arXiv subjects

C. Kouveliotou

Publications and source records attributed to C. Kouveliotou.

At least 163 records · Page 9Linked to original sources

arXiv2005

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 J1810-197. The burst morphologies consist of a short spike or multiple spikes lasting ~1 s each followed by extended tails of emission where the pulsed flux from XTE J1810-197 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. During the brightest X-ray tail, we detect a narrow emission line at 12.6 keV with an equivalent width of 1.4 keV and a probability of chance occurrence less than 4 x 10^-6. 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 significantly different 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.

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arXiv2005

Late-epoch optical and near-infrared observations of the GRB000911 afterglow and its host galaxy

We present the results of an optical and near-infrared (NIR) monitoring campaign of the counterpart of Gamma-Ray Burst (GRB) 000911, located at redshift z=1.06, from 5 days to more than 13 months after explosion. Our extensive dataset is a factor of 2 larger and spans a time interval about 4 times longer than the ones considered previously for this GRB afterglow; this allows a more thorough analysis of its light curve and of the GRB host galaxy properties. The afterglow light curves show a single power-law temporal decline, modified at late times by light from a host galaxy with moderate intrinsic extinction, and possibly by an emerging supernova (SN). The afterglow evolution is interpreted within the classical "fireball" scenario as a weakly collimated adiabatic shock propagating in the interstellar medium. The presence of a SN light curve superimposed on the non-thermal afterglow emission is investigated: while in the optical bands no significant contribution to the total light is found from a SN, the NIR J-band data show an excess which is consistent with a SN as bright as the known hypernova SN1998bw. If the SN interpretation is true, this would be the farthest GRB-associated SN, as well as the farthest core-collapse SN, discovered to date. However, other possible explanations of this NIR excess are also investigated. Finally, we studied the photometric properties of the host, and found that it is likely to be a slightly reddened, subluminous, extreme starburst compact galaxy, with luminosity about 0.1 L*, an age of about 0.5 Gyr and a specific Star Formation Rate (SFR) of approximately 30 Msol yr-1 (L/L*)-1. This is the highest specific SFR value for a GRB host inferred from optical/NIR data.

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arXiv2005

SCUBA sub-millimeter observations of gamma-ray bursters III. GRB 030329: The brightest sub-millimeter afterglow to date

We discuss our ongoing program of Target of Opportunity (ToO) sub-millimeter observations of gamma-ray bursts (GRBs) using the Sub-millimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). In this paper, we present all the ToO observations of GRB 030329. This was by far the brightest sub-millimeter afterglow seen to date. The flux density at 850 microns was approximately constant up to a break that took place ~7 days after the burst. This was consistent with being a jet break. The 850 micron results agree with those at longer wavelengths that show a brighter flux ~7 days after the burst, right at the time of the break. No short-lived large-scale brightenings were detected in the sub-millimeter light curve. However, the 850 micron light curve may have had a drop <~ 16 days after the burst. The peak of the afterglow emission was at ~90 GHz in the days before the break in the light curve. A simple modeling is consistent with the spectral indices remaining the same as the afterglow evolved, with the breaks in the spectrum moving to longer wavelengths at later times and the flux at the peak falling. No significant sub-millimeter emission was detected from the host galaxy.

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arXiv2005

SCUBA sub-millimeter observations of gamma-ray bursters IV. GRB 021004, 021211, 030115, 030226, 041006

We discuss our ongoing program of Target of Opportunity (ToO) sub-millimeter observations of gamma-ray bursts (GRBs) using the Sub-millimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). In this paper, we present the ToO observations of GRBs 021004, 021211, 030115, 030226, and 041006. The observations of GRBs 021004, 021211, 030226, and 041006 all started within ~1 day of the burst, but did not detect any significant sub-millimeter emission from the reverse shock and/or afterglow. These observations put some constraints on the models for the early emission, although the generally poor observing conditions and/or the faintness of these afterglows at other wavelengths limit the inferences that can be drawn from these lack of detections. However, these observations demonstrate that SCUBA can perform rapid observations of GRBs, and provide encouragement for future observations in the Swift era. None of these GRBs had significant sub-millimeter emission from their host galaxies. This adds to the indication that GRBs are not closely linked to the most luminous dusty star-forming galaxies.

