arXiv subjects

Chryssa Kouveliotou

Publications and source records attributed to Chryssa Kouveliotou.

At least 91 records · Page 5Linked to original sources

arXiv2011

Extended Tails from SGR 1806-20 Bursts

In 2004, SGR 1806-20 underwent a period of intense and long-lasting burst activity that included the giant flare of 27 December 2004 -- the most intense extra-solar transient event ever detected at Earth. During this active episode, we routinely monitored the source with Rossi X-ray Timing Explorer and occasionally with Chandra. During the course of these observations, we identified two relatively bright bursts observed with Konus-Wind in hard X-rays that were followed by extended X-ray tails or afterglows lasting hundreds to thousands of seconds. Here, we present detailed spectral and temporal analysis of these events observed about 6 and 1.5 months prior to the 27 December 2004 Giant Flare. We find that both X-ray tails are consistent with a cooling blackbody of constant radius. These spectral results are qualitatively similar to those of the burst afterglows recorded from SGR 1900+14 and recently from SGR 1550-5418. However, the latter two sources exhibit significant increase in their pulsed X-ray intensity following the burst, while we did not detect any significant changes in the RMS pulsed amplitude during the SGR 1806-20 events. Moreover, we find that the fraction of energy partitioned to the burst (prompt energy release) and the tail (afterglow) differs by an order of magnitude between SGR 1900+14 and SGR 1806-20. We suggest that such differences can be attributed to differences in the crustal heating mechanism of these neutron stars combined with the geometry of the emitting areas.

astro-ph.HE↗

arXiv2011

Fermi/GBM Observations of SGR J0501+4516 Bursts

We present our temporal and spectral analyses of 29 bursts from SGR J0501+4516, detected with the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope during the 13 days of the source activation in 2008 (August 22 to September 3). We find that the T90 durations of the bursts can be fit with a log-normal distribution with a mean value of ~ 123 ms. We also estimate for the first time event durations of Soft Gamma Repeater (SGR) bursts in photon space (i.e., using their deconvolved spectra) and find that these are very similar to the T90s estimated in count space (following a log-normal distribution with a mean value of ~ 124 ms). We fit the time-integrated spectra for each burst and the time-resolved spectra of the five brightest bursts with several models. We find that a single power law with an exponential cutoff model fits all 29 bursts well, while 18 of the events can also be fit with two black body functions. We expand on the physical interpretation of these two models and we compare their parameters and discuss their evolution. We show that the time-integrated and time-resolved spectra reveal that Epeak decreases with energy flux (and fluence) to a minimum of ~30 keV at F=8.7e-6 erg/cm2/s, increasing steadily afterwards. Two more sources exhibit a similar trend: SGRs J1550-5418 and 1806-20. The isotropic luminosity corresponding to these flux values is roughly similar for all sources (0.4-1.5 e40 erg/s).

astro-ph.HE↗

arXiv2011

The missing link: Merging neutron stars naturally produce jet-like structures and can power short Gamma-Ray Bursts

Short Gamma-Ray Bursts (SGRBs) are among the most luminous explosions in the universe, releasing in less than one second the energy emitted by our Galaxy over one year. Despite decades of observations, the nature of their "central-engine" remains unknown. Considering a binary of magnetized neutron stars and solving Einstein equations, we show that their merger results in a rapidly spinning black hole surrounded by a hot and highly magnetized torus. Lasting over 35 ms and much longer than previous simulations, our study reveals that magnetohydrodynamical instabilities amplify an initially turbulent magnetic field of ~ 10^{12} G to produce an ordered poloidal field of ~ 10^{15} G along the black-hole spin-axis, within a half-opening angle of ~ 30 deg, which may naturally launch a relativistic jet. The broad consistency of our ab-initio calculations with SGRB observations shows that the merger of magnetized neutron stars can provide the basic physical conditions for the central-engine of SGRBs.

