408 research outputs found
Inner Disk Structure of Dwarf Novae in the Light of X-ray Observations
Diversity of the X-ray observations of dwarf nova are still not fully
understood. I review the X-ray spectral characteristics of dwarf novae during
the quiescence in general explained by cooling flow models and the outburst
spectra that show hard X-ray emission dominantly with few sources that reveal
soft X-ray/EUV blackbody emission. The nature of aperiodic time variability of
brightness of dwarf novae shows band limited noise, which can be adequately
described in the framework of the model of propagating fluctuations. The
frequency of the break (1-6 mHz) indicates inner disk truncation of the
optically thick disk with a range of radii (3.0-10.0)10 cm. The
RXTE and optical (RTT150) data of SS Cyg in outburst and quiescence reveal that
the inner disk radius moves towards the white dwarf and receeds as the outburst
declines to quiescence. A preliminary analysis of SU UMa indicates a similar
behaviour. In addition, I find that the outburst spectra of WZ Sge shows two
component spectrum of only hard X-ray emission, one of which may be fitted with
a power law suggesting thermal Comptonization occuring in the system.
Cross-correlations between the simultaneous UV and X-ray light curves
(XMM-Newton) of five DNe in quiescence show time lags in the X-rays of 96-181
sec consistent with travel time of matter from a truncated inner disk to the
white dwarf surface. All this suggests that dwarf novae and other plausible
nonmagnetic systems have truncated accretion disks indicating that the disks
may be partially evaporated and the accretion may occur through hot (coronal)
flows in the disk.Comment: 7 pages and 10 figures, accepted for publication in Acta Polytechnica
as proceedings of the 2013 workshop on "The Golden Age of Cataclysmic
Variables and Related Objects - II", Franco Giovannelli & Lola Sabau-Graziati
(eds.
A search for periodicities from a ULX in the LINER galaxy NGC 4736
We report our findings on a new quasi-periodic oscillation (QPO) and a long
period from the ultraluminous X-ray source (ULX) X-2 in nearby galaxy NGC 4736
based on the Chandra and XMM-Newton archival data. To examine the timing
properties, power density spectra of the source have been obtained using Fast
Fourier Transform. Also the spectral parameters of the source have been
calculated by obtaining and fitting the energy spectra. Power density spectrum
of this source reveals a QPO peak at mHz with an
fractional rms variability of 16% using the Chandra data (in the year
2000-lower state of the source). The XMM-Newton data analysis indicates a peak
at mHz with a fractional rms variation of 5% (in the
year 2006-higher state of the source). These recovered QPOs overlap within
errors and may be the same oscillation. In addition, we detect a long
periodicity or a QPO in the Chandra data of about Hz
( 5.4 hrs) over 3 confidence level. If this is a QPO, it is the
lowest QPO detected from a ULX. The mass of the compact object in ULX X-2 is
estimated using the Eddington luminosity and a disk blackbody model in the
range (1080) M_{\sun}.Comment: 6 pages, 4 Figures; Accepted for publication in Astrophysics and
Space Scienc
Data transfer scheduling with advance reservation and provisioning
Over the years, scientific applications have become more complex and more data intensive. Although through the use of distributed resources the institutions and organizations gain access to the resources needed for their large-scale applications, complex middleware is required to orchestrate the use of these storage and network resources between collaborating parties, and to manage the end-to-end processing of data. We present a new data scheduling paradigm with advance reservation and provisioning. Our methodology provides a basis for provisioning end-to-end high performance data transfers which require integration between system, storage and network resources, and coordination between reservation managers and data transfer nodes. This allows researchers/users and higher level meta-schedulers to use data placement as a service where they can plan ahead and reserve time and resources for their data movement operations. We present a novel approach for evaluating time-dependent structures with bandwidth guaranteed paths. We present a practical online scheduling model using advance reservation in dynamic network with time constraints. In addition, we report a new polynomial algorithm presenting possible reservation options and alternatives for earliest completion and shortest transfer duration. We enhance the advance network reservation system by extending the underlying mechanism to provide a new service in which users submit their constraints and the system suggests possible reservation requests satisfying users\u27 requirements. We have studied scheduling data transfer operation with resource and time conflicts. We have developed a new scheduling methodology considering resource allocation in client sites and bandwidth allocation on network link connecting resources. Some other major contributions of our study include enhanced reliability, adaptability, and performance optimization of distributed data placement tasks. While designing this new data scheduling architecture, we also developed other important methodologies such as early error detection, failure awareness, job aggregation, and dynamic adaptation of distributed data placement tasks. The adaptive tuning includes dynamically setting data transfer parameters and controlling utilization of available network capacity. Our research aims to provide a middleware to improve the data bottleneck in high performance computing systems
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