arXiv subjects

Jochen Greiner

Publications and source records attributed to Jochen Greiner.

At least 91 records · Page 5Linked to original sources

arXiv2012

The Fermi GBM Gamma-Ray Burst Catalog: The First Two Years

The Fermi Gamma-ray Burst Monitor (GBM) is designed to enhance the scientific return from Fermi in studying gamma-ray bursts (GRBs). In its first two years of operation GBM triggered on 491 GRBs. We summarize the criteria used for triggering and quantify the general characteristics of the triggered GRBs, including their locations, durations, peak flux, and fluence. This catalog is an official product of the Fermi GBM science team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center (HEASARC).

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arXiv2012

The Fermi GBM Gamma-Ray Burst Spectral Catalog: The First Two Years

We present systematic spectral analyses of GRBs detected by the Fermi Gamma-Ray Burst Monitor (GBM) during its first two years of operation. This catalog contains two types of spectra extracted from 487 GRBs, and by fitting four different spectral models, this results in a compendium of over 3800 spectra. The models were selected based on their empirical importance to the spectral shape of many GRBs, and the analysis performed was devised to be as thorough and objective as possible. We describe in detail our procedure and criteria for the analyses, and present the bulk results in the form of parameter distributions. This catalog should be considered an official product from the Fermi GBM Science Team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center (HEASARC).

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arXiv2011

The Fast Evolution of SN 2010bh associated with XRF 100316D

(...) Since 1998, only half a dozen spectroscopically confirmed associations have been discovered and XRF 100316D associated with the type-Ic SN 2010bh at z = 0.059 is among the latest. We began observations with GROND 12 hours after the GRB trigger and continued until 80 days after the burst. GROND provided excellent photometric data in six filter bands covering a wavelength range from approx. 350 to 1800 nm, significantly expanding the pre-existing data set for this event. Combining GROND and Swift data, the early broad-band SED is modelled with a blackbody and afterglow component attenuated by dust and gas absorption. The best-fit SED models imply moderate reddening along the line of sight through the host galaxy (A_V = 1.2 \pm 0.1 mag). The temperature and radius evolution of the thermal component are analysed and combined with earlier measurements available from the literature. This reveals a cooling envelope at an apparent initial radius of 7 \times 10^11 cm, which is compatible with a dense wind surrounding a Wolf-Rayet star. Multicolour templates of SN 1998bw are fitted to the SN to directly compare the light-curve properties. This shows that SN 2010bh is ~65% as bright as SN 1998bw. Reaching maximum brightness at ~8 days after the burst in the blue bands, SN 2010bh proves to be the most rapidly evolving GRB-SNe to date. Finally, a two-component parametrised model is fitted to the quasi-bolometric light curve, which delivers physical parameters of the explosion. This yields M_Ni = 0.21 \pm 0.03 M\bigodot and M_ej = 2.6 \pm 0.2 M\bigodot, typical of values within the GRB-SN population. The kinetic energy is 2.4 \pm 0.7 \times 10^52 erg, which is making this SN the second most energetic GRB-SN after SN 1998bw. This supernova has one of the earliest peaks ever recorded and thereafter fades more rapidly than other GRB-SNe, hypernovae, or typical type-Ic SNe. (...)

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arXiv2011

Temporal Deconvolution study of Long and Short Gamma-Ray Burst Light curves

The light curves of Gamma-Ray Bursts (GRBs) are believed to result from internal shocks reflecting the activity of the GRB central engine. Their temporal deconvolution can reveal potential differences in the properties of the central engines in the two populations of GRBs which are believed to originate from the deaths of massive stars (long) and from mergers of compact objects (short). We present here the results of the temporal analysis of 42 GRBs detected with the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope. We deconvolved the profiles into pulses, which we fit with lognormal functions. The distributions of the pulse shape parameters and intervals between neighboring pulses are distinct for both burst types and also fit with lognormal functions. We have studied the evolution of these parameters in different energy bands and found that they differ between long and short bursts. We discuss the implications of the differences in the temporal properties of long and short bursts within the framework of the internal shock model for GRB prompt emission.