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arXiv2005

Gamma-Ray Observations of a Giant Flare from The Magnetar SGR 1806-20

Magnetars comprise two classes of rotating neutron stars (Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars), whose X-ray emission is powered by an ultrastrong magnetic field, B ~ 10^15 G. Occasionally SGRs enter into active episodes producing many short X-ray bursts; extremely rarely (about once per 50 years per source), SGRs emit a giant flare, an event with total energy at least 1000 times higher than their typical bursts. Here we report that, on 2004 December 27, SGR 1806-20 emitted the brightest extra-solar transient event ever recorded, even surpassing the full moon brightness for 0.2 seconds. The total (isotropic) flare energy is 2x10^46 erg, 100 times higher than the only two previous events, making this flare a once in a century event. This colossal energy release likely occurred during a catastrophic reconfiguration of the magnetar's magnetic field. Such an event would have resembled a short, hard Gamma Ray Burst (GRB) if it had occurred within 40 Mpc, suggesting that extragalactic SGR flares may indeed form a subclass of GRBs.

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arXiv2005

How Special Are Dark Gamma-Ray Bursts: 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 (e.g. Rees & Meszaors 1992; Meszaros & Rees 1997), 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-ray 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 GRBs 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.

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arXiv2004

The Interplanetary Network Supplement to the BATSE Catalogs of Untriggered Cosmic Gamma Ray Bursts

We present Interplanetary Network (IPN) detection and localization information for 211 gamma-ray bursts (GRBs) observed as untriggered events by the Burst and Transient Source Experiment (BATSE), and published in catalogs by Kommers et al. (2001) and Stern et al. (2001). IPN confirmations have been obtained by analyzing the data from 11 experiments. For any given burst observed by BATSE and one other distant spacecraft, arrival time analysis (or ``triangulation'') results in an annulus of possible arrival directions whose half-width varies between 14 arcseconds and 5.6 degrees, depending on the intensity, time history, and arrival direction of the burst, as well as the distance between the spacecraft. This annulus generally intersects the BATSE error circle, resulting in a reduction of the area of up to a factor of ~650. When three widely separated spacecraft observed a burst, the result is an error box whose area is as much as 30000 times smaller than that of the BATSE error circle. Because the IPN instruments are considerably less sensitive than BATSE, they generally did not detect the weakest untriggered bursts, but did detect the more intense ones which failed to trigger BATSE when the trigger was disabled. In a few cases, we have been able to identify the probable origin of bursts as soft gamma repeaters. The vast majority of the IPN-detected events, however, are GRBs, and the confirmation of them validates many of the procedures utilized to detect BATSE untriggered bursts.

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arXiv2004

Massive star-formation rates of gamma-ray burst host galaxies: an unobscured view in X-rays

The hard X-ray (2-10keV) luminosity of a star-forming galaxy tracks its population of high mass X-ray binaries and is essentially unobscured. It is therefore a practically unbiased measure of star-formation in the host galaxies of gamma-ray bursts (GRBs). Using recent and archival observations of GRBs with the XMM-Newton and Chandra X-ray observatories, limits are placed on the underlying X-ray emission from GRB hosts. Useful limits on the current massive star-formation rates (SFRs), unaffected by obscuration, are obtained for the hosts of three low redshift GRBs: GRB980425, GRB030329 and GRB031203. These limits show that though the specific SFRs may be high (as in dwarf starburst galaxies), none have massive obscured star-formation at the levels implied by the sub-mm detection of some GRB hosts. It is also shown that in cases where the faint luminosities of the late time afterglow or supernova emission are of interest, the contribution of the host galaxy to the X-ray flux may be significant.

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arXiv2004

Probing a GRB progenitor at a redshift of z=2: a comprehensive observing campaign of the afterglow of GRB 030226

We report results from a comprehensive follow-up observing campaign of the afterglow of GRB 030226, including VLT spectroscopy, VLT polarimetry, and Chandra X-ray observations. In addition, we present BOOTES-1 wide-field observations at the time of the occurrence of the burst. First observations at ESO started 0.2 days after the event when the GRB afterglow was at a magnitude of R~19 and continued until the afterglow had faded below the detection threshold (R>26). No underlying host galaxy was found. The optical light curve shows a break around 0.8 days after the burst, which is achromatic within the observational errors, supporting the view that it was due to a jetted explosion. Close to the break time the degree of linear polarization of the afterglow light was less than 1.1%, which favors a uniform jet model rather than a structured one. VLT spectra show two absorption line systems at redshifts z=1.962+/-0.001 and at z=1.986+/-0.001, placing the lower limit for the redshift of the GRB close to 2. We emphasize that the kinematics and the composition of the absorbing clouds responsible for these line systems is very similar to those observed in the afterglow of GRB 021004. This corroborates the picture in which at least some GRBs are physically related to the explosion of a Wolf-Rayet star.

