Engineering topics

Kouveliotou, Chryssa

Publications and source records attributed to Kouveliotou, Chryssa.

At least 73 records · Page 4

NASA NTRS2006

Understanding the Continuum Spectra of Short Soft Gamma Repeater Bursts

The spectra of short soft gamma repeater (SGR) bursts at photon energies above -15 keV are often well described by an optically thin thermal bremsstrahlung model (i.e., F(E) - E^-1 * exp(-E/kT) ) with kT=20-40 keV. However, the spectral shape burst continuum at lower photon energies (down to -2 keV) is not well established. It is important to better understand the SGR burst spectral properties at lower energies since inadequate description of the burst spectral continuum could lead to incorrect conclusions, such as existence of spectral lines. Here, we present detailed spectral investigations (in 2-200 keV) of 163 bursts from SGR 1806-20, all detected with Rossi X-ray Timing Explorer during the 2004 active episode that included the giant flare on 27 December 2004. We find that the great majority of burst spectra are well represented by the combination of a blackbody plus a OTTB models.

Gogus, Ersin↗

NASA NTRS2006

Characteristics of the Extended Burst Tails from SGR 1806-20 During the 2004 Active Episode

In 2004, SGR 1806-20 underwent an episode of extreme activities, exhibiting thousands of recurrent bursts and finally the 27 December giant flare, which was the most energetic extra-solar event ever detected near Earth. During this active episode, we routinely monitored the source with Rossi X-ray Timing Explorer and occasionally with Chandra. We identified two relatively strong event that were followed by extended (few hundreds-thousands seconds long) tail emission. Here, we present detailed spectral investigations of these two strong SGR 1806-20 events with extended tail observed about 6 and 1.5 months prior to the 27 December 2004 giant flare. We find that both extended burst tail spectra exhibit thermal behavior. Similar thermal spectral trend was also seen in two SGR 1900+14 bursts with extended tails, therefore, it may be a general characteristic of extended SGR events.

Gogus, Ersin↗

NASA NTRS2006

A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356

We report observations with the Chandra X-ray Observatory of the single, cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray observations of single white dwarfs, we also re-analyzed archival ROSAT data for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ 339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD 358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting the most restrictive upper limit to the X-ray luminosity from any cool white dwarf - Lx less than 6.0 x 10(exp 25) erg s(sup -1), at 99.7% confidence, for a 1- keV thermal-bremsstrahlung spectrum. The corresponding limit to the electron density is no less than 4.4x10(exp 11) per cubic centimeter. Our re-analysis of the archival data confirmed the non-detections reported by the original investigators. We discuss the implications of our and prior observations on models for coronal emission from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent constraints imposed by cyclotron radiation. In addition, we describe (in an appendix) a statistical methodology for detecting a source and for constraining the strength of a source, which applies even when the number of source or background events is small.

Weisskopf, Martin C.↗

NASA NTRS2005

A Deep Search with the Hubble Space Telescope for Late-Time Supernova Signatures in the Hosts of XRF 011030 and XRF 020427

X-ray flashes (XRFs) are, like gamma-ray bursts (GRBs), thought to signal the collapse of massive stars in distant galaxies. Many models posit that the isotropic equivalent energies of XRFs are lower than those for GRBs, such that they are visible fiom a reduced range of distances when compared with GRBs. Here we present the results of two-epoch Hubble Space Telescope imaging of two XRFs. These images, taken approximately 45 and 200 days postburst, reveal no evidence of an associated supernova in either case. Supernovae such as SN 1998bw would have been visible out to z approximately 1.5 in each case, while fainter supernovae such as SN 2002ap would have been visible to z approximately 1. If the XRFs lie at such large distances, their energies would not fit the observed correlation between the GRB peak energy and isotropic energy release (E(sub p) proportional to E(sub iso)(sup 1/2), in which soft bursts are less energetic. We conclude that, should these XRFs reside at low redshifts (z less than 0.6), either their line of sight is heavily extinguished, they are associated with extremely faint supernovae, or, unlike GRBs, these XRFs do not have temporally coincident supernovae.