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arXiv2011

GRB 091127: The cooling break race on magnetic fuel

Using high-quality, broad-band afterglow data for GRB 091127, we investigate the validity of the synchrotron fireball model for gamma-ray bursts, and infer physical parameters of the ultra-relativistic outflow. We used multi-wavelength follow-up observations obtained with GROND and the XRT onboard the Swift satellite. The resulting afterglow light curve is of excellent accuracy, and the spectral energy distribution is well-sampled over 5 decades in energy. These data present one of the most comprehensive observing campaigns for a single GRB afterglow and allow us to test several proposed emission models and outflow characteristics in unprecedented detail. Both the multi-color light curve and the broad-band SED of the afterglow of GRB 091127 show evidence of a cooling break moving from high to lower energies. The early light curve is well described by a broken power-law, where the initial decay in the optical/NIR wavelength range is considerably flatter than at X-rays. Detailed fitting of the time-resolved SED shows that the break is very smooth with a sharpness index of 2.2 +- 0.2, and evolves towards lower frequencies as a power-law with index -1.23 +- 0.06. These are the first accurate and contemporaneous measurements of both the sharpness of the spectral break and its time evolution. The measured evolution of the cooling break (nu_c propto t^-1.2) is not consistent with the predictions of the standard model, wherein nu_c propto t^-0.5 is expected. A possible explanation for the observed behavior is a time dependence of the microphysical parameters, in particular the fraction of the total energy in the magnetic field epsilon_B. This conclusion provides further evidence that the standard fireball model is too simplistic, and time-dependent micro-physical parameters may be required to model the growing number of well-sampled afterglow light curves.

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arXiv2011

Constraints on the Synchrotron Shock Model for the Fermi GBM Gamma-Ray Burst 090820A

Discerning the radiative dissipation mechanism for prompt emission in Gamma-Ray Bursts (GRBs) requires detailed spectroscopic modeling that straddles the $νF_ν$ peak in the 100 keV - 1 MeV range. Historically, empirical fits such as the popular Band function have been employed with considerable success in interpreting the observations. While extrapolations of the Band parameters can provide some physical insight into the emission mechanisms responsible for GRBs, these inferences do not provide a unique way of discerning between models. By fitting physical models directly this degeneracy can be broken, eliminating the need for empirical functions; our analysis here offers a first step in this direction. One of the oldest, and leading, theoretical ideas for the production of the prompt signal is the synchrotron shock model (SSM). Here we explore the applicability of this model to a bright {\it Fermi} GBM burst with a simple temporal structure, GRB {\it 090820}A. Our investigation implements, for the first time, thermal and non-thermal synchrotron emissivities in the RMFIT forward-folding spectral analysis software often used in GBM burst studies. We find that these synchrotron emissivities, together with a blackbody shape, provide at least as good a match with the data as the Band GRB spectral fitting function. This success is achieved in both time-integrated and time-resolved spectral fits.

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arXiv2011

Gamma-Ray Bursts: The multiwavelength context in 2015 and beyond

We collect information as complete as possible about the upcoming telescopes and facilities relevant to gamma-ray bursts, both in the electromagnetic bands as well as for non-electromagnetic messengers. We describe the expected synergy between these new facilities and the SVOM mission and predict possible progress in the field of gamma-ray bursts over the next years.

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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.

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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.

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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.

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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.

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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.

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arXiv2010

Optical/IR counterpart to the resolved X-ray jet source CXO J172337.5-373442 and its distance

