276 research outputs found
Intra-day Variability of Sagittarius A* at 3 Millimeters
We report observations and analysis of flux monitoring of Sagittarius A* at
3-mm wavelength using the OVRO millimeter interferometer over a period of eight
days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes)
with the nearby calibrator source J1744-312 was employed to control for
instrumental and atmospheric effects. Time variations are sought by computing
and subtracting, from each visibility in the database, an average visibility
obtained from all the data acquired in our monitoring program having similar uv
spacings. This removes the confusing effects of baseline-dependent, correlated
flux interference caused by the static, thermal emission from the extended
source Sgr A West. Few-day variations up to ~20% and intra-day variability of
\~20% and in some cases up to ~40% on few-hour time scales emerge from the
differenced data on SgrA*. Power spectra of the residuals indicate the presence
of hourly variations on all but two of the eight days. Monte Carlo simulation
of red-noise light curves indicates that the hourly variations are well
described by a red-noise power spectrum with P(f) ~ f^(-1). Of particular
interest is a ~2.5 hour variation seen prominently on two consecutive days. An
average power spectrum from all eight days of data reveals noteworthy power on
this time scale. There is some indication that few-hour variations are more
pronounced on days when the average daily flux is highest. We briefly discuss
the possibility that these few-hour variations are due to the dynamical
modulation of accreting gas around the central supermassive black hole, as well
as the implications for the structure of the SgrA* photosphere at 3 mm.
Finally, these data have enabled us to produce a high sensitivity 3-mm map of
the extended thermal emission surrounding SgrA*.Comment: Accepted for publication in The Astrophysical Journal Letters, 8
pages, 4 figure
Development of full in-flight acoustic design criteria scaling effects Final report, 29 Jun. 1968 - 29 Jun. 1969
Scaling fluctuating pressure environments for high performance aerospace vehicles in acoustic design criteri
Electron Acceleration around the Supermassive Black Hole at the Galactic Center
The recent detection of variable infrared emission from Sagittarius A*,
combined with its previously observed flare activity in X-rays, provides
compelling evidence that at least a portion of this object's emission is
produced by nonthermal electrons. We show here that acceleration of electrons
by plasma wave turbulence in hot gases near the black hole's event horizon can
account both for Sagittarius A*'s mm and shorter wavelengths emission in the
quiescent state, and for the infrared and X-ray flares, induced either via an
enhancement of the mass accretion rate onto the black hole or by a
reorganization of the magnetic field coupled to the accretion gas. The
acceleration model proposed here produces distinct flare spectra that may be
compared with future coordinated multi-wavelength observations. We further
suggest that the diffusion of high energy electrons away from the acceleration
site toward larger radii might be able to account for the observed
characteristics of Sagittarius A*'s emission at cm and longer wavelengths.Comment: 13 pages, 2 figures and 1 table, submitted to ApJ
A Constant Spectral Index for Sagittarius A* During Infrared/X-ray Intensity Variations
We report the first time-series of broadband infrared (IR) color measurements
of Sgr A*, the variable emission source associated with the supermassive black
hole at the Galactic Center. Using the laser and natural guide star AO systems
on the Keck II telescope, we imaged Sgr A* in multiple near-infrared broadband
filters with a typical cycle time of ~3 min during 4 observing runs
(2005-2006), two of which were simultaneous with Chandra X-ray measurements. In
spite of the large range of dereddened flux densities for Sgr A* (2-30 mJy),
all of our near-IR measurements are consistent with a constant spectral index
of alpha = -0.6+-0.2. Furthermore, this value is consistent with the spectral
indices observed at X-ray wavelengths during nearly all outbursts; which is
consistent with the synchrotron self-Compton model for the production of the
X-ray emission. During the coordinated observations, one IR outburst occurs <36
min after a possibly associated X-ray outburst, while several similar IR
outbursts show no elevated X-ray emission. A variable X-ray to IR ratio and
constant infrared spectral index challenge the notion that the IR and X-ray
emission are connected to the same electrons. We, therefore, posit that the
population of electrons responsible for both the IR and X-ray emission are
generated by an acceleration mechanism that leaves the bulk of the electron
energy distribution responsible for the IR emission unchanged, but has a
variable high-energy cutoff. Occasionally a tail of electrons >1 GeV is
generated, and it is this high-energy tail that gives rise to the X-ray
outbursts. One possible explanation for this type of variation is from the
turbulence induced by a magnetorotational instability, in which the outer scale
length of the turbulence varies and changes the high-energy cutoff.Comment: 11 pages, 7 figures (color), Accepted for publication in ApJ.