Levan, Andrew↗

NASA NTRS2005

X-Ray Bursts from the Transient Magnetar Candidate XTE J1810-197

We have discovered four X-ray bursts, recorded with the Rossi X-ray Timing Explorer Proportional Counter Array between 2003 September and 2004 April, that we show to originate from the transient magnetar candidate XTE 51810-197. The burst morphologies consist of a short spike or multiple spikes lasting approx. 1 s each followed by extended tails of emission where the pulsed flux from XTE 51810-197 is significantly higher. The burst spikes are likely correlated with the pulse maxima, having a chance probability of a random phase distribution of 0.4%. The burst spectra are best fit to a blackbody with temperatures 4-8 keV, considerably harder than the persistent X-ray emission. During the X-ray tails following these bursts, the temperature rapidly cools as the flux declines, maintaining a constant emitting radius after the initial burst peak. The temporal and spectral characteristics of these bursts closely resemble the bursts seen from 1E 1048.1-5937 and a subset of the bursts detected from 1E 2259+586, thus establishing XTE J1810-197 as a magnetar candidate. The bursts detected from these three objects are sufficiently similar to one another, yet si,g&cantly differe2t from those seen from soft gamma repeaters, that they likely represent a new class of bursts from magnetar candidates exclusive (thus far) to the anomalous X-ray pulsar-like sources.

Kouveliotou, Chryssa↗

NASA NTRS2005

Magnetars

"Magnetars (Soft Gamma Repeaters and Anomalous X-ray Pulsars) are a subclass of neutron stars characterized by their recurrent X-ray bursts. While in an active (bursting) state, they are emitting hundreds of predominantly soft (kT=30 keV), short (0.1-100 ms long) events. Active states last anywhere between days and years. Their quiescent source X-ray light curves exhibit pulsations in the narrow range of 5-11 s; estimates of these rotational period rate changes (spin-down) indicate that their magnetic fields are extremely high, of the order of 10^14-10^15 G. Such high B-field objects, dubbed "magnetars", had been predicted to exist in 1992, but the first concrete observational evidence was obtained in 1998 for two of these sources. I will discuss here the recent spectacular results from SGR 1806-20, as well as the history of magnetars, and their spectral, timing and flux characteristics both in the persistent and their burst emission."

Kouveliotou, Chryssa↗

NASA NTRS2005

The Amazing SGR 1806-20

On 2004 December 27, the brightest gamma-ray flare ever recorded was detected by several instruments to be coming from the Soft Gamma Repeater SGR 1806-20. The flare even caused an ionospheric disturbance recorded around the globe by ionizing the Earth's upper atmosphere. SGRs belong to a class of slowly spinning (P = approx. 5-12 seconds) neutron stars with extremely high surface magnetic fields, B approx. 10(exp 15) G. The high magnetic fields of these objects were suggested by Duncan & Thompson, who called them "magnetars" to reflect the nature of the energy source that produces their quiescent and flaring X-ray luminosities. Very rarely, magnetars emit giant flares, extreme events with total energies typically approx. 10(exp 44) ergs, at least a factor of 1000 higher than the more frequent, repeating SGR bursts. Previously, only two giant flares have been recorded: from SGR 0526-66 in 1979 and from SGR 1900+14 in 1998. In this letter we report the detection of a very bright but rapidly fading radio afterglow that indicates a moderately relativistic expansion of plasma ejected from the magnetar. The radio afterglow is estimated to be at least 700 times brighter than that of the Aug 27 event, and is consistent with the hypothesis (also suggested by gamma-ray and X-ray data) that it was intrinsically a much more powerful event. I will discuss here the VLA results and their implications for the SGR flare energetics.

