| 1. |
- Beal, Jacob, et al.
(författare)
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Robust estimation of bacterial cell count from optical density
- 2020
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Ingår i: Communications Biology. - : Springer Science and Business Media LLC. - 2399-3642. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data.
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| 2. |
- Li, Liang, et al.
(författare)
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A Large Catalog of Multiwavelength GRB Afterglows. I. Color Evolution and Its Physical Implication
- 2018
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Ingår i: Astrophysical Journal Supplement Series. - : Institute of Physics Publishing (IOPP). - 0067-0049 .- 1538-4365. ; 234:2
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Tidskriftsartikel (refereegranskat)abstract
- The spectrum of gamma-ray burst (GRB) afterglows can be studied with color indices. Here, we present a large comprehensive catalog of 70 GRBs with multiwavelength optical transient data on which we perform a systematic study to find the temporal evolution of color indices. We categorize them into two samples based on how well the color indices are evaluated. The Golden sample includes 25 bursts mostly observed by GROND, and the Silver sample includes 45 bursts observed by other telescopes. For the Golden sample, we find that 96% of the color indices do not vary over time. However, the color indices do vary during short periods in most bursts. The observed variations are consistent with effects of (i) the cooling frequency crossing the studied energy bands in a wind medium (43%) and in a constant-density medium (30%), (ii) early dust extinction (12%), (iii) transition from reverse-shock to forward-shock emission (5%), or (iv) an emergent SN emission (10%). We also study the evolutionary properties of the mean color indices for different emission episodes. We find that 86% of the color indices in the 70 bursts show constancy between consecutive ones. The color index variations occur mainly during the late GRB-SN bump, the flare, and early reverse-shock emission components. We further perform a statistical analysis of various observational properties and model parameters (spectral index beta(CI)(o), electron spectral indices p(CI), etc.) using color indices. Overall, we conclude that similar to 90% of colors are constant in time and can be accounted for by the simplest external forward-shock model, while the varying color indices call for more detailed modeling.
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| 3. |
- Acuner, Zeynep, et al.
(författare)
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Non-dissipative photospheres in GRBs : spectral appearance in the Fermi/GBM catalogue
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 487:4, s. 5508-5519
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Tidskriftsartikel (refereegranskat)abstract
- A large fraction of gamma-ray burst (GRB) spectra are very hard below the peak. Indeed, the observed distribution of sub-peak power-law indices, alpha, has been used as an argument for a photospheric origin of GRB spectra. Here, we investigate what fraction of GRBs have spectra that are consistent with emission from a photopshere in a non-dissipative outflow. This is the simplest possible photospheric emission scenario. We create synthetic spectra, with a range of peak energies, by folding the theoretical predictions through the detector response of the FERMI/GBM detector. These simulated spectral data are fitted with typically employed empirical models. We find that the low-energy photon indices obtain values ranging -0.4 < alpha < 0.0, peaking at around -0.1, thus covering a non-negligible fraction of observed values. These values are significantly softer than the asymptotic value of the theoretical spectrum of alpha similar to 0.4. The reason for the alpha values to be much softer than expected, is the limitation of the empirical functions to capture the true curvature of the theoretical spectrum. We conclude that more than a quarter of the bursts in the GBM catalogue have at least one time-resolved spectrum, whose alpha values are consistent with spectra from a non-dissipative outflow, releasing its thermal energy at the photosphere. The fraction of spectra consistent with emission from the photosphere will increase even more if dissipation of kinetic energy in the flow occurs below the photosphere.
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| 4. |
- Bhat, P. Narayana, et al.