astro-ph.HE↗

arXiv2011

When A Standard Candle Flickers

The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma ray telescopes. Although small-scale variations in the nebula are well-known, since the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in August 2008, a ~ 7% (70 mcrab) decline has been observed in the overall Crab Nebula flux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline is independently confirmed with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the INTErnational Gamma-Ray Astrophysics Laboratory Imager on Board INTEGRAL (IBIS). A similar decline is also observed in the ~3 - 15 keV data from the RXTE/PCA and INTEGRAL Joint European Monitor (JEM-X) and in the 50 - 100 keV band with GBM and INTEGRAL/IBIS. Observations from 100 to 500 keV with GBM suggest that the decline may be larger at higher energies. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ~3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in April 2007. As of August 2010, the current flux has declined below the 2007 minimum.

astro-ph.HE↗

arXiv2011

First-year Results of Broadband Spectroscopy of the Brightest Fermi-GBM Gamma-Ray Bursts

We present here our results of the temporal and spectral analysis of a sample of 52 bright and hard gamma-ray bursts (GRBs) observed with the Fermi Gamma-ray Burst Monitor (GBM) during its first year of operation (July 2008-July 2009). Our sample was selected from a total of 253 GBM GRBs based on each event peak count rate measured between 0.2 and 40MeV. The final sample comprised 34 long and 18 short GRBs. These numbers show that the GBM sample contains a much larger fraction of short GRBs, than the CGRO/BATSE data set, which we explain as the result of our (different) selection criteria and the improved GBM trigger algorithms, which favor collection of short, bright GRBs over BATSE. A first by-product of our selection methodology is the determination of a detection threshold from the GBM data alone, above which GRBs most likely will be detected in the MeV/GeV range with the Large Area Telescope (LAT) onboard Fermi. This predictor will be very useful for future multiwavelength GRB follow ups with ground and space based observatories. Further we have estimated the burst durations up to 10MeV and for the first time expanded the duration-energy relationship in the GRB light curves to high energies. We confirm that GRB durations decline with energy as a power law with index approximately -0.4, as was found earlier with the BATSE data and we also notice evidence of a possible cutoff or break at higher energies. Finally, we performed time-integrated spectral analysis of all 52 bursts and compared their spectral parameters with those obtained with the larger data sample of the BATSE data. We find that the two parameter data sets are similar and confirm that short GRBs are in general harder than longer ones.

astro-ph.HE↗

arXiv2011

A New Derivation of GRB Jet Opening Angles from the Prompt Gamma-Ray Emission

The jet opening angle of gamma-ray bursts (GRBs) is an important parameter for determining the characteristics of the progenitor, and the information contained in the opening angle gives insight into the relativistic outflow and the total energy that is contained in the burst. Unfortunately, a confident inference of the jet opening angle usually requires broadband measurement of the afterglow of the GRB, from the X-ray down to the radio and from minutes to days after the prompt gamma-ray emission, which may be difficult to obtain. For this reason, very few of all detected GRBs have constrained jet angles. We present an alternative approach to derive jet opening angles from the prompt emission of the GRB, given that the GRB has a measurable Epeak and fluence, and which does not require any afterglow measurements. We present the distribution of derived jet opening angles for the first two years of the Fermi Gamma-ray Burst Monitor (GBM) operation, and we compare a number of our derived opening angles to the reported opening angles using the traditional afterglow method. We derive the collimation-corrected gamma-ray energy, E_γ, for GRBs with redshift and find that some of the GRBs in our sample are inconsistent with a proto-magnetar progenitor. Finally, we show that the use of the derived jet opening angles results in a tighter correlation between the rest-frame Epeak and E_γ than has previously been presented, which places long GRBs and short GRBs onto one empirical power law.