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arXiv2004

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

We present precise localizations of AXP 1E1841-045 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 AXP and 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 to have detectable counterparts with the observational limits currently available for these sources. To confirm the reported counterpart of SGR 1806-20, we obtained new IR observations during the July 2003 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.

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arXiv2004

Discovery of the Near-IR Afterglow and of the Host of GRB 030528

The rapid dissemination of an arcmin-sized HETE-2 localization of the long-duration X-ray flash GRB 030528 led to a ground-based multi-observatory follow-up campaign. We report the discovery of the near-IR afterglow, and also describe the detection of the underlying host galaxy in the optical and near-IR bands. The afterglow is classified as "optically dark" as it was not detected in the optical band. The K-band photometry presented here suggests that the lack of optical detection was simply the result of observational limitations (lack of rapid and deep observations plus high foreground extinction). Simple power law fits to the afterglow in the K-band suggest a typically decay with a slope of alpha=1.2. The properties of the host are consistent with the idea that GRB hosts are star forming blue galaxies. The redshift of GRB 030528 can not be determined accurately, but the data favour redshifts less than unity. In addition, we present an optical and near-IR analysis of the X-ray source CXOU J170354.0--223654 from the vicinity of GRB 030528.

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arXiv2004

The line-of-sight towards GRB 030429 at z = 2.66: Probing the matter at stellar, galactic and intergalactic scales

We report the discovery of the optical afterglow (OA) of the long-duration gamma-ray burst GRB 030429, and present a comprehensive optical/near-infrared dataset used to probe the matter at different distance scales, i.e. in the burst environment, in the host galaxy and in an intervening absorber. A break in the afterglow light curve is seen approximately 1 day from the onset of the burst. The light curve displays a significant deviation from a simple broken power-law with a bright 1.5 magnitude bump with a duration of 2-3 days. The optical/near-infrared spectral energy distribution is best fit with a power-law with index beta = -0.36 +/- 0.12 reddened by an SMC-like extinction law with (a modest) A_V = 0.34 +/- 0.04. In addition, we present deep spectroscopic observations obtained with the Very Large Telescope. The redshift measured via metal absorption lines in the OA is z = 2.658 +/- 0.004. Based on the damped Ly-alpha absorption line in the OA spectrum we measure the HI column density to be log N(HI) = 21.6 +/- 0.2. This confirms the trend that GRBs tend to be located behind very large HI column densities. The resulting dust-to-gas ratio is consistent with that found in the SMC, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. We find that a neighbouring galaxy, at a separation of only 1.2", has z = 0.841 +/- 0.001, ruling it out as the host of GRB 030429. The small impact parameter of this nearby galaxy, which is responsible for MgII absorption in the OA spectrum, is in contrast to previous identifications of most QSO absorption-selected galaxy counterparts. Finally, we demonstrate that the OA was not affected by strong gravitational lensing via the nearby galaxy.

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arXiv2004

Measurements of Gamma-Ray Bursts (GRBs) with Glast

One of the scientific goals of the main instrument of GLAST is the study of Gamma-Ray Bursts (GRBs) in the energy range from ~20 MeV to ~300 GeV. In order to extend the energy measurement towards lower energies a secondary instrument, the GLAST Burst Monitor (GBM), will measure GRBs from ~10 keV to ~25 MeV and will therefore allow the investigation of the relation between the keV and the MeV-GeV emission from GRBs over six energy decades. These unprecedented measurements will permit the exploration of the unknown aspects of the high-energy burst emission and the investigation of their connection with the well-studied low-energy emission. They will also provide ne insights into the physics of GRBs in general. In addition the excellent localization of GRBs by the LAT will stimulate follow-up observations at other wavelengths which may yield clues about the nature of the burst sources.

