactic Center: A Window on the
Nuclear Environment of Disk Galaxies, ASP Conference Series, eds: M. Morris,
D. Q. Wang and F. Yua
Accurate OH maser positions II. the Galactic Center region
We present high spatial resolution observations of ground-state OH masers,
achieved using the Australia Telescope Compact Array (ATCA). These observations
were conducted towards 171 pointing centres, where OH maser candidates were
identified previously in the Southern Parkes Large-Area Survey in Hydroxyl
(SPLASH) towards the Galactic Center region, between Galactic longitudes of
and and Galactic latitudes of and
. We detect maser emission towards 162 target fields and suggest
that 6 out of 9 non-detections are due to intrinsic variability. Due to the
superior spatial resolution of the follow-up ATCA observations, we have
identified 356 OH maser sites in the 162 of the target fields with maser
detections. Almost half (161 of 356) of these maser sites have been detected
for the first time in these observations. After comparing the positions of
these 356 maser sites to the literature, we find that 269 (76\%) sites are
associated with evolved stars (two of which are planetary nebulae), 31 (9\%)
are associated with star formation, four are associated with supernova remnants
and we were unable to determine the origin of the remaining 52 (15\%) sites.
Unlike the pilot region (\citealt{Qie2016a}), the infrared colors of evolved
star sites with symmetric maser profiles in the 1612 MHz transition do not show
obvious differences compared with those of evolved star sites with asymmetric
maser profiles.Comment: 24 pages, 12 figures, accepted by ApJ
A Constraint on the Organization of the Galactic Center Magnetic Field Using Faraday Rotation
We present new 6 and 20 cm Very Large Array (VLA) observations of polarized
continuum emission of roughly 0.5 square degrees of the Galactic center (GC)
region. The 6 cm observations detect diffuse linearly-polarized emission
throughout the region with a brightness of roughly 1 mJy per 15"x10" beam. The
Faraday rotation measure (RM) toward this polarized emission has structure on
degree size scales and ranges from roughly +330 rad/m2 east of the dynamical
center (Sgr A) to -880 rad/m2 west of the dynamical center. This RM structure
is also seen toward several nonthermal radio filaments, which implies that they
have a similar magnetic field orientation and constrains models for their
origin. Modeling shows that the RM and its change with Galactic longitude are
best explained by the high electron density and strong magnetic field of the GC
region. Considering the emissivity of the GC plasma shows that while the
absolute RM values are indirect measures of the GC magnetic field, the RM
longitude structure directly traces the magnetic field in the central
kiloparsec of the Galaxy. Combining this result with previous work reveals a
larger RM structure covering the central ~2 degrees of the Galaxy. This RM
structure is similar to that proposed by Novak and coworkers, but is shifted
roughly 50 pc west of the dynamical center of the Galaxy. If this RM structure
originates in the GC region, it shows that the GC magnetic field is organized
on ~300 pc size scales. The pattern is consistent with a predominantly poloidal
field geometry, pointing from south to north, that is perturbed by the motion
of gas in the Galactic disk.Comment: Accepted to ApJ. emulateapj style, 14 pages, 15 figure
Jet-lag in Sgr A*: What size and timing measurements tell us about the central black hole in the Milky Way
The black hole at the Galactic Center, Sgr A*, is the prototype of a galactic
nucleus at a very low level of activity. Its radio through submm-wave emission
is known to come from a region close to the event horizon, however, the source
of the emission is still under debate. A successful theory explaining the
emission is based on a relativistic jet model scaled down from powerful
quasars. We want to test the predictive power of this established jet model
against newly available measurements of wavelength-dependent time lags and the
size-wavelength structure in Sgr A*. Using all available closure amplitude VLBI
data from different groups, we again derived the intrinsic wavelength-dependent
size of Sgr A*. This allowed us to calculate the expected frequency-dependent
time lags of radio flares, assuming a range of in- and outflow velocities.
