arXiv subjects

E. Kalemci

Publications and source records attributed to E. Kalemci.

28 records · Page 2Linked to original sources

arXiv2005

Gamma-Ray Polarimetry of Two X-Class Solar Flares

We have performed the first polarimetry of solar flare emission at gamma-ray energies (0.2-1 MeV). These observations were performed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) for two large flares: the GOES X4.8-class solar flare of 2002 July 23, and the X17-class flare of 2003 October 28. We have marginal polarization detections in both flares, at levels of 21% +/- 9% and -11% +/- 5% respectively. These measurements significantly constrain the levels and directions of solar flare gamma-ray polarization, and begin to probe the underlying electron distributions.

astro-ph↗

arXiv2004

Multi-wavelength observations of the Galactic black hole transient 4U1543-47 during outburst decay: state transitions and jet contribution

Multiwavelength observations of Galactic black hole (GBH) transients during the state transitions and in the low/hard state may provide detailed information on the accretion structure of these systems. 4U1543-47 is a GBH transient that was covered exceptionally well in X-ray and infrared (daily observations) and reasonably well in optical and radio during its outburst decay in 2002. When all the available information is gathered in the intermediate and the low/hard state, 4U1543-47 makes an important contribution to our understanding of state transitions and the role of outflows on the high energy emission properties of black hole binaries. The evolution of the X-ray spectral and temporal properties and the IR light curve place strong constraints on different models for explaining the overall emission from accreting black holes. The overall spectral energy distribution is consistent with synchrotron origin for the optical and infrared emission, however, the X-ray flux is above the power-law continuation of the optical and infrared flux. The infrared light curve, the HEXTE light curve and the evolution of the X-ray photon index indicate that the major source of hard X-rays cannot be direct synchrotron radiation from an acceleration region in a jet for most of the outburst decay.

astro-ph↗

arXiv2004

An X-ray Timing Study of XTE J1550--564: Evolution of the Low Frequency QPO for the Complete 2000 Outburst

We report on RXTE observations of the microquasar XTE J1550 during a ~70 day outburst in April-June 2000. We focus here on the temporal properties of the source and study the behavior of low frequency (0.1-10 Hz) quasi periodic oscillations (LFQPO), which seem to be of different types. We focus on the so-called type C (according to the classification of Remillard and collaborators), which corresponds to a strong 0.1-6 Hz LFQPO, found to be present during at least 17 observations. We find that the frequency of the QPO is better correlated with the soft X-ray (< 7 keV) flux than with the hard flux (> 7 keV). If soft X-rays represent the behavior of an accretion disk, the relation shows that the disk may set the LFQPO frequency. In two cases, the identification of the type of QPO is not straightforward. If the QPOs in those two cases are type A (or B), then we may be seeing the QPO type alternate between type C and type A (or B), and this may represent some rapid changes in the physical properties of the accretion flow, before the system stabilizes and slowly decays toward the end of the outburst. On the other hand, if all the QPOs are of type C, we may be observing an inversion in the frequency vs. flux relation, similar to that seen in GRO J1655-40. We discuss the QPO behavior in the framework of theoretical models.

astro-ph↗

arXiv2003

Transitions of black hole transients to the low/hard state under the microscope

We characterized the evolution of spectral and temporal properties of several Galactic black hole transients observed between 1996-2001 using the data from well sampled PCA observations close to the transition to the low/hard state. We showed that the changes in temporal properties are much sharper than the changes in the spectral properties, and it is much easier to identify a state transition with the temporal properties. The ratio of the power-law flux to the total flux in the 3-25 keV band increases close to the transition, and the power-law flux shows a sharp increase along with the changes in temporal properties during the transitions. In this work we concentrate on the decay of two recent outbursts, from 4U 1543$-$47, and H1743$-$322 and discuss the state transitions by tracking their daily, and sometimes hourly evolution, and interpret results based on the expectations from our earlier observations.

astro-ph↗

arXiv2003

A close look at the state transitions of Galactic black hole transients during outburst decay

We characterize the evolution of spectral and temporal properties of several Galactic black hole transients during outburst decay using the data from well sampled PCA/\rxte observations close to the transition to the low/hard state. We find several global patterns of evolution for spectral and temporal parameters before, during, and after the transition. We show that the changes in temporal properties (sudden increase or decrease in the rms amplitude of variability) are much sharper than the changes in the spectral properties, and it is much easier to identify a state transition with the temporal properties. The spectral index shows a drop 3-5 days before the transition for some of our sources. The ratio of the power-law flux to the total flux in the 3-25 keV band increases close to the transition, which may mean that the system must be dominated by the coronal emission for the transition to occur. We also show that the power-law flux shows a sharp change along with the temporal properties during the transitions which may indicate a threshold transition volume for the corona. The evolution of the spectral and temporal properties after the transition is consistent with the idea that the inner accretion disk moves away from the black hole. Based on the evolution of spectral and temporal parameters and changes during the transitions, we discuss possible scenarios of how the transition is happening.