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arXiv2004

The bright optical afterglow of the long GRB 001007

We present optical follow up observations of the long GRB 001007 between 6.14 hours and ~468 days after the event. An unusually bright optical afterglow (OA) was seen to decline following a steep power law decay with index alpha = -2.03 +/- 0.11, possibly indicating a break in the light curve at t - to < 3.5 days, as found in other bursts. Upper limits imposed by the LOTIS alerting system 6.14 hours after the gamma ray event provide tentative (1.2 sigma) evidence for a break in the optical light curve. The spectral index beta of the OA yields -1.24 +/- 0.57. These values may be explained both by several fireball jet models and by the cannonball model. Fireball spherical expansion models are not favoured. Late epoch deep imaging revealed the presence of a complex host galaxy system, composed of at least two objects located 1.2" (1.7 sigma) and 1.9" (2.7 sigma) from the afterglow position.

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arXiv2004

The submm properties of GRB host galaxies

Long duration gamma-ray bursts (GRBs) accompany the deaths of some massive stars and hence, since massive stars are short lived, are a tracer of star formation activity. Given that GRBs are bright enough to be seen to very high redshifts, and detected even in dusty environments, they should therefore provide a powerful probe of the global star formation history of the universe. The potential of this approach can be investigated via submm photometry of GRB host galaxies. Submm luminosity also correlates with star formation rate, so the distribution of host galaxy submm fluxes should allow us to test the two methods for consistency. Here, we report new JCMT/SCUBA 850 micron measurements for 15 GRB hosts. Combining these data with results from previous studies we construct a sample of 21 hosts with <1.4 mJy errors. We show that the distribution of apparent 850 micron flux densities of this sample is reasonably consistent with model predictions, but there is tentative evidence of a dearth of submm bright (>4 mJy) galaxies. Furthermore, the optical/infrared properties of the submm brightest GRB hosts are not typical of the galaxy population selected in submm surveys, although the sample size is still small. Possible selection effects and physical mechanisms which may explain these discrepancies are discussed.

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arXiv2004

A near-infrared survey of the N49 region around the Soft Gamma-Ray Repeater 0526-66

We report the results of a deep near-infrared survey with VLT/ISAAC of the environment of the supernova remnant N49 in the Large Magellanic Cloud, which contains the soft gamma-ray repeater SGR 0526-66. Two of the four confirmed SGRs are potentially associated with compact stellar clusters. We thus searched for a similar association of SGR 0526-66, and imaged a young stellar cluster at a projected distance of ~30 pc from the SGR. This constitutes the third cluster-SGR link, and lends support to scenarios in which SGR progenitors originate in young, dusty clusters. If confirmed, the cluster-SGR association constrains the age and thus the initial mass of SGR progenitors.

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arXiv2004

The INTEGRAL view of the Soft Gamma-ray Repeater SGR 1806-20

We present the results obtained by INTEGRAL on the Soft-Gamma Ray Repeater SGR 1806-20. In particular we report on the temporal and spectral properties of the bursts detected during a moderately active period of the source in September and October 2003 and on the search for quiescent emission.

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arXiv2004

GRB 020813: polarization in the case of a smooth optical decay

We present the results of a VLT polarimetric monitoring campaign of the GRB 020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days after the gamma-ray event. The mean values of the degree of linear polarization (P) and its position angle (Theta) resulting from our campaign are =1.18+/-0.10% and Theta = 148.7+/-2.3 deg after correcting for Galactic interstellar polarization. Our VLT data set is most naturally described by a constant degree of linear polarization and position angle, nonetheless a slow Theta evolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant Theta rotation respect to the Keck spectropolarimetric observations performed ~0.25 days after the GRB (Barth et al. 2003). However, is a factor of two lower than the polarization degree measured from Keck. Additionally, the VLT polarization data allowed us to construct an accurate V-band light curve. The V-band photometric data revealed a smooth light curve with a break located between the last Keck and our first VLT polarimetric measurement, 0.33 < t_(break, V) < 0.88 days after the GRB. The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag, 0.010 mag and 0.016 mag for our three observing nights, respectively. We speculate that the stability of Theta might be related to the smoothness of the light curve.

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