528 research outputs found
Studies of Quasi Periodic Oscillations in the Black Hole Transient XTE J 1817-330
We have used archival RXTE PCA data to investigate timing and spectral
characteristics of the transient XTE J1817-330. The data pertains to 160 PCA
pointed observations made during the outburst period 2006, January 27 to August
2. A detailed analysis of Quasi-Periodic Oscillations (QPOs) in this black hole
X-ray binary is carried out. Power density spectra were obtained using the
light curves of the source. QPOs have been detected in the 2-8 keV band in 10
of the observations. In 8 of these observations, QPOs are present in the 8-14
keV and in 5 observations in the 15-25 keV band. XTE J1817-330 is the third
black hole source from which the low frequency QPOs are clearly detected in
hard X-rays. The QPO frequency lies in ~ 4-9 Hz and the rms amplitude in
1.7-13.3% range, the amplitude being higher at higher energies. We have fitted
the PDS of the observations with Lorentzian and power law models. Energy
spectra are derived for those observations in which the QPOs are detected to
investigate any dependence of the QPO characteristic on the spectral
parameters. These spectra are well fitted with a two component model that
includes the disk black body component and a power law component. The QPO
characteristics and their variations are discussed and its implication on the
origin of the QPOs are examinedComment: 16 page, 9 figures and 2 tables. Accepted in MNRAS Journa
The Weak Blue Bump of H2106-099 and AGN De-Reddening
We present multi-frequency spectra of the Seyfert 1 galaxy H2106-099, from
radio to hard X-rays, spanning over a decade of observations. The hard X-ray
(2-20 keV) spectrum measured with Ginga had a Log slope of -0.80 +/- 0.02 on
1988 May 18 and -1.02 +/- 0.10 on 1988 May 22 / 23 UT, with no observed flux
changes. Other measurements showed variability and unusual spectral features:
The V band flux changed by a factor of 1.8 (> 10 sigma) in six weeks. Only
moderate optical Fe II emission is present, but strong [FeVII] and [Fe X] lines
are present in some epochs. The Balmer lines show > 25% variations in flux
relative to the mean, and He I changed by more than 100% relative to the mean
in <~ six years. The most surprising finds came from the composite UV through
near-IR spectrum: If the spectrum is de-reddened by the galactic extinction
value (from 21 cm observations), a residual 2175 Angstrom absorption feature is
present. Additional de-reddening to correct the feature yields E(B-V)=0.07 mag
due to material outside our galaxy, most probably associated with the AGN or
its host galaxy. No other clear indications of reddening are observed in this
object, suggesting that blue bump strength measurements in low and intermediate
red-shift AGN could be incorrect if derived without UV observations of the
region near 2175 Angstrom in the AGN frame. After all reddening corrections are
performed, the log slope of H2106-099 from the near IR (~12500 Angstrom) to the
UV (~1400 Angstrom), -0.94 +/- 0.05, is steep compared to other AGN, suggesting
that the blue bump in this object is intrinsically weak. Weak blue bumps are,
therefore, not always an artifact caused by reddening.Comment: Three parts: A. 33 pages text, B. one landscape table, C. 8 figure
Discovery of two simultaneous non-harmonically related Quasi-Periodic Oscillations in the 2005 outburst of the black-hole binary GRO J1655-40
We studied the low-frequency quasi-periodic oscillations (LFQPOs) in the
black hole GRO J1655-40 during the 2005 outburst, using data from the Rossi
X-ray Timing Explorer. All LFQPOs could be identified as either type B or type
C using previously proposed classification schemes. In the soft state of the
outburst the type-C LFQPOs reached frequencies that are among the highest ever
seen for LFQPOs in black holes. At the peak of the outburst, in the
ultra-luminous state, the power spectrum showed two simultaneous,
non-harmonically related peaks which we identified as a type-B and a type-C
QPO. The simultaneous presence of a type-C and type-B QPO shows that at least
two of the three known LFQPO types are intrinsically different and likely the
result of distinct physical mechanisms. We also studied the properties of a
broad peaked noise component in the power spectra of the ultra-luminous state.
This noise component becomes more coherent with count rate and there are strong
suggestions that it evolves into a type-B QPO at the highest observed count
rates.Comment: 14 pages, 6 figures, 2 tables, accepted for publication in MNRA
An Accretion-Jet Model for Black Hole Binaries: Interpreting the Spectral and Timing Features of XTE J1118+480
Multi-wavelength observations of the black hole X-ray binary XTE J1118+480
have offered abundant spectral and timing information about the source, and
have thus provided serious challenges to theoretical models. We propose a
coupled accretion-jet model to interpret the observations. We model the
accretion flow as an outer standard thin accretion disk truncated at a
transition radius by an inner hot accretion flow. The accretion flow accounts
for the observed UV and X-ray emission, but it substantially under-predicts the
radio and infrared fluxes, even after we allow for nonthermal electrons in the
hot flow. We attribute the latter components to a jet. We model the jet
emission by means of the internal shock scenario which is widely employed for
gamma-ray bursts. In our accretion-jet model of XTE J1118+480, the jet
dominates the radio and infrared emission, the thin disk dominates the UV
emission, and the hot flow produces most of the X-ray emission. The optical
emission has contributions from all three components: jet, thin disk, and hot
flow. The model qualitatively accounts for timing features, such as the
intriguing positive and negative time lags between the optical and X-ray
emission, and the wavelength-dependent variability amplitude.Comment: 27 pages, 4 figures (one in color); to appear in ApJ in Feb. 200
The X-ray Outburst of H1743-322: High-Frequency QPOs with a 3:2 Frequency Ratio
We observed the 2003 X-ray outburst of H1743-322 in a series of 130 pointed
observation with RXTE. We searched individual observations for high-frequency
QPOs (HFQPOs) and found only weak or marginal detections near 240 and 160 Hz.
We next grouped the observations in several different ways and computed the
average power-density spectra (PDS) in a search for further evidence of HFQPOs.
This effort yielded two significant results for those observations defined by
the presence of low-frequency QPOs (0.1-20 Hz) and an absence of
``band-limited'' power continua: (1) The 9 time intervals with the highest 7-35
keV count rates yielded an average PDS with a QPO at Hz. (; 3--35 keV); and (2) a second group with lower 7-35 keV count rates (26
intervals) produced an average PDS with a QPO at Hz (;
7--35 keV). The ratio of these two frequencies is . This finding
is consistent with results obtained for three other black hole systems that
exhibit commensurate HFQPOs in a 3:2 ratio. Furthermore, the occurrence of
H1743-322's slower HFQPO at times of higher X-ray luminosity closely resembles
the behavior of XTE J1550-564 and GRO J1655-40. We discuss our results in terms
of a resonance model that invokes frequencies set by general relativity for
orbital motions near a black-hole event horizon.Comment: 12 pages, 3 figures, submitted to Ap
Experimental Impacts of the Ongoing Assessment Project on Teachers and Students
In this report, we describe the results of a rigorous two-year study of the impacts of a mathematics initiative called Ongoing Assessment Project (OGAP) on teacher and student learning in grades 3-5 in two Philadelphia area school districts. OGAP is a mathematics program which combines teacher formative assessment practices with knowledge of student developmental progressions to build deeper student understanding of mathematics content. OGAP includes teacher professional development, classroom resources, school-based routines for regular practice, and ongoing school-based supports. The study was conducted in 61 schools during the 2014-15 and 2015-16 school years, with OGAP randomly assigned to 31 schools and the remaining 30 serving as comparison sites. The results of this study showed that OGAP produced meaningful impacts on both teacher knowledge and student learning
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