Dirk Grupe

Evidence for a canonical GRB afterglow light curve in the Swift/XRT data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) detected with the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments. These power law segments are separated by two corresponding break times. On top of this canonical behavior of the early X-ray light curve, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long lasting sporadx activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission: from photons that are radiated at large angles relative to our line of sight. The first break in the light curve takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve. This energy injection increases the energy of the afterglow shock by at least a factor of f greater than or approx. equal to 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.

Evidence for a canonical gamma-ray burst afterglow light curve in the Swift XRT data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) well observed by the Swift X-Ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power-law segments: (1) an initial very steep decay (/t � � with 3P � 1 P5), followed by (2) a very shallow decay (0:5P � 2 P1:0), and finally (3) a somewhat steeper decay (1P � 3 P1:5). These power-law segments are separated by two corresponding break times, tbreak;1 P500 s and 10 3 sPtbreak;2P 10 4 s. On top of this canonical behavior, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long-lasting sporadic activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission, from photons that are radiated at large angles relative to our line of sight. The first break in the light curve (tbreak;1) takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay (� 2) likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve (tbreak;2). This energy injection increases the energy of the afterglow shock by at least a factor of f k4 and augments the already severe requirements for the efficiency of the prompt gamma-ray emission. Subject headingg gamma rays: bursts — radiation mechanisms: nonthermal

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

The early x-ray emission from grbs

We present observations of the early X-ray emission for a sample of 40 gamma-ray bursts (GRBs) obtained using the Swift satellite, for which the narrow-field instruments were pointed at the burst within 10 minutes of the trigger. Using data from the Burst Alert Telescope and the X-Ray Telescope, we show that the X-ray light curve can be well described by an exponential that relaxes into a power law, often with flares superimposed. The transition time between the exponential and the power law provides a physically defined timescale for the burst duration. In most bursts, the power law breaks to a shallower decay within the first hour, and a late emission hump is observed, which can last for many hours. In other GRBs the hump is weak or absent. The observed variety in the shape of the early X-ray light curve can be explained as a combination of three components: prompt emission from the central engine, afterglow, and the late hump. In this scenario, afterglow emission begins during or soon after the burst, and the observed shape of the X-ray light curve depends on the relative strengths of the emission due to the central engine and that of the afterglow. There is a strong correlation such that those GRBs with stronger afterglow components have brighter early optical emission. The late emission hump can have a total fluence equivalent to that of the prompt phase. GRBs with the strongest late humps have weak or no X-ray flares.

The 22-Month Swift-BAT All-Sky Hard X-ray Survey

We present the catalog of sources detected in the first 22 months of data from the hard X-ray survey (14-195 keV) conducted with the Burst Alert Telescope (BAT) coded mask imager on the Swift satellite. The catalog contains 461 sources detected above the 4.8σ level with BAT. High angular resolution X-ray data for every source from Swift-XRT or archival data have allowed associations to be made with known counterparts in other wavelength bands for over 97% of the detections, including the discovery of ~30 galaxies previously unknown as active galactic nuclei and several new Galactic sources. A total of 266 of the sources are associated with Seyfert galaxies (median redshift z ~ 0.03) or blazars, with the majority of the remaining sources associated with X-ray binaries in our Galaxy. This ongoing survey is the first uniform all-sky hard X-ray survey since HEAO-1 in 1977. Since the publication of the nine-month BAT survey we have increased the number of energy channels from four to eight and have substantially increased the number of sources with accurate average spectra. The BAT 22 month catalog is the product of the most sensitive all-sky survey in the hard X-ray band, with a detection sensitivity (4.8σ) of 2.2 × 10–11 erg cm–2 s–1 (1 mCrab) over most of the sky in the 14-195 keV band.

The First Survey of X-Ray Flares from Gamma-Ray Bursts Observed by Swift: Temporal Properties and Morphology

We present the first systematic investigation of the morphological and timing properties of flares in GRBs observed by Swift XRT. We consider a large sample drawn from all GRBs detected by Swift, INTEGRAL, and HETE-2 prior to 2006 January 31, which had an XRT follow-up and which showed significant flaring. Our sample of 33 GRBs includes long and short, at low and high redshift, and a total of 69 flares. The strongest flares occur in the early phases, with a clear anticorrelation between the flare peak intensity and the flare time of occurrence. Fitting each X-ray flare with a Gaussian model, we find that the mean ratio of the width and peak time is --> ? t/t = 0.13 ? 0.10, albeit with a large scatter. Late flares at times >2000 s have long durations, -->? t > 300 s, and can be very energetic compared to the underlying continuum. We further investigated whether there is a clear link between the number of pulses detected in the prompt phase by BAT and the number of X-ray flares detected by XRT, finding no correlation. However, we find that the distribution of intensity ratios between successive BAT prompt pulses and that between successive XRT flares is the same, an indication of a common origin for gamma-ray pulses and X-ray flares. All evidence indicates that flares are indeed related to the workings of the central engine and, in the standard fireball scenario, originate from internal shocks rather than external shocks. While all flares can be explained by long-lasting engine activity, 29/69 flares may also be explained by refreshed shocks. However, 10 can only be explained by prolonged activity of the central engine.