astro-ph.HE↗

arXiv2011

Long Term Radiative Behavior of SGR 1900+14

The prolific magnetar SGR 1900+14 showed two outbursts in the last decade and has been closely monitored in the X-rays to track the changes in its radiative properties. We use archival Chandra and XMM-Newton observations of SGR 1900+14 to construct a history of its spectrum and persistent X-ray flux spanning a period of about seven years. We show that the decline of its X-ray flux in these two outburst episodes follows the same trend. The flux begins to decline promptly and rapidly subsequent to the flares, then decreases gradually for about 600 days, at which point it resumes a more rapid decline. Utilizing the high quality spectral data in each epoch, we also study the spectral coevolution of the source with its flux. We find that neither the magnetic field strength nor the magnetospheric properties change over the period spanned by the observations, while the surface temperature as well as the inferred emitting area both decline with time following both outbursts. We also show that the source reached the same minimum flux level in its decline from these two subsequent outbursts, suggesting that this flux level may be its steady quiescent flux.

astro-ph.HE↗

arXiv2010

Time-Resolved Spectroscopy of the 3 Brightest and Hardest Short Gamma-Ray Bursts Observed with the FGST Gamma-Ray Burst Monitor

From July 2008 to October 2009, the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope (FGST) has detected 320 Gamma-Ray Bursts (GRBs). About 20% of these events are classified as short based on their T90 duration below 2 s. We present here for the first time time-resolved spectroscopy at timescales as short as 2 ms for the three brightest short GRBs observed with GBM. The time-integrated spectra of the events deviate from the Band function, indicating the existence of an additional spectral component, which can be fit by a power-law with index ~-1.5. The time-integrated Epeak values exceed 2 MeV for two of the bursts, and are well above the values observed in the brightest long GRBs. Their Epeak values and their low-energy power-law indices (α) confirm that short GRBs are harder than long ones. We find that short GRBs are very similar to long ones, but with light curves contracted in time and with harder spectra stretched towards higher energies. In our time-resolved spectroscopy analysis, we find that the Epeak values range from a few tens of keV up to more than 6 MeV. In general, the hardness evolutions during the bursts follows their flux/intensity variations, similar to long bursts. However, we do not always see the Epeak leading the light-curve rises, and we confirm the zero/short average light-curve spectral lag below 1 MeV, already established for short GRBs. We also find that the time-resolved low-energy power-law indices of the Band function mostly violate the limits imposed by the synchrotron models for both slow and fast electron cooling and may require additional emission processes to explain the data. Finally, we interpreted these observations in the context of the current existing models and emission mechanisms for the prompt emission of GRBs.

astro-ph.HE↗

arXiv2010

Detection of a Thermal Spectral Component in the Prompt Emission of GRB 100724B

Observations of GRB 100724B with the Fermi Gamma-Ray Burst Monitor (GBM) find that the spectrum is dominated by the typical Band functional form, which is usually taken to represent a non-thermal emission component, but also includes a statistically highly significant thermal spectral contribution. The simultaneous observation of the thermal and non-thermal components allows us to confidently identify the two emission components. The fact that these seem to vary independently favors the idea that the thermal component is of photospheric origin while the dominant non-thermal emission occurs at larger radii. Our results imply either a very high efficiency for the non-thermal process, or a very small size of the region at the base of the flow, both quite challenging for the standard fireball model. These problems are resolved if the jet is initially highly magnetized and has a substantial Poynting flux.

astro-ph.HE↗

arXiv2010

Late time observations of GRB080319B: jet break, host galaxy and accompanying supernova

The Swift discovered GRB080319B was by far the most distant source ever observed at naked eye brightness, reaching a peak apparent magnitude of 5.3 at a redshift of z=0.937. We present our late-time optical (HST, Gemini & VLT) and X-ray (Chandra) observations, which confirm that an achromatic break occurred in the power-law afterglow light curve at ~11 days post-burst. This most likely indicates that the gamma-ray burst (GRB) outflow was collimated, which for a uniform jet would imply a total energy in the jet E_{jet} \gsim 10^{52} erg. Our observations also show a late-time excess of red light, which is well explained if the GRB was accompanied by a supernova (SN), similar to those seen in some other long-duration GRBs. The latest observations are dominated by light from the host and show that the GRB took place in a faint dwarf galaxy (r(AB)\approx27.0, rest-frame M_B\approx-17.2). This galaxy is small even by the standards of other GRB hosts, which is suggestive of a low metallicity environment. Intriguingly, the properties of this extreme event - a small host and bright supernova - are entirely typical of the very low-luminosity bursts such as GRB980425 and GRB060218.

