764 research outputs found
Tributes to Professor Alice Brumbaugh
Tributes to Professor Alice Brumbaugh upon her retirement from the University of Maryland School of Law
XMM-Newton observation of the brightest X-ray flare detected so far from SgrA*
We report the high S/N observation on October 3, 2002 with XMM-Newton of the
brightest X-ray flare detected so far from SgrA* with a duration shorter than
one hour (~ 2.7 ks). The light curve is almost symmetrical with respect to the
peak flare, and no significant difference between the soft and hard X-ray range
is detected. The overall flare spectrum is well represented by an absorbed
power-law with a soft photon spectral index of Gamma=2.5+/-0.3, and a peak 2-10
keV luminosity of 3.6 (+0.3-0.4) x 10^35 erg/s, i.e. a factor 160 higher than
the Sgr A* quiescent value. No significant spectral change during the flare is
observed. This X-ray flare is very different from other bright flares reported
so far: it is much brighter and softer. The present accurate determination of
the flare characteristics challenge the current interpretation of the physical
processes occuring inside the very close environment of SgrA* by bringing very
strong constraints for the theoretical flare models.Comment: Accepted for publication in A&A Letters. 4 pages, 2 figures, 1 table
Accretion models of Sgr A*
The supermassive black hole in the center of our Galaxy, Sgr A*, is unique
because the angular size of the black hole is the largest in the sky thus
providing detailed boundary conditions on, and much less freedom for, accretion
flow models. In this paper I review advection-dominated accretion flow (ADAF;
another name is radiatively inefficient accretion flow) models for Sgr A*. This
includes the developments and dynamics of ADAFs, and how to explain
observational results including the multi-waveband spectrum, radio
polarization, IR and X-ray flares, and the size measurements at radio
wavebands.Comment: 9 pages, 6 figures; invited talk presented at the "Galactic Center
Workshop 2006: From the Center of the Milky Way to Nearby Low-Luminosity
Galactic Nuclei", April 18-22, 2006; Bad Honnef, German
The extreme luminosity states of Sagittarius A*
We discuss mm-wavelength radio, 2.2-11.8um NIR and 2-10 keV X-ray light
curves of the super massive black hole (SMBH) counterpart of Sagittarius A*
(SgrA*) near its lowest and highest observed luminosity states. The luminosity
during the low state can be interpreted as synchrotron emission from a
continuous or even spotted accretion disk. For the high luminosity state SSC
emission from THz peaked source components can fully account for the flux
density variations observed in the NIR and X-ray domain. We conclude that at
near-infrared wavelengths the SSC mechanism is responsible for all emission
from the lowest to the brightest flare from SgrA*. For the bright flare event
of 4 April 2007 that was covered from the radio to the X-ray domain, the SSC
model combined with adiabatic expansion can explain the related peak
luminosities and different widths of the flare profiles obtained in the NIR and
X-ray regime as well as the non detection in the radio domain.Comment: 18 pages, 13 figures, accepted by A&
Evidence for X-ray synchrotron emission from simultaneous mid-IR to X-ray observations of a strong Sgr A* flare
This paper reports measurements of Sgr A* made with NACO in L' -band (3.80
um), Ks-band (2.12 um) and H-band (1.66 um) and with VISIR in N-band (11.88 um)
at the ESO VLT, as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On
4 April, 2007, 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 SED has a blue slope (beta > 0 for nuL_nu ~ nu^beta, consistent with
nuL_nu ~ nu^0.4) between 12 micron and 3.8 micron.
For the first time our high quality data allow a detailed comparison of
infrared 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 micron light curve are clearly not
seen in the X-ray data. From the shortest timescale variations in the L'-band
lightcurve we find that the flaring region must be no more than 1.2 R_S in
size.