(författare)
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THE THIRD FERMI GBM GAMMA-RAY BURST CATALOG : THE FIRST SIX YEARS
- 2016
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Ingår i: Astrophysical Journal Supplement Series. - : Institute of Physics Publishing (IOPP). - 0067-0049 .- 1538-4365. ; 223:2
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Tidskriftsartikel (refereegranskat)abstract
- Since its launch in 2008, the Fermi Gamma-ray Burst Monitor (GBM) has triggered and located on average approximately two.-ray bursts (GRBs) every three days. Here, we present the third of a series of catalogs of GRBs detected by GBM, extending the second catalog by two more years through the middle of 2014 July. The resulting list includes 1405 triggers identified as GRBs. The intention of the GBM GRB catalog is to provide information to the community on the most important observables of the GBM-detected GRBs. For each GRB, the location and main characteristics of the prompt emission, the duration, peak flux, and fluence are derived. The latter two quantities are calculated for the 50-300 keV energy band where the maximum energy release of GRBs in the instrument reference system is observed, and also for a broader energy band from 10 to 1000 keV, exploiting the full energy range of GBM's low-energy [NaI[Tl)] detectors. Using statistical methods to assess clustering, we find that the hardness and duration of GRBs are better fit by a two-component model with short-hard and long-soft bursts than by a model with three components. Furthermore, information is provided on the settings and modifications of the triggering criteria and exceptional operational conditions during years five and six in the mission. This third 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.
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| 5. |
- Burgess, J. Michael, et al.
(författare)
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AN OBSERVED CORRELATION BETWEEN THERMAL AND NON-THERMAL EMISSION IN GAMMA-RAY BURSTS
- 2014
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 784:2
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Tidskriftsartikel (refereegranskat)abstract
- Recent observations by the Fermi Gamma-ray Space Telescope have confirmed the existence of thermal and non-thermal components in the prompt photon spectra of some gamma-ray bursts (GRBs). Through an analysis of six bright Fermi GRBs, we have discovered a correlation between the observed photospheric and non-thermal gamma-ray emission components of several GRBs using a physical model that has previously been shown to be a good fit to the Fermi data. From the spectral parameters of these fits we find that the characteristic energies, E-p and kT, of these two components are correlated via the relation E-p proportional to T-alpha which varies from GRB to GRB. We present an interpretation in which the value of the index alpha indicates whether the jet is dominated by kinetic or magnetic energy. To date, this jet composition parameter has been assumed in the modeling of GRB outflows rather than derived from the data.
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| 6. |
- Burgess, J. Michael, et al.
(författare)
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Awakening the BALROG : BAyesian Location Reconstruction Of GRBs
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 476:2, s. 1427-1444
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Tidskriftsartikel (refereegranskat)abstract
- The accurate spatial location of gamma-ray bursts (GRBs) is crucial for both accurately characterizing their spectra and follow-up observations by other instruments. The Fermi Gamma-ray Burst Monitor (GBM) has the largest field of view for detecting GRBs as it views the entire unocculted sky, but as a non-imaging instrument it relies on the relative count rates observed in each of its 14 detectors to localize transients. Improving its ability to accurately locate GRBs and other transients is vital to the paradigm of multimessenger astronomy, including the electromagnetic follow-up of gravitational wave signals. Here we present the BAyesian Location Reconstruction Of GRBs (BALROG) method for localizing and characterizing GBM transients. Our approach eliminates the systematics of previous approaches by simultaneously fitting for the location and spectrum of a source. It also correctly incorporates the uncertainties in the location of a transient into the spectral parameters and produces reliable positional uncertainties for both well-localized sources and those for which the GBM data cannot effectively constrain the position. While computationally expensive, BALROG can be implemented to enable quick follow-up of all GBM transient signals. Also, we identify possible response problems that require attention and caution when using standard, public GBM detector response matrices. Finally, we examine the effects of including the uncertainty in location on the spectral parameters of GRB080916C. We find that spectral parameters change and no extra components are required when these effects are included in contrast to when we use a fixed location. This finding has the potential to alter both the GRB spectral catalogues and the reported spectral composition of some well-known GRBs.
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| 7. |
- Burgess, J. Michael, et al.