astro-ph↗

arXiv2010

Spatial, Temporal and Spectral Properties of X-ray Emission from the Magnetar SGR~0501+4516

SGR~0501+4516 was discovered with the Swift satellite on 2008 August 22, after it emitted a series of very energetic bursts. Since then, the source was extensively monitored with Swift, the Rossi X-ray Timing Explorer (RXTE) and observed with Chandra and XMM-Newton, providing a wealth of information about its outburst behavior and burst induced changes of its persistent X-ray emission. Here we report the most accurate location of SGR~0501+4516 (with an accuracy of 0.11'') derived with Chandra. Using the combined RXTE, Swift/X-ray Telescope, Chandra and XMM-Newton observations we construct a phase connected timing solution with the longest time baseline (~240 days) to date for the source. We find that the pulse profile of the source is energy dependent and exhibits remarkable variations associated with the SGR~0501+4516 bursting activity. We also find significant spectral evolution (hardening) of the source persistent emission associated with bursts. Finally, we discuss the consequences of the SGR~0501+4516 proximity to the supernova remnant, SNR G160.9+2.6 (HB9).

astro-ph.HE↗

arXiv2010

Photospheric radius expansion during magnetar bursts

On August 24th 2008 the new magnetar SGR 0501+4516 (discovered by SWIFT) emitted a bright burst with a pronounced double-peak structure in hard X-rays, reminiscent of the double-peak temporal structure seen in some bright thermonuclear bursts on accreting neutron stars. In the latter case this is due to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington limit, the photosphere expands and cools so that emission becomes softer and drops temporarily out of the X-ray band, re-appearing as the photosphere settles back down. We consider the factors necessary to generate double-peaked PRE events, and show that such a mechanism could plausibly operate in magnetar bursts, despite the vastly different emission process. Identification of the magnetic Eddington limit in a magnetar would constrain magnetic field and distance and could, in principle, enable a measurement of gravitational redshift. It would also locate the emitting region at the neutron star surface, constraining the burst trigger mechanism. Conclusive confirmation of PRE events will require more detailed radiative models for bursts. However for SGR 0501+4516 the predicted critical flux (using the magnetic field strength inferred from timing and the distance suggested by its probable location in the Perseus arm of our Galaxy) is consistent with that observed in the August 24th burst.

astro-ph.HE↗

arXiv2009

Measuring the Accreting Stellar and Intermediate Mass Black Hole Populations in the Galaxy and Local Group

The population of stellar black holes (SBHs) in the Galaxy and galaxies generally is poorly known in both number and distribution. SBHs are the fossil record of the massive stars in galaxy evolution and may have produced some (if not all) of the intermediate mass (\gsim100\Msun) black holes (IMBHs) and, in turn, the central supermassive black holes (SMBHs) in galactic nuclei. For the first time, a Galaxy-wide census of accreting black holes, and their more readily recognizable tracer population, accreting neutron stars (NSs), could be measured with a wide-field hard X-ray imaging survey and soft X-ray and optical/IR prompt followup -- as proposed for the EXIST mission.