Resolution (Fig 1&2) downgraded for astro-ph. For full resolution, see
http://casa.colorado.edu/~hornstei/sgracolor.pd
Around 200 new X-ray binary IDs from 13 years of Chandra observations of the M31 center
We have created 0.3--10 keV, 13 year, unabsorbed luminosity lightcurves for
528 X-ray sources in the central 20' of M31. We have 174 Chandra observations
spaced at ~1 month intervals thanks to our transient monitoring program, deeper
observations of the M31 nucleus, and some public data from other surveys. We
created 0.5--4.5 keV structure functions (SFs) for each source, for comparison
with the ensemble structure function of AGN. We find 220 X-ray sources with
luminosities > ~1E+35 erg/s that have SFs with significantly more variability
than the ensemble AGN SF, and are likely X-ray binaries (XBs). A further 30
X-ray sources were identified as XBs using other methods. We therefore have 250
probable XBs in total, including ~200 new identifications. This result
represents great progress over the ~50 XBs and ~40 XB candidates previously
identified out of the ~2000 X-ray sources within the D_25 region of M31; it
also demonstrates the power of SF analysis for identifying XBs in external
galaxies. We also identify a new transient black hole candidate, associated
with the M31 globular cluster B128.Comment: Accepted for publication in ApJ; 20 pages, 7 figures; Tables 1 and 2
continue after the references (8 pages
The Chandra Dust Scattering Halo of Galactic Center transient Swift J174540.7-290015
We report the detection of a dust scattering halo around a recently
discovered X-ray transient, Swift J174540.7-290015, which in early February of
2016 underwent one of the brightest outbursts (F_X ~ 5e-10 erg/cm^2/s) observed
from a compact object in the Galactic Center field. We analyze four Chandra
images that were taken as follow-up observations to Swift discoveries of new
Galactic Center transients. After adjusting our spectral extraction for the
effects of detector pileup, we construct a point spread function for each
observation and compare it to the GC field before the outburst. We find
residual surface brightness around Swift J174540.7-290015, which has a shape
and temporal evolution consistent with the behavior expected from X-rays
scattered by foreground dust. We examine the spectral properties of the source,
which shows evidence that the object transitioned from a soft to hard spectral
state as it faded below L_X ~ 1e36 erg/s. This behavior is consistent with the
hypothesis that the object is a low mass X-ray binary in the Galactic Center.Comment: Accepted for publication in Ap
The X-ray Binary GRS 1741.9-2853 in Outburst and Quiescence
We report Chandra and XMM-Newton observations of the transient neutron star
low-mass X-ray binary GRS 1741.9-2853. Chandra detected the source in outburst
on 2000 October 26 at an X-ray luminosity of ~10^{36} erg/s (2--8 keV; 8 kpc),
and in quiescence on 2001 July 18 at ~10^{32} erg/s. The latter observation is
the first detection of GRS 1741.9-2853 in quiescence. We obtain an accurate
position for the source of 17h 45m 2.33s, -28o 54' 49.7" (J2000), with an
uncertainty of 0.7". GRS 1741.9-2853 was not detected significantly in three
other Chandra observations, nor in three XMM-Newton observations, indicating
that the luminosity of the source in quiescence varies by at least a factor of
5 between (< 0.9 - 5.0) \times 10^{32} erg/s (2--8 keV). A weak X-ray burst
with a peak luminosity of 5 \times 10^{36} erg/s above the persistent level was
observed with Chandra during the outburst on 2000 October 26. The energy of
this burst, 10^{38} erg, is unexpectedly low, and may suggest that the accreted