Kouveliotou, Chryssa↗

NASA NTRS2005

How Special Are Dark Gamma-Ray Burst: A Diagnostic Tool

We present here a comprehensive study of the optical/near-infrared (IR) upper limits for gamma-ray bursts that have an X-ray afterglow. We have extrapolated the X-ray afterglows to optical wavelengths based on the physics of the fireball blast wave model and compared these results with optical upper limits for a large sample of bursts. We find a small set of only three bursts out of a sample of 20 for which the upper limits are not compatible with their X-ray afterglow properties within the context of any blast wave model. This sparse sample does not allow us to conclusively determine the cause of this optical/near-IR deficit. Extinction in the host galaxy is a likely cause, but high redshifts and different afterglow mechanisms might also explain the deficit in some cases. We note that the three bursts appear to have higher than average gamma-my peak fluxes. In a magnitude versus time diagram the bursts are separated from the majority of bursts with a detected optical/near-IR afterglow. However, two gamma- ray bursts with an optical afterglow (one of which is highly reddened) also fall in this region with dark bursts, making it likely that dark bursts are at the faint end of the set of optically detected bursts, and therefore the dark bursts likely form a continuum with the bursts with a detected optical afterglow. Our work provides a useful diagnostic tool for follow-up observations for potentially dark bursts; applied to the events detected with the Swift satellite, it will significantly increase our sample of truly dark bursts and shed light upon their nature. Subject headings: dust, extinction - gamma rays: bursts Online material: machine-readable tables

Rod, Evert↗

NASA NTRS2005

Discovery of X-ray and Optical/Infrared Counterpart of XTE J1906+09

We present the Chandra localization of the transient X-ray source XTE J1906+09 and the discovery of optical and infrared counterparts. We analysed the archival Chandra ACIS-I observations of the region and determined the precise source position, that is 8$\arcmin$ away from position determined with the RXTE/PCA. We performed optical observations of the I field around the new location and identified a coincident ~ optical source (R=18.7). There exists an infrared point source in the Two Micron All Sky Survey catalogue with J=15.2, H=14.2 and K=13.5 whose location is also coincident with our Chandra and optical astrometry solutions. We additionally comment on the likely Be/X-ray binary nature of the system.

Gogus, Ersin↗

NASA NTRS2005

Observations of Soft Gamma Repeaters

Magnetars (Soft Gamma Repeaters and Anomalous X-ray Pulsars) are a subclass of neutron stars characterized by their recurrent X-ray bursts. While in an active (bursting) state (lasting anywhere between days and years), they are emitting hundreds of predominantly soft (kl'=30 kev), short (0.1 - 100 ms long) events. Their quiescent source X-ray light curves exhibit pulsations in the narrow range of 5-1 1 s; estimates of these rotational period rate changes (spin-down) indicate that their magnetic fields are extremely high, of the order of 10A14-10A15 G. Such high B-field objects, dubbed "magnetars", had been predicted to exist in 1992, but the first concrete observational evidence was obtained in 1998 for two of these sources. Very recently, SGR1806-20 emitted a giant flare, which was detected in the radio with a multitude of telescopes under an extensive international campaign. These observations have revealed exciting new results, never seen before in any of the other magnetar sources. I will discuss here these results and their relevance to our understanding of the nature of magnetars.

Kouveliotou, Chryssa↗

NASA NTRS2005

Magnetars

Magnetars (Soft Gamma Repeaters and Anomalous X-ray Pulsars) are a subclass of neutron stars characterized by their recurrent X-ray bursts. While in an active (bursting) state, they are emitting hundreds of predominantly soft (kT=30 keV), short (0.1-100 ms long) events. Active states last anywhere between days and years. Their quiescent source X-ray light curves exhibit pulsations in the narrow range of 5-11 s; estimates of their rotational period rate changes (spin-down) indicate that their magnetic fields are extremely high, of the order of 10^14-10^15 G. Such high B-field objects, dubbed "magnetars", had been predicted to exist in 1992, but the first ccncrete observational evidence was obtained in 1998 for two of these sources. I will discuss here the recent spectacular results from SGR 1806-20, as well as the history of magnetars, and their spectral, timing and flux characteristics both in the persistent and their burst emission.

