Search results for " X-rays"
showing 10 items of 234 documents
Discovery of 105 Hz coherent pulsations in the ultracompact binary IGR J16597-3704
2018
We report the discovery of X-ray pulsations at 105.2 Hz (9.5 ms) from the transient X-ray binary IGR J16597-3704 using NuSTAR and Swift. The source was discovered by INTEGRAL in the globular cluster NGC 6256 at a distance of 9.1 kpc. The X-ray pulsations show a clear Doppler modulation implying an orbital period of ~46 minutes and a projected semi-major axis of ~5 lt-ms, which makes IGR J16597-3704 an ultra-compact X-ray binary system. We estimated a minimum companion mass of 0.0065 solar masses, assuming a neutron star mass of 1.4 solar masses, and an inclination angle of <75 degrees (suggested by the absence of eclipses or dips in its light-curve). The broad-band energy spectrum of the…
An XMM-Newton and INTEGRAL view on the hard state of EXO 1745-248 during its 2015 outburst
2017
CONTEXT - Transient low-mass X-ray binaries (LMXBs) often show outbursts lasting typically a few-weeks and characterized by a high X-ray luminosity ($L_{x} \approx 10^{36}-10^{38}$ erg/sec), while for most of the time they are found in X-ray quiescence ($L_X\approx10^{31} -10^{33}$ erg/sec). EXO 1745-248 is one of them. AIMS - The broad-band coverage, and the sensitivity of instrument on board of {\xmm} and {\igr}, offers the opportunity to characterize the hard X-ray spectrum during {\exo} outburst. METHODS - In this paper we report on quasi-simultaneous {\xmm} and {\igr} observations of the X-ray transient {\exo} located in the globular cluster Terzan 5, performed ten days after the begin…
The role of radiative losses in the late evolution of pulse-heated coronal loops/strands
2012
Radiative losses from optically thin plasma are an important ingredient for modeling plasma confined in the solar corona. Spectral models are continuously updated to include the emission from more spectral lines, with significant effects on radiative losses, especially around 1 MK. We investigate the effect of changing the radiative losses temperature dependence due to upgrading of spectral codes on predictions obtained from modeling plasma confined in the solar corona. The hydrodynamic simulation of a pulse-heated loop strand is revisited comparing results using an old and a recent radiative losses function. We find significant changes in the plasma evolution during the late phases of plas…
An iterative method in a probabilistic approach to the spectral inverse problem - Differential emission measure from line spectra and broadband data
2010
Inverse problems are of great importance in astrophysics for deriving information about the physical characteristics of hot optically thin plasma sources from their EUV and X-ray spectra. We describe and test an iterative method developed within the framework of a probabilistic approach to the spectral inverse problem for determining the thermal structures of the emitting plasma. We also demonstrate applications of this method to both high resolution line spectra and broadband imaging data. Our so-called Bayesian iterative method (BIM) is an iterative procedure based on Bayes' theorem and is used to reconstruct differential emission measure (DEM) distributions. To demonstrate the abilities …
The Role Of General Relativity in the Evolution of Low-Mass X-ray Binaries
2005
We study the evolution of Low Mass X-ray Binaries (LMXBs) and of millisecond binary radio pulsars (MSPs), with numerical simulations that keep into account the evolution of the companion, of the binary system and of the neutron star. According to general relativity, when energy is released, the system loses gravitational mass. Moreover, the neutron star can collapse to a black hole if its mass exceeds a critical limit, that depends on the equation of state. These facts have some interesting consequences: 1) In a MSP the mass-energy is lost with a specific angular momentum that is smaller than the one of the system, resulting in a positive contribution to the orbital period derivative. If th…
Chandra Observation of the Dipping Source XB 1254-690
2006
We present the results of a 53 ks long Chandra observation of the dipping source XB 1254--690. During the observation neither bursts or dips were observed. From the zero-order image we estimated the precise X-ray coordinates of the source with a 90% uncertainty of 0.6\arcsec. Since the lightcurve did not show any significant variability, we extracted the spectrum corresponding to the whole observation. We confirmed the presence of the \ion{Fe}{xxvi} K$_\alpha$ absorption lines with a larger accuracy with respect to the previous XMM EPIC pn observation. Assuming that the line width were due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines were p…
Effects of Flaring Activity on Dynamics of Accretion Disks in YSOs
2009
We investigate the effects of strong flares on the accretion phenomena in YSOs. Among all classical assumptions, the model accounts magnetic-field oriented thermal conduction. We study the global dynamics of the system for two positions of the heating release triggering the flare.
Disappearance of Hard X-Ray Emission in the Last BeppoSAX Observation of the Z Source GX 349+2
2003
We report on the results from two BeppoSAX observations of the Z source GX 349+2 performed in February 2001 and covering the broad energy range 0.12-200 keV. The light curve obtained from these observations shows a large flaring activity, the count rate varying from ~130 to ~260 counts/s, indicating that the source was in the flaring branch during these observations. The average spectrum is well described by a soft blackbody and a Comptonized component. To well fit the energy spectrum three gaussian lines are needed at 1.2 keV, 2.6 keV, and 6.7 keV with corresponding equivalent widths of 13 eV, 10 eV, and 39 eV, probably associated to L-shell emission of Fe XXIV, Ly-alpha S XVI, and Fe XXV,…
Does GRS 1915+105 exhibit "canonical" black-hole states?
2003
We have analysed RXTE data of the superluminal source GRS 1915+105 in order to investigate if, despite its extreme variability, it also exhibits the canonical source states that characterise other black-hole candidates. The phenomenology of GRS 1915+105 has been described in terms of three states (named A, B and C) based on their hardness ratios and position in the colour-colour diagram. We have investigated the connection between these states and the canonical behaviour and found that the shape of the power spectral continuum and the values of the best-fit model parameters to the noise components in all three states indicate that the source shows properties similar to the canonical very hi…
Fe Kα and Hydrodynamic Loop Model Diagnostics for a Large Flare on II Pegasi
2008
The observation by the Swift X-ray Telescope of the Fe K alpha_1, alpha_2 doublet during a large flare on the RS CVn binary system II Peg represents one of only two firm detections to date of photospheric Fe K alpha from a star other than our Sun. We present models of the Fe K alpha equivalent widths reported in the literature for the II Peg observations and show that they are most probably due to fluorescence following inner shell photoionisation of quasi-neutral Fe by the flare X-rays. Our models constrain the maximum height of flare the to 0.15 R_* assuming solar abundances for the photospheric material, and 0.1 R_* and 0.06 R_* assuming depleted photospheric abundances ([M/H]=-0.2 and […