(författare)
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Taking the band function too far : a tale of two alpha's
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 451:2, s. 1511-1521
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Tidskriftsartikel (refereegranskat)abstract
- The long standing problem of identifying the emission mechanism operating in gamma-ray bursts (GRBs) has produced a myriad of possible models that have the potential of explaining the observations. Generally, the empirical Band function is fit to the observed gamma-ray data and the fit parameters that are used to infer which radiative mechanisms are at work in GRB outflows. In particular, the distribution of the Band function's low-energy power-law index, alpha, has led to the so-called synchrotron 'line-of-death' (LOD) which is a statement that the distribution cannot be explained by the simplest of synchrotron models alone. As an alternatively fitting model, a combination of a blackbody in addition to the Band function is used, which in many cases provide a better or equally good fit. It has been suggested that such fits would be able to alleviate the LOD problem for synchrotron emission in GRBs. However, these conclusions rely on the Band function's ability to fit a synchrotron spectrum within the observed energy band. In order to investigate if this is the case, we simulate synchrotron and synchrotron+blackbody spectra and fold them through the instrumental response of the Fermi Gamma-ray Burst Monitor (GBM). We then perform a standard data analysis by fitting the simulated data with both Band and Band+blackbody models. We find two important results: the synchrotron LOD is actually more severe than the original predictions: alpha(LOD) similar to -0.8. Moreover, we find that intrinsic synchrotron+blackbody emission is insufficient to account for the entire observed alpha distribution. This implies that some other emission mechanism(s) are required to explain a large fraction of observed GRBs.
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| 8. |
- Li, Liang, et al.
(författare)
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Bayesian Time-resolved Spectroscopy of Multipulse GRBs : Variations of Emission Properties among Pulses
- 2021
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Ingår i: Astrophysical Journal Supplement Series. - : IOP PUBLISHING LTD. - 0067-0049 .- 1538-4365. ; 254:2
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray bursts (GRBs) are highly variable and exhibit strong spectral evolution. In particular, the emission properties vary from pulse to pulse in multipulse bursts. Here we present a time-resolved Bayesian spectral analysis of a compilation of GRB pulses observed by the Fermi/Gamma-ray Burst Monitor. The pulses are selected to have at least four time bins with a high statistical significance, which ensures that the spectral fits are well determined and spectral correlations can be established. The sample consists of 39 bursts, 117 pulses, and 1228 spectra. We confirm the general trend that pulses become softer over time, with mainly the low-energy power-law index alpha becoming smaller. A few exceptions to this trend exist, with the hardest pulse occurring at late times. The first pulse in a burst is clearly different from the later pulses; three-fourths of them violate the synchrotron line of death, while around half of them significantly prefer photospheric emission. These fractions decrease for subsequent pulses. We also find that in two-thirds of the pulses, the spectral parameters (alpha and peak energy) track the light-curve variations. This is a larger fraction compared to what is found in previous samples. In conclusion, emission compatible with the GRB photosphere is typically found close to the trigger time, while the chance of detecting synchrotron emission is greatest at late times. This allows for the coexistence of emission mechanisms at late times.
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| 9. |
- Lin, En-Tzu, et al.
(författare)
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Bayesian analysis on the X-ray spectra of the binary neutron star merger GW170817
- 2019
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Ingår i: Journal of High Energy Astrophysics. - : ELSEVIER SCIENCE BV. - 2214-4048 .- 2214-4056. ; 21, s. 1-5
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Tidskriftsartikel (refereegranskat)abstract
- For the first time, we present a Bayesian time-resolved spectral study of the X-ray afterglow datasets of GW170817/GRB17017A observed by the Chandra X-ray Observatory. These include all 12 public datasets, from the earliest observation taken at t similar to 9 d to the newest observation at similar to 359 d post-merger. While our results are consistent with the other works using Cash statistic within uncertainty, the Bayesian analysis we performed in this work have yielded Gaussian-like parameter distributions. We also obtained the parameter uncertainties directly from their posterior probability distributions. We are able to confirm that the power-law photon index has remained constant of Gamma similar to 1.6 throughout the entire year-long observing period, except for the first dataset observed at t = 8.9 d when Gamma = 1.04 +/- 0.44 is marginally harder. We also found that the unabsorbed X-ray flux peaked at t similar to 155 d, temporally consistent with the X-ray flare model suggested recently by Piro et al. (2019). The X-ray flux has been fading since similar to 160 days after the merger and has returned to the level as first discovered after one year. Our result shows that the X-ray spectrum of GW170817/GRB170817A is well-described by a simple power-law originated from non-thermal slow-cooling synchrotron radiation.