Kouveliotou, Chryssa↗

NASA NTRS2005

Tomography of a Gamma-ray Burst Progenitor and its Host Galaxy

We have obtained near-infrared and high-resolution optical spectroscopy of the bright afterglow of the very intense gamma-ray burst recorded on 2002, October 4 (GRB 021004). Besides of line emission in the near-IR allowing an independent measurement of the systemic redshift (z = 2.3304 plus or minus 0.0005), we find several absorption line groups spanning a range of about 3,000 kilometers per second in velocity relative to the redshift of the host galaxy. The absorption profiles are very complex with both velocity-broadened components extending over several 100 kilometers per second and narrow lines with velocity widths of only approximately 20 kilometers per second. By analogy with QSO absorption line studies, the relative velocities, widths, and degrees of ionization of the lines ("line-locking", "ionization-velocity correlation") show that the progenitor had both an extremely strong radiation field and several distinct mass loss phases (winds). These results are consistent with GRB progenitors being massive stars, such as Luminous Blue Variables (LBVs) or Wolf-Rayet stars, providing a detailed picture of the spatial and velocity structure of the GRB progenitor star at the time of explosion. The host galaxy is a prolific star-forming galaxy with a SFR of approximately 10 solar mass yr(sup -l).

Castro-Tirado, Alberto J.↗

NASA NTRS2004

An Off-Axis Model for GRB 031203

The low luminosity radio emission of the unusually faint GRB 031203 has been argued to support the idea of a class of intrinsically sub-energetic gamma-ray bursts (GRBs), currently comprising two members. While low energy GRBs probably exist, we show that the collective prompt and multiwavelength observations of the afterglow of GRB 031203, do not necessarily require a sub-energetic nature for that event. In fact, the data are more consistent with a typical, powerful GRB seen at an angle of about twice the opening angle of the central jet. The (redshift corrected) peak energy, E(sub p), of GRB 031203 then becomes approximately 2 MeV, similar to many other GRBs.

Ramirez-Ruiz, Enrico↗

NASA NTRS2004

Precise Localization of the Soft Gamma Repeater SGR 1627-41 with Chandra and the Anomalous X-Ray Pulsar AXP 1E1841-045 with Chandra

We present precise localizations of AXP 1E184-045 and SGR 1627-41 with Chandra. We obtained new infrared observations of SGR 1627-41 and reanalyzed archival observations of AXP 1E1841-045 in order to refine their positions and search for infrared counterparts. A faint source is detected inside the error circle of AXP 1E1841-045. In the case of SGR 1627-41, several sources are located within the error radius of the X-ray position, and we discuss the likelihood of one of them being the counterpart. We compare the properties of our candidates to those of other known anomalous X-ray pulsar (AXP) and soft gamma repeater (SGR) counterparts. We find that the counterpart candidates for SGR 1627-41 and SGR 1806-20 would have to be intrinsically much brighter than AXPs in order to have counterparts detectable with the observational limits currently available for these sources. To confirm the reported counterpart of SGR 1806-20, we obtained new infrared observations during the 2003 July burst activation of the source. No brightening of the suggested counterpart is detected, implying that the counterpart of SGR 1806-20 remains yet to be identified.

Wachter, Stefanie↗

NASA NTRS2004

Long-Term Spectral and Timing Behavior of the Black Hole Candidate XTE J1908+094

We present the long-term X-ray light curves and detailed spectral and timing analyses of XTE J1908+094 using the Rossi X-Ray Timing Explorer Proportional Counter Array observations covering two outbursts in 2002 and early 2003. At the onset of the first outburst, the source was found in a spectrally low/hard state lasting for approx.40 days, followed by a 3 day long transition to the high/soft state. The source flux (in 2- 10 keV) reached approx.100 mcrab on 2002 April 6, then decayed rapidly. In power spectra, we detect strong band-limited noise and varying low- frequency quasi-periodic oscillations that evolved from approx.0.5 to approx.5 Hz during the initial low/hard state of the source. We find that the second outburst closely resembled the spectral evolution of the first. The X-ray transient s overall outburst characteristics led us to classify XTE J1908+094 as a black hole candidate. Here we also derive precise X-ray position of the source using Chandra observations that were performed during the decay phase of the first outburst and following the second outburst.