The high X-ray to infrared flux ratio, blue nuL_nu slope MIR to L' -band, and
the soft nuL_nu 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.Comment: ApJ, 49 pages, 9 figure
An evolving hot spot orbiting around Sgr A*
Here we report on recent near-infrared observations of the Sgr A* counterpart
associated with the super-massive ~ 4x10^6 M_sun black hole at the Galactic
Center. We find that the May 2007 flare shows the highest sub-flare contrast
observed until now, as well as evidence for variations in the profile of
consecutive sub-flares. We modeled the flare profile variations according to
the elongation and change of the shape of a spot due to differential rotation
within the accretion disk.Comment: 7 pages, 5 figures, contribution for the conference "The Universe
under the Microscope" (AHAR 2008), to be published in Journal of Physics:
Conference Series by Institute of Physics Publishin
An Inverse Compton Scattering Origin of X-ray Flares from Sgr A*
The X-ray and near-IR emission from Sgr A* is dominated by flaring, while a
quiescent component dominates the emission at radio and sub-mm wavelengths. The
spectral energy distribution of the quiescent emission from Sgr A* peaks at
sub-mm wavelengths and is modeled as synchrotron radiation from a thermal
population of electrons in the accretion flow, with electron temperatures
ranging up to \,MeV. Here we investigate the mechanism by which
X-ray flare emission is produced through the interaction of the quiescent and
flaring components of Sgr A*. The X-ray flare emission has been interpreted as
inverse Compton, self-synchrotron-Compton, or synchrotron emission. We present
results of simultaneous X-ray and near-IR observations and show evidence that
X-ray peak flare emission lags behind near-IR flare emission with a time delay
ranging from a few to tens of minutes. Our Inverse Compton scattering modeling
places constraints on the electron density and temperature distributions of the
accretion flow and on the locations where flares are produced. In the context
of this model, the strong X-ray counterparts to near-IR flares arising from the
inner disk should show no significant time delay, whereas near-IR flares in the
outer disk should show a broadened and delayed X-ray flare.Comment: 22 pages, 6 figures, 2 tables, AJ (in press
Overtly anaphoric control in type logical grammar
In this paper we analyse anaphoric pronouns in control sentences and we investigate the implications of these kinds of sentences in relation to the Propositional Theory versus Property Theory question. For these purposes, we invoke the categorial calculus with limited contraction, a conservative extension of Lambek calculus that builds contraction into the logical rules for a customized slash type-constructor.Peer ReviewedPostprint (author's final draft
A Remarkable Low-Mass X-ray Binary within 0.1 pc of the Galactic Center
Recent X-ray and radio observations have identified a transient low-mass
X-ray binary (LMXB) located only 0.1 pc in projection from the Galactic center,
CXOGC J174540.0-290031. In this paper, we report the detailed analysis of X-ray
and infrared observations of the transient and its surroundings. Chandra
bservations detect the source at a flux of F_X = 2e-12 erg cm^-2 s^-1 (2-8
keV). After accounting for absorption both in the interstellar medium and in
material local to the source, the implied luminosity of the source is only L_X
= 4e34 erg/s (2-8 keV; D=8 kpc). However, the diffuse X-ray emission near the
source also brightened by a factor of 2. The enhanced diffuse X-ray emission
lies on top of a known ridge of dust and ionized gas that is visible infrared
images. We interpret the X-ray emission as scattered flux from the outburst,
and determine that the peak luminosity of CXOGC J174540.0-290031 was >2e36
erg/s. We suggest that the relatively small observed flux results from the fact
that the system is observed nearly edge-on, so that the accretion disk
intercepts most of the flux emitted along our line of sight. We compare the
inferred peak X-ray luminosity to that of the radio jet. The ratio of the X-ray
to radio luminosities, L_X/L_R<1e4, is considerably smaller than in other known
LMXBs (> 1e5). This is probably because the jets are radiating with unusually
high efficiency at the point where they impact the surrounding interstellar
medium. This hypothesis is supported by a comparison with mid-infrared images
of the surrounding dust. Finally, we find that the minimum power required to
produce the jet, L_jet~1e37 erg/s, is comparable to the inferred peak X-ray
luminosity. This is the most direct evidence yet obtained that LMXBs accreting
at low rates release about half of their energy as jets.Comment: 11 pages, including 9 figures (6 color). submitted to Ap
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