Moreover, we calculated the time lags expected in the previously published
pressure-driven jet model. The predicted lags are then compared to radio
monitoring observations at 22, 43, and 350 GHz. The combination of time lags
and size measurements imply a mildly relativistic outflow with bulk outflow
speeds of gamma*beta ~ 0.5-2. The newly measured time lags are reproduced well
by the jet model without any major fine tuning. The results further strengthen
the case for the cm-to-mm wave radio emission in Sgr A* as coming from a mildly
relativistic jet-like outflow. The combination of radio time lag and VLBI
closure amplitude measurements is a powerful new tool for assessing the flow
speed and direction in Sgr A*. Future VLBI and time lag measurements over a
range of wavelengths will reveal more information about Sgr A*, such as the
existence of a jet nozzle, and measure the detailed velocity structure of a
relativistic jet near its launching point for the first time.Comment: Latex, 7 pages, accepted for publication in Astronomy & Astrophysic
G359.87+0.18: An FR II Radio Galaxy 15 Arcminutes from Sgr A*. Implications for the Scattering Region in the Galactic Center
G359.87+0.18 is an enigmatic object located 15' from Sgr A*. It has been
variously classified as an extragalactic source, Galactic jet source, and young
supernova remnant. We present new observations of G359.87+0.18 between 0.33 and
15 GHz and use these to argue that this source is an Faranoff-Riley II radio
galaxy. We are able to place a crude limit on its redshift of z > 0.1. The
source has a spectral index \alpha < -1 (S \propto \nu^\alpha), suggestive of a
radio galaxy with a redshift z >~ 2.
The scattering diameters of Sgr A* and several nearby OH masers (~ 1" at 1
GHz) indicate that a region of enhanced scattering is along the line of sight
to the Galactic center. If the region covers the Galactic center uniformly, the
implied diameter for a background source is at least 600" at 0.33 GHz, in
contrast with the observed 20" diameter of G359.87+0.18. Using the scattering
diameter of a nearby OH maser OH 359.762+0.120 and the widths of two, nearby,
non-thermal threads, G0.08+0.15 and G359.79+0.17, we show that a uniform
scattering region should cover G359.87+0.18. We therefore conclude that the
Galactic center scattering region is inhomogeneous on a scale of 5' (~ 10 pc at
a distance of 8.5 kpc). This scale is comparable to the size scale of molecular
clouds in the Galactic center. The close agreement between these two lengths
scales is an indication that the scattering region is linked intimately to the
Galactic center molecular clouds.Comment: Accepted for publication in the ApJ, vol. 515, LaTeX2e manuscript
using aaspp4 macro, 19 pages, 8 figures in 11 PostScript file
A Nonthermal Radio Filament Connected to the Galactic Black Hole?
Using the Very Large Array, we have investigated a non-thermal radio filament
(NTF) recently found very near the Galactic black hole and its radio
counterpart, SgrA*. While this NTF -- the Sgr A West Filament (SgrAWF) --
shares many characteristics with the population of NTFs occupying the central
few hundred parsecs of the Galaxy, the SgrAWF has the distinction of having an
orientation and sky location that suggest an intimate physical connection to
SgrA*. We present 3.3 and 5.5 cm images constructed using an innovative
methodology that yields a very high dynamic range, providing an unprecedentedly
clear picture of the SgrAWF. While the physical association of the SgrAWF with
SgrA* is not unambiguous, the images decidedly evoke this interesting
possibility. Assuming that the SgrAWF bears a physical relationship to SgrA*,
we examine the potential implications. One is that SgrA* is a source of
relativistic particles constrained to diffuse along ordered local field lines.
The relativistic particles could also be fed into the local field by a
collimated outflow from SgrA*, perhaps driven by the Poynting flux accompanying
the black hole spin in the presence of a magnetic field threading the event
horizon. Second, we consider the possibility that the SgrAWF is the
manifestation of a low-mass-density cosmic string that has become anchored to
the black hole. The simplest form of these hypotheses would predict that the
filament be bi-directional, whereas the SgrAWF is only seen on one side of
SgrA*, perhaps because of the dynamics of the local medium.Comment: 9 pages, 4 figures, accepted for ApJ Letter
Turbulent Origin of the Galactic-Center Magnetic Field: Nonthermal Radio Filaments
A great deal of study has been carried out over the last twenty years on the
origin of the magnetic activity in the Galactic center. One of the most popular
hypotheses assumes milli-Gauss magnetic field with poloidal geometry, pervading
the inner few hundred parsecs of the Galactic-center region. However, there is
a growing observational evidence for the large-scale distribution of a much
weaker field of B \lesssim 10 micro G in this region. Here, we propose that the
Galactic-center magnetic field originates from turbulent activity that is known
to be extreme in the central hundred parsecs. In this picture the spatial
distribution of the magnetic field energy is highly intermittent, and the
regions of strong field have filamentary structures. We propose that the
observed nonthermal radio filaments appear in (or, possibly, may be identified
with) such strongly magnetized regions. At the same time, the large-scale
diffuse magnetic field is weak. Both results of our model can explain the
magnetic field measurements of the the Galactic-center region. In addition, we
discuss the role of ionized outflow from stellar clusters in producing the long
magnetized filaments perpendicular to the Galactic plane.Comment: 11 pages, accepted to ApJ Letter
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