astro-ph.HE↗

arXiv2009

The progenitor mass of the magnetar SGR1900+14

Magnetars are young neutron stars with extreme magnetic fields (B > 10^{14}-10^{15}G). How these fields relate to the properties of their progenitor stars is not yet clearly established. However, from the few objects associated with young clusters it has been possible to estimate the initial masses of the progenitors, with results indicating that a very massive progenitor star (M_prog >40Msun) is required to produce a magnetar. Here we present adaptive-optics assisted Keck/NIRC2 imaging and Keck/NIRSPEC spectroscopy of the cluster associated with the magnetar SGR 1900+14, and report that the initial progenitor star mass of the magnetar was a factor of two lower than this limit, M_prog=17 \pm 2 Msun. Our result presents a strong challenge to the concept that magnetars can only result from very massive progenitors. Instead, we favour a mechanism which is dependent on more than just initial stellar mass for the production of these extreme magnetic fields, such as the "fossil-field" model or a process involving close binary evolution.

astro-ph.SR↗

arXiv2009

The Fermi Gamma-Ray Burst Monitor

The Gamma-Ray Burst Monitor (GBM) will significantly augment the science return from the Fermi Observatory in the study of Gamma-Ray Bursts (GRBs). The primary objective of GBM is to extend the energy range over which bursts are observed downward from the energy range of the Large Area Telescope (LAT) on Fermi into the hard X-ray range where extensive previous data exist. A secondary objective is to compute burst locations on-board to allow re-orientiong the spacecraft so that the LAT can observe delayed emission from bright bursts. GBM uses an array of twelve sodium iodide scintillators and two bismuth germanate scintillators to detect gamma rays from ~8 keV to ~40 MeV over the full unocculted sky. The on-board trigger threshold is ~0.7 photons/cm2/s (50-300 keV, 1 s peak). GBM generates on-board triggers for ~250 GRBs per year.

astro-ph.IM↗

arXiv2009

The Dynamic X-ray Sky of the Local Universe

Over the next decade, we can expect time domain astronomy to flourish at optical and radio wavelengths. In parallel with these efforts, a dedicated transient "machine" operating at higher energies (X-ray band through soft gamma-rays) is required to reveal the unique subset of events with variable emission predominantly visible above 100 eV. Here we focus on the transient phase space never yet sampled due to the lack of a sensitive, wide-field and triggering facility dedicated exclusively to catching high energy transients and enabling rapid coordinated multi-wavelength follow-up. We first describe the advancements in our understanding of known X-ray transients that can only be enabled through such a facility and then focus on the classes of transients theoretically predicted to be out of reach of current detection capabilities. Finally there is the exciting opportunity of revealing new classes of X-ray transients and unveiling their nature through coordinated follow-up observations at longer wavelengths.

astro-ph.HE↗

arXiv2009

In Situ Probes of the First Galaxies and Reionization: Gamma-ray Bursts

The first structures in the Universe formed at z>7, at higher redshift than all currently known galaxies. Since GRBs are brighter than other cosmological sources at high redshift and exhibit simple power-law afterglow spectra that is ideal for absorption studies, they serve as powerful tools for studying the early universe. New facilities planned for the coming decade will be able to obtain a large sample of high-redshift GRBs. Such a sample would constrain the nature of the first stars, galaxies, and the reionization history of the Universe.

astro-ph.CO↗

arXiv2008

Ground-based calibration and characterization of the Fermi Gamma-Ray Burst Monitor Detectors

One of the scientific objectives of NASA's Fermi Gamma-ray Space Telescope is the study of Gamma-Ray Bursts (GRBs). The Fermi Gamma-Ray Burst Monitor (GBM) was designed to detect and localize bursts for the Fermi mission. By means of an array of 12 NaI(Tl) (8 keV to 1 MeV) and two BGO (0.2 to 40 MeV) scintillation detectors, GBM extends the energy range (20 MeV to > 300 GeV) of Fermi's main instrument, the Large Area Telescope, into the traditional range of current GRB databases. The physical detector response of the GBM instrument to GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on-ground individual detector calibration measurements. We present the principal instrument properties, which have been determined as a function of energy and angle, including the channel-energy relation, the energy resolution, the effective area and the spatial homogeneity.

astro-ph↗