Radio-loud Narrow-Line Type 1 Quasars

We present the first systematic study of (non-radio-selected) radio-loud narrow-line Seyfert 1 (NLS1) galaxies. Cross-correlation of the Catalogue of Quasars and Active Nuclei with several radio and optical catalogs led to the identification of ~11 radio-loud NLS1 candidates, including four previously known ones. This study almost triples the number of known radio-loud NLS1 galaxies if all candidates are confirmed. Most of the radio-loud NLS1 galaxies are compact, steep-spectrum sources accreting close to or above the Eddington limit. The radio-loud NLS1 galaxies of our sample are remarkable in that they occupy a previously rarely populated regime in NLS1 multiwavelength parameter space. While their [O III]/Hβ and Fe II/Hβ intensity ratios almost cover the whole range observed in NLS1 galaxies, their radio properties extend the range of radio-loud objects to those with small widths of the broad Balmer lines. Their black hole masses are generally at the upper observed end among NLS1 galaxies but are still unusually small in view of the radio loudness of the sources. Among the radio-detected NLS1 galaxies, the radio index R is distributed quite smoothly up to the critical value of R 10 and covers about 4 orders of magnitude in total. Statistics show that ~7% of the NLS1 galaxies are formally radio-loud, while only 2.5% exceed a radio index R > 100. Implications for NLS1 models are discussed. Several mechanisms are considered as explanations for the radio loudness of the NLS1 galaxies and for the lower frequency of radio-loud galaxies among NLS1 galaxies than among quasars. While properties of most sources (with two to three exceptions) generally do not favor relativistic beaming, the combination of accretion mode and spin may explain the observations.

The First Survey of X-Ray Flares from Gamma-Ray Bursts Observed by Swift: Spectral Properties and Energetics

GRB observations with Swift produced the initially surprising result that many bursts have large, late-time X-ray flares. The flares were sometimes intense, had rapid rise and decay phases, and occurred late relative to the prompt phase. Many GRBs have had several flares, which were sometimes overlapping. The origin of the flares can be investigated by comparing the spectra during the flares to those of the afterglow and the initial prompt emission. In this work we have analyzed all significant X-ray flares from the first 110 GRBs observed by Swift. Significant X-ray flares (>3 -->σ ) were found in 33 of these GRBs, with 77 flares detected. A variety of spectral models have been fit to each flare. We find that the spectral fits sometimes favor a Band function model, which is more akin to the prompt emission than to that of the afterglow. While some flares are approximately as energetic as the prompt GRB emission, we find that the average fluence of the flares is approximately 10 times below the average prompt GRB fluence. We also find that the peak energy of the observed flares is typically in the soft X-ray band, as one might expect due to the X-ray selection of the sample. These results, when combined with those presented in the companion paper on temporal properties of flares, support the hypothesis that many X-ray flares are from late-time activity of the internal engine that spawned the initial GRB, not from an afterglow-related effect.

Jet Breaks in Short Gamma-Ray Bursts. II. The Collimated Afterglow of GRB 051221A

We report the results of the Chandra observations of the Swift-discovered short gamma-ray burst GRB 050724. Chandra observed this burst twice, about 2 days after the burst and a second time 3 weeks later. The first Chandra pointing occurred at the end of a strong late-time flare. About 150 photons were detected during this 49.3 ks observation in the 0.4-10.0 keV range. The spectral fit is in good agreement with spectral analysis of earlier Swift XRT data. In the second Chandra pointing the afterglow was clearly detected with eight background-subtracted photons in 44.6 ks. From the combined Swift XRT and Chandra-ACIS-S light curve we find significant flaring superposed on an underlying power-law decay slope of α = 0.98. There is no evidence for a break between about 1 ks after the burst and the last Chandra pointing about 3 weeks after the burst. The nondetection of a jet break places a lower limit of 25° on the jet opening angle, indicating that the outflow is less strongly collimated than most previously reported long GRBs. This implies that the beaming corrected energy of GRB 050724 is at least 4 × 1049 ergs.

MBH- Relation for a Complete Sample of Soft X-Ray-selected Active Galactic Nuclei

We present black hole mass--bulge velocity dispersion relation for a complete sample of 75 soft X-ray selected AGNs: 43 broad line Seyfert 1s and 32 narrow line Seyfert 1s. We use luminosity and FWHM(H-beta) as surrogates for black hole mass and FWHM([OIII]) as a surrogate for the bulge velocity dispersion. We find that NLS1s lie below the M-BH - sigma relation of BLS1s, confirming the Mathur et al. (2001) result. The statistical result is robust and not due to any systematic measurement error. This has important consequences towards our understanding of black hole formation and growth: black holes grow by accretion in well formed bulges, possibly after a major merger. As they grow, they get closer to the M-BH - sigma relation for normal galaxies. The accretion is highest in the beginning and dwindles as the time goes by. Our result does not support theories of M-BH - sigma relation in which the black hole mass is a constant fraction of the bulge mass/ velocity dispersion at all times or those in which bulge growth is controlled by AGN feedback.We present the black hole mass-bulge velocity dispersion relation for a complete sample of 75 soft X-ray-selected active galactic nuclei (AGNs): 43 broad-line Seyfert 1 galaxies (BLS1s) and 32 narrow-line Seyfert 1 galaxies (NLS1s). We use luminosity and FWHM(Hβ) as surrogates for black hole mass and FWHM([O III]) as a surrogate for the bulge velocity dispersion. We find that NLS1s lie below the MBH-σ relation of BLS1s, confirming the Mathur et al. result. The statistical result is robust and not due to any systematic measurement error. This has important consequences toward our understanding of black hole formation and growth: black holes grow by accretion in well-formed bulges, possibly after a major merger. As they grow, they get closer to the MBH-σ relation for normal galaxies. The accretion is highest in the beginning and dwindles as the time goes by. Our result does not support theories of MBH-σ relation in which the black hole mass is a constant fraction of the bulge mass/velocity dispersion at all times or those in which bulge growth is controlled by AGN feedback.