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| 10. |
- Ryde, Felix, 1970-, et al.
(författare)
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On the alpha-intensity correlation in gamma-ray bursts : subphotospheric heating with varying entropy
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 484:2, s. 1912-1925
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Tidskriftsartikel (refereegranskat)abstract
- The emission mechanism during the prompt phase in gamma-ray bursts (GRBs) can be investigated through correlations between spectral properties. Here, we revisit the correlation relating the instantaneous flux, F, and the photon index below the spectral break, alpha, in individual emission pulses, by studying the 38 most prominent pulses in the Fermi/Gamma-ray Burst Monitor GRB catalogue. First, we search for signatures of the bias in the determination of alpha due to the limited spectral coverage (window effect) expected in the synchrotron case. The absence of such a characteristic signature argues against the simplest synchrotron models. We instead find that the observed correlation between F and alpha can, in general, be described by the relation F(t) proportional to e(k alpha(t)), for which the median k = 3. We suggest that this correlation is a manifestation of subphotospheric heating in a flow with a varying entropy. Around the peak of the light curve, a large entropy causes the photosphere to approach the saturation radius, leading to an intense emission with a narrow spectrum. As the entropy decreases the photosphere secedes from the saturation radius, and weaker emission with a broader spectrum is expected. This simple scenario naturally leads to a correlated variation of the intensity and spectral shape, covering the observed range.
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| 11. |
- von Kienlin, Andreas, et al.
(författare)
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The Second Fermi GBM Gamma-Ray Burst Catalog : The First Four Years
- 2014
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Ingår i: Astrophysical Journal Supplement Series. - : Institute of Physics Publishing (IOPP). - 0067-0049 .- 1538-4365. ; 211:1
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Tidskriftsartikel (refereegranskat)abstract
- This is the second of a series of catalogs of gamma-ray bursts (GRBs) observed with the Fermi Gamma-ray Burst Monitor (GBM). It extends the first two-year catalog by two more years, resulting in an overall list of 953 GBM triggered GRBs. The intention of the GBM GRB catalog is to provide information to the community on the most important observables of the GBM detected GRBs. For each GRB the location and main characteristics of the prompt emission, the duration, peak flux and fluence are derived. The latter two quantities are calculated for the 50-300 keV energy band, where the maximum energy release of GRBs in the instrument reference system is observed and also for a broader energy band from 10-1000 keV, exploiting the full energy range of GBMs low-energy detectors. Furthermore, information is given on the settings and modifications of the triggering criteria and exceptional operational conditions during years three and four in the mission. This second 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.
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| 12. |
- Yu, Hoi-Fung, et al.
(författare)
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Bayesian Inference on the Radio-quietness of Gamma-ray Pulsars
- 2018
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 857:2
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Tidskriftsartikel (refereegranskat)abstract
- For the first time we demonstrate using a robust Bayesian approach to analyze the populations of radio-quiet (RQ) and radio-loud (RL) gamma-ray pulsars. We quantify their differences and obtain their distributions of the radiocone opening half-angle d and the magnetic inclination angle a by Bayesian inference. In contrast to the conventional frequentist point estimations that might be non-representative when the distribution is highly skewed or multi-modal, which is often the case when data points are scarce, Bayesian statistics displays the complete posterior distribution that the uncertainties can be readily obtained regardless of the skewness and modality. We found that the spin period, the magnetic field strength at the light cylinder, the spin-down power, the gamma-rayto-X-ray flux ratio, and the spectral curvature significance of the two groups of pulsars exhibit significant differences at the 99% level. Using Bayesian inference, we are able to infer the values and uncertainties of d and a from the distribution of RQ and RL pulsars. We found that d is between 10 degrees and 35 degrees and the distribution of a is skewed toward large values.
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| 13. |
- Yu, Hoi-Fung, et al.