astro-ph↗

arXiv2002

Evolution of a low frequency QPO during the 2000 outburst of XTE J1550-564

We follow the evolution of a low frequency QPO during the 2000 outburst of the microquasar XTE J1550-564, which was found to be present in the PCA energy range (2-65 keV) in 19 of 43 observations. The frequency of the QPO varies from 0.1 Hz to 6 Hz, and appears to follow the evolution of the soft X-ray flux. If we assume the soft X-rays represent the behavior of an accretion disk, the relation indicates that this low frequency QPO is linked to the accretion disk. We show that the non-trivial relation between the QPO frequency and the soft flux may be as expected from the Accretion Ejection Instability (AEI), when the disk approaches its last stable orbit. Furthermore, the energy dependence of the QPO may indicate the presence of a hot spot rotating in the disk as predicted by the AEI.

astro-ph↗

arXiv2002

X-ray temporal properties of XTE J1650-500 during outburst decay

We investigated the temporal behavior of the new black hole transient XTE J1650-500 with the Rossi X-ray Timing Explorer (RXTE) as the source made a transition to the low/hard state during the decay of the 2001 outburst. We find QPOs in the 4 - 9 Hz range, enhanced time lags and reduced coherence during the state transition. We also observe a shift in the peak frequency of the noise component with energy during the transition. The evolution of the power spectrum as well as the lag and coherence behavior during the state transition are similar to the state transitions observed for other black hole sources, especially Cyg X-1. The temporal properties during the transition to the low state put constraints on the accretion geometry of XTE J1650-500 and may have implications for all black hole binary systems. We suggest a possible geometry and evolution of a jet+corona+disk system based on enhanced lags and peak frequency shift during the transition.

astro-ph↗

arXiv2002

RXTE observations of XTE J1550-564 during its 2000 outburst

We report on timing and spectral observations of the microquasar XTE J1550-564 during its 2000 outburst made by the Rossi X-ray Timing Experiment (RXTE). We study the spectral properties of the source during both the rise to the outburst and its decline. We observe transitions from a low hard state to an intermediate state, and vice versa, before the source returns to quiescence. We show that the first transition likely reflects a change in the relative importance of the emitting media, instead of a change in the total accretion rate, contrary to the second transition. We investigate the temporal properties of the source, and follow the evolution of a low frequency Quasi Periodic Oscillations (QPO).

astro-ph↗

arXiv2001

Investigation of charge sharing among electrode strips for a CdZnTe detector

We have investigated charge sharing among the anode strips of a CdZnTe (CZT) detector using a 30 micrometer collimated gamma-ray beam. We compared the laboratory measurements with the predictions from our modeling of the charge transport within the detector. The results indicate that charge sharing is a function of the interaction depth and the energy of the incoming photon. Also, depending on depth, a fraction of the electrons might drift to the inter-anode region causing incomplete charge collection. Here, we show that photoelectron range and diffusion of the charge cloud are the principal causes of charge sharing and obtain limits on the size of the electron cloud as a function of position in the detector.

astro-ph↗

arXiv2001

X-ray states and radio emission in the black hole candidate XTE J1550-564

We report on radio and X-ray observations of the black hole candidate (BHC) XTE J1550-564 performed during its 2000 X-ray outburst. Observations have been conducted with the Australia Telescope Compact Array (ATCA) and have allowed us to sample the radio behavior of XTE J1550-564 in the X-ray Low Hard and Intermediate/Very High states. We observed optically thin radio emission from XTE J1550-564 five days after a transition to an Intermediate/Very High state, but we observed no radio emission six days later, while XTE J1550-564 was still in the Intermediate/Very High state. In the Low Hard state, XTE J1550-564 is detected with an inverted radio spectrum. The radio emission in the Low Hard state most likely originates from a compact jet; optical observations suggest that the synchrotron emission from this jet may extend up to the optical range. The total power of the compact jet might therefore be a significant fraction of the total luminosity of the system. We suggest that the optically thin synchrotron radio emission detected five days after the transition to the Intermediate/Very High state is due to a discrete ejection of relativistic plasma during the state transition. Subsequent to the decay of the optically thin radio emission associated with the state transition, it seems that in the Intermediate/Very High state the radio emission is quenched by a factor greater than 50, implying a suppression of the outflow. We discuss the properties of radio emission in the X-ray states of BHCs.

astro-ph↗