We present results of observations in the optical to mid-infrared wavelengths of the X-ray source CXO J172337.5-373442, which was serendipitously discovered in the Chandra images and was found to have a fully resolved X-ray jet. The observations include a combination of photometry and spectroscopy in the optical using ground-based telescopes and mid-infrared photometry using Spitzer. We detect the optical/IR counterpart of CXO J172337.5-373442 and identify it to be a G9-V star located at a distance of 334+-60~pc. Comparable values of the hydrogen column densities determined independently from the optical/IR observations and X-ray observations indicate that the optical source is associated with the X-ray source. Since the X-ray luminosity can not be explained in terms of emission from a single G9-V star, it is likely that CXO J172337.5-373442 is an accreting compact object in a binary system. Thus, CXO J172337.5-373442 is the nearest known resolved X-ray jet from a binary system, which is not a symbiotic star. Based on the existing X-ray data, the nature of the compact object can not be confirmed. However the low luminosity of the X-ray point source, 7.1x10^{30} Lsun combined with estimates of the age of the jet and a lack of detection of bright outburst, suggests that the X-ray jet was launched during extreme quiescence of the object. The measured low X-ray luminosity of the jet suggests the likelihood of such jets being more ubiquitous than our current understanding.

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arXiv2009

V5116 Sgr: a disc-eclipsed SSS post-outburst nova?

Nova V5116 Sgr 2005 No. 2, discovered on 2005 July 4, was observed with XMM-Newton in March 2007, 20 months after the optical outburst. The X-ray spectrum showed that the nova had evolved to a pure supersoft X-ray source, indicative of residual H-burning on top of the white dwarf. The X-ray light-curve shows abrupt decreases and increases of the flux by a factor 8 with a periodicity of 2.97h, consistent with the possible orbital period of the system. The EPIC spectra are well fit with an ONe white dwarf atmosphere model, with the same temperature both in the low and the high flux periods. This rules out an intrinsic variation of the X-ray source as the origin of the flux changes, and points to a possible partial eclipse as the origin of the variable light curve. The RGS high resolution spectra support this scenario showing a number of emission features in the low flux state, which either disappear or change into absorption features in the high flux state. A new XMM-Newton observation in March 2009 shows the SSS had turned off and V5116 Sgr had evolved into a weaker and harder X-ray source.

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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.

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arXiv2009

GRB 060605: multi-wavelength analysis of the first GRB observed using integral field spectroscopy

The long and relatively faint gamma-ray burst GRB 060605 detected by \emph{Swift}/BAT lasted about 20 sec. Its afterglow could be observed with \emph{Swift}/XRT for nearly 1 day, while \emph{Swift}/UVOT could detect the afterglow during the first 6 hours after the event. Here, we report on integral field spectroscopy of its afterglow performed with PMAS/PPak mounted at the Calar Alto 3.5 m telescope. In addition, we report on a detailed analysis of XRT and UVOT data and on the results of deep late-time VLT observations that reveal the GRB host galaxy. We find that the burst occurred at a redshift of $z$=3.773, possibly associated with a faint, $R_C=26.4 \pm 0.3$ host. Based on the optical and X-ray data, we deduce information on the SED of the afterglow, the position of the cooling frequency in the SED, the nature of the circumburst environment, its collimation factor, and its energetics. We find that the GRB fireball was expanding into a constant-density medium and that the explosion was collimated with a narrow half-opening angle of about 2.4 degrees. The initial Lorentz factor of the fireball was about 250; however, its beaming-corrected energy release in the gamma-ray band was comparably low. The optical, X-ray afterglow, on the other hand, was rather luminous. Finally, we find that the data are consistent within the error bars with an achromatic evolution of the afterglow during the suspected jet break time at about 0.27 days after the burst.

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arXiv2009

Intermediate polars in the Swift/BAT survey: Spectra and white dwarf masses

White dwarf masses in cataclysmic variables are difficult to determine accurately, but are fundamental for understanding binary system parameters, as well as binary evolution. We investigate the X-ray spectral properties of a sample of Intermediate Polars detected above 15 keV to derive the masses of their accreting white dwarfs. We use data from the Swift/BAT instrument which during the first 2.5 yrs of operation has detected 22 known intermediate polars. The X-ray spectra of these sources are used to estimate the mass of the white dwarfs. We are able to produce a mass estimate for 22 out of 29 of the confirmed intermediate polars. Comparison with previous mass measurements shows good agreement. For GK Per, we were able to detect spectral changes due to the changes in the accretion rate. The Swift/BAT detector with its combination of sensitivity and all-sky coverage provides an ideal tool to determine accurate white dwarf masses in intermediate polars.

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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.

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