(författare)
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Bayesian Time-resolved Spectroscopy of GRB Pulses
- 2019
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Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 886:1
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Tidskriftsartikel (refereegranskat)abstract
- We performed time-resolved spectroscopy on a sample of 38 single pulses from 37 gamma-ray bursts detected by the Fermi/Gamma-ray Burst Monitor during the first 9 yr of its mission. For the first time a fully Bayesian approach is applied. A total of 577 spectra are obtained and their properties studied using two empirical photon models, namely the cutoff power law (CPL) and Band model. We present the obtained parameter distributions, spectral evolution properties, and parameter relations. We also provide the result files containing this information for usage in further studies. It is found that the CPL model is the preferred model, based on the deviance information criterion and the fact that it consistently provides constrained posterior density maps. In contrast to previous works, the high-energy power-law index of the Band model, ?, has in general a lower value for the single pulses in this work. In particular, we investigate the individual spectrum in each pulse, that has the largest value of the low-energy spectral indexes, ?. For these 38 spectra, we find that 60% of the ? values are larger than ?2/3, and thus incompatible with synchrotron emission. Finally, we find that the parameter relations show a variety of behaviors. Most noteworthy is the fact that the relation between ? and the energy flux is similar for most of the pulses, independent of any evolution of the other parameters.
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| 14. |
- Yu, Hoi-Fung, et al.
(författare)
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Synchrotron cooling in energetic gamma-ray bursts observed by the Fermi Gamma-Ray Burst Monitor
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 573
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Tidskriftsartikel (refereegranskat)abstract
- Context. We study the time-resolved spectral properties of energetic gamma-ray bursts (GRBs) with good high-energy photon statistics observed by the Gamma-Ray Burst Monitor ((IBM) onboard the Fermi Gamma-Ray Space Telescope. Aims. We aim to constrain in detail the spectral properties of GRB prompt emission on a time-resolved basis and to discuss the theoretical implications of the fitting results in the context of various prompt emission models. Methods. Our sample comprises eight GRBs observed by the Fermi (IBM in its first five years of mission, with 1 keV-1 MeV fluence f > 1.0 x 10(-4) erg cm(-2) and a signal-to-noise ratio level of S/N >= 10.0 above 900 keV. We performed a time-resolved spectral analysis using a variable temporal binning technique according to optimal S/N criteria, resulting in a total of 299 time-resolved spectra. We performed Band function fits to all spectra and obtained the distributions for the low-energy power-lay index alpha, the high-energy power-law index beta, the peak energy in the observed nu F-nu, spectrum E-p, and the difference between the low- and high-energy power-law indices Delta s = alpha-beta. We also applied a physically motivated synchrotron model, which is a triple power-law with constrained power-law indices and a blackbody component, to test the prompt emission for consistency with a synchrotron origin and obtain the distributions for the two break energies E-b,E-1 and E-b,E-2 the middle segment power-law index beta, and the Planck function temperature kT. Results. The Band function parameter distributions are alpha = -0.73(-0.21)(+0.16), beta = -2.13(-0.56)(+0.28), E-p = 374.47(-187.7)(+307.3) keV (log(10) E-p = 2.577(-0.30)(+0.26)), and Delta s = 1.38(-0.31)(+0.54), with average errors sigma(alpha) similar to 0.1, sigma(beta) similar to 0.2, and sigma(Ep) similar to 0.1E(p). Using the distributions of Delta s and beta, the electron population index p is found to be consistent with the "moderately fast" scenario, in which fast- and slow-cooling scenarios cannot be distinguished. The physically motivated synchrotron-fitting function parameter distributions are E-b,E-1 = 129.6(-32.4)(+132.2) keV, E-b,E-2 = 631.4(-309.6)(+582) keV, beta = 1.721(-0.25)(+0.48), and kT = 10.4(-3.7)(+4.9) keV, with average errors sigma(beta) similar to 0.2, sigma E-b,E-1 similar to 0.1E(b,1), sigma E-b,E-2 similar to 0.4E(b,2,) and sigma(kT) similar to 0.1kT. This synchrotron function requires the synchrotron injection and cooling break (i.e., E-min and E-cool) to be close to each other within a factor of ten, often in addition to a Planck function. Conclusions. A synchrotron model is found that is consistent with most of the time-resolved spectra for eight energetic Fermi (IBM bursts with good high-energy photon statistics as long as both the cooling and injection break are included and the leftmost spectral slope is lifted either by including a thermal component or when an evolving magnetic field is accounted for.
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