Gogus, Ersin↗

NASA NTRS2004

Observations of X-ray Afterglows

Discussion of X-ray studies of the environs of Gamma Ray Burst (GRB) associated Supernovae using the Chandra and XMM X-Ray Observatories, in particular the long term evolution of their X-ray afterglow lightcurves. For SN 1998bw, we have combined all available observations and derived a smooth X-ray light curve spanning -1300 days, assuming that GRB 980425 and SN 1998bw were coincident at 35.6 Mpc. Perform comparison of this X-ray light curve with those of the X-ray afterglows of ordinary GRBs, X-ray Flashes, and ordinary supernovae, and discuss the emerging evidence for at least two classes of lightcurves, perhaps bounding a continuum. By three to ten years, all these phenomena seem to converge on a common X-ray luminosity, possibly indicative of the supernova underlying them all. One possible explanation for the two classes is a (nearly) standard GRB observed at different angles. If this interpretation is correct, X-ray afterglows with intermediate luminosities should eventually be discovered; GRB 03 1203 maybe just such a case.

Kouveliotou, Chryssa↗

NASA NTRS2004

A Deep Search with HST for Late Time Supernova Signatures in the Hosts of XRF 011030 and XRF 020427

X-ray Flashes (XRFs), are, like Gamma-Ray Bursts (GRBs) thought to signal the collapse of massive stars in distant galaxies. Many models posit that the isotropic equivalent energies of XRFs are lower than those for GRBs, such that they are visible hom a reduced range of distances when compared with GRBs. Here we present the results of two epoch Hubble Space Telescope imaging of two XRFs. These images taken approximately 45 and 200 days post bust reveal no evidence for an associated supernova in either case. Supernovae such as SN 1998bw would have been visible out to z approximately 1.5 in each case, while faint supernovae such as SN 2002ap would be visible to z approximately 1. At these distances the bursts would not fit the observed correlations between E(sub p) and E(sub iso) and would have required extremely luminous X-ray afterglows. We conclude that should these XRFs reside at low redshift, it is necessary either that their line of sight is heavily extinguished, or that XRFs, unlike GRBs do not have temporally coincident supernovae.

Patel, Sandeep↗

NASA NTRS2004

Infrared and Optical Observations of GRB 030115 and its Extremely Red Host Galaxy: Implications for Dark Bursts

We present near-infrared (a) and optical observations of the afterglow of GRB 030115. Discovered in an infrared search at Kitt Peak 5 hours after the burst trigger, this afterglow is the faintest ever observed in the R-band at such an early epoch, and exhibits very red colors, with R-K approximately equal to 6. The magnitude of the optical afterglow of GRB 030115 is fainter than many upper limits for other bursts, suggesting that without early nIR observations it would have been classified as a "dark" burst. Both the color and optical magnitude of the afterglow are likely due to dust extinction and indicate that at least some optical afterglows are observations were also taken of the host galaxy and the surrounding field. Photometric redshifts imply that the host, and a substantial number of faint galaxies in the field are at z approximately 2.5. The overdensity of galaxies is sufficiently great that GRB 030115 may have occurred in a rich high-redshift cluster. The host galaxy shows extremely red colors (R-K=5) and is the first GRB host to be classified as an Extreme Red Object (ERO). Some of the galaxies surrounding the host also show very red colors, while the majority of the cluster are much bluer, indicating ongoing unobscured star formation. As it is thought that much of high redshift star formation occurs in highly obscured environments it may well be that GRB 030115 represents a transition object, between the relatively unobscured afterglows seen to date and a population which are very heavily extinguished, even in the nIR.

Levan, Andrew↗