material is confined to the polar caps of the neutron star. A search of the
literature reveals that GRS 1741.9-2853 was observed in outburst with ASCA in
Fall 1996 as well, when the BeppoSAX WFC detected the three previous X-ray
bursts from this source. The lack of X-ray bursts from GRS 1741.9-2853 at other
epochs suggests that it produces bursts only during transient outbursts when
the accretion rate onto the surface of the neutron star is about 10^{-10}
M_sun/yr. A similar situation may hold for other low-luminosity bursters
recently identified from WFC data.Comment: Submitted to ApJ. 9 pages, including 5 figure
Flaring Activity of Sgr A*: Expanding Hot Blobs
Sgr A* is considered to be a massive black hole at the Galactic center and is
known to be variable in radio, millimeter, near-IR and X-rays. Recent
multi-wavelength observing campaigns show a simultaneous X-ray and near-IR
flare, as well as sub-millimeter and near-IR flares from Sgr A*. The flare
activity is thought to be arising from the innermost region of Sgr A*. We have
recently argued that the duration of flares in near-IR and submillimeter
wavelengths implies that the burst of emission expands and cools on a dynamical
time scale before the flares leave Sgr A*. The detection of radio flares with a
time delay in the range of 20 and 40 minutes between 7 and 12mm peak emission
implies adiabatic expansion of a uniform, spherical hot blob due to flare
activity. We suspect that this simple outflow picture shows some of the
characteristics that are known to take place in microquasars, thus we may learn
much from comparative study of Sgr A* and its environment vs. microquasars.Comment: 10 pages, 5 figures, to be published in IV Microquasar Workshop:
Microquasars and Beyond, September 18-22 2006, Como, Ital
The Variability of Sagittarius A* at Centimeter Wavelengths
We present the results of a 3.3-year project to monitor the flux density of
Sagittarius A* at 2.0, 1.3, and 0.7 cm with the VLA. The fully calibrated light
curves for Sgr A* at all three wavelengths are presented. Typical errors in the
flux density are 6.1%, 6.2%, and 9.2% at 2.0, 1.3, and 0.7 cm, respectively.
There is preliminary evidence for a bimodal distribution of flux densities,
which may indicate the existence of two distinct states of accretion onto the
supermassive black hole. At 1.3 and 0.7 cm, there is a tail in the distribution
towards high flux densities. Significant variability is detected at all three
wavelengths, with the largest amplitude variations occurring at 0.7 cm. The rms
deviation of the flux density of Sgr A* is 0.13, 0.16, and 0.21 Jy at 2.0, 1.3,
and 0.7 cm, respectively. During much of this monitoring campaign, Sgr A*
appeared to be relatively quiescent compared to results from previous
campaigns. At no point during the monitoring campaign did the flux density of
Sgr A* more than double its mean value. The mean spectral index of Sgr A* is
alpha=0.20+/-0.01, with a standard deviation of 0.14. The spectral index
appears to depend linearly on the observed flux density at 0.7 cm with a
steeper index observed during outbursts. This correlation is consistent with
the expectation for outbursts that are self-absorbed at wavelengths of 0.7 cm
or longer and inconsistent with the effects of simple models for interstellar
scintillation. Much of the variability of Sgr A*, including possible time lags
between flux density changes at the different wavelengths, appears to occur on
time scales less than the time resolution of our observations (8 days). Future
observations should focus on the evolution of the flux density on these time
scales.Comment: 16 pages, 10 figures, accepted for publication in A
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