scholarly journals Detection of gamma-ray transients with wild binary segmentation

2020 ◽  
Vol 493 (3) ◽  
pp. 4428-4441
Author(s):  
S Antier ◽  
K Barynova ◽  
P Fryzlewicz ◽  
C Lachaud ◽  
G Marchal-Duval
Keyword(s):  
Gamma Ray ◽  
Change Point Detection ◽  
Archival Data ◽  
Data Set ◽  
Temporal Properties ◽  
Background Removal ◽  
Segmentation Analysis ◽  
Binary Segmentation ◽  
Blind Search ◽  
Detection Search

ABSTRACT In the context of time domain astronomy, we present an offline detection search of gamma-ray transients using a wild binary segmentation analysis called F-WBSB targeting both short and long gamma-ray bursts (GRBs) and covering the soft and hard gamma-ray bands. We use NASA Fermi/GBM archival data as a training and testing data set. This paper describes the analysis applied to the 12 NaI detectors of the Fermi/GBM instrument. This includes background removal, change-point detection that brackets the peaks of gamma-ray flares, the evaluation of significance for each individual GBM detector, and the combination of the results among the detectors. We also explain the calibration of the ∼ 10 parameters present in the method using one week of archival data. Finally, we present our detection performance result for 60 d of a blind search analysis with F-WBSB by comparing to both the onboard and offline GBM search as well as external events found by others surveys such as Swift-BAT. We detect 42/44 onboard GBM events but also other gamma-ray flares at a rate of 1 per hour in the 4–50 keV band. Our results show that F-WBSB is capable of recovering gamma-ray flares, including the detection of soft X-ray long transients. FWBSB offers an independent identification of GRBs in combination with methods for determining spectral and temporal properties of the transient as well as localization. This is particularly useful for increasing the GRB rate and that will help the joint detection with gravitational-wave events.

scholarly journals Consistency of an Estimator for Change Point in Volatility of Financial Returns

2021 ◽  
Vol 13 (1) ◽  
pp. 56
Author(s):  
Josephine Njeri Ngure ◽  
Anthony Gichuhi Waititu
Keyword(s):  
Exchange Rate ◽  
Change Point ◽  
Point Estimation ◽  
Fourth Moment ◽  
Financial Returns ◽  
Test Statistic ◽  
Data Set ◽  
United States Dollar ◽  
Binary Segmentation ◽  
Non Parametric

A non parametric Auto-Regressive Conditional Heteroscedastic model for financial returns series is considered in which the conditional mean and volatility functions are estimated non-parametrically using Nadaraya Watson kernel. A test statistic for unknown abrupt change point in volatility which takes into consideration conditional heteroskedasticity, dependence, heterogeneity and the fourth moment of financial returns, since kurtosis is a function of the fourth moment is considered. The test is based on L2norm of the conditional variance functions of the squared residuals. A non-parametric change point estimator in volatility of financial returns is further obtained. The consistency of the estimator is shown theoretically and through simulation. An application of the estimator in change point estimation in volatility of United States Dollar/Kenya Shilling exchange rate returns data set is made. Through binary segmentation procedure, three change points in volatility of the exchange rate returns are estimated and further accounted for.

scholarly journals The properties of prompt emission in short gamma-ray bursts with extended emission observed by Fermi/GBM

2020 ◽  
Vol 492 (3) ◽  
pp. 3622-3630
Author(s):  
Lin Lan ◽  
Rui-Jingi Lu ◽  
Hou-Jun Lü ◽  
Jun Shen ◽  
Jared Rice ◽  
...  
Keyword(s):  
Gamma Ray ◽  
Gamma Ray Bursts ◽  
Large Scatter ◽  
Intrinsic Properties ◽  
Temporal Properties ◽  
Peak Energy ◽  
Peak Flux ◽  
Short Grbs ◽  
Prompt Emission ◽  
Extended Emission

ABSTRACT Short gamma-ray bursts (GRB) with extended emission (EE) that are composed of an initial short hard spike followed by a long-lasting EE are thought to comprise a sucategory of short GRBs. The narrow energy band available during the Swift era, combined with a lack of spectral information, prevented the discovery of the intrinsic properties of these events. In this paper, we perform a systematic search of short GRBs with EE using all available Fermi/GBM data. The search identified 26 GBM-detected short GRBs with EE that are similar to GRB 060614 observed by Swift/BAT. We focus on investigating the spectral and temporal properties of both the hard spike and the EE component of all 26 GRBs, and explore differences and possible correlations between them. We find that while the peak energy (Ep) of the hard spikes is slightly harder than that of the EE, their fluences are comparable. The harder Ep seems to correspond to a larger fluence and peak flux, with a large scatter for both the hard spike and the EE component. Moreover, the Ep of both the hard spike and the EE are compared with other short GRBs. Finally, we also compare the properties of GRB 170817A with those of short GRBs with EE and find no significant statistical differences between them. We find that GRB 170817A has the lowest Ep, probably because it is off-axis.

scholarly journals Advanced hodograph-based analysis technique to derive gravity-wave parameters from lidar observations

2020 ◽  
Vol 13 (2) ◽  
pp. 479-499
Author(s):  
Irina Strelnikova ◽  
Gerd Baumgarten ◽  
Franz-Josef Lübken
Keyword(s):  
Gravity Wave ◽  
Wave Theory ◽  
Linear Wave ◽  
Data Set ◽  
Background Removal ◽  
Analysis Technique ◽  
Lidar Measurements ◽  
Meridional Winds ◽  
Linear Fit ◽  
Removal Technique

Abstract. An advanced hodograph-based analysis technique to derive gravity-wave (GW) parameters from observations of temperature and winds is developed and presented as a step-by-step recipe with justification for every step in such an analysis. As the most adequate background removal technique the 2-D FFT is suggested. For an unbiased analysis of fluctuation whose amplitude grows with height exponentially, we propose applying a scaling function of the form exp (z∕(ςH)), where H is scale height, z is altitude, and the constant ς can be derived by a linear fit to the fluctuation profile and should be in the range 1–10. The most essential part of the proposed analysis technique consists of fitting cosine waves to simultaneously measured profiles of zonal and meridional winds and temperature and subsequent hodograph analysis of these fitted waves. The linear wave theory applied in this analysis is extended by introducing a wave packet envelope term exp⁡(-(z-z0)2/2σ2) that accounts for limited extent of GWs in the observational data set. The novelty of our approach is that its robustness ultimately allows for automation of the hodograph analysis and resolves many more GWs than can be inferred by the manually applied hodograph technique. This technique allows us to unambiguously identify upward- and downward-propagating GWs and their parameters. This technique is applied to unique lidar measurements of temperature and horizontal winds measured in an altitude range of 30 to 70 km.

Tests for spatial isotropy of three thousand gamma-ray bursts found in BATSE archival data

2000 ◽  
Vol 26 (10) ◽  
pp. 672-678
Author(s):  
Ya. Yu. Tikhomirova ◽  
B. E. Stern
Keyword(s):  
Gamma Ray ◽  
Gamma Ray Bursts ◽  
Archival Data

scholarly journals Large-amplitude late-time radio variability in GRB 151027B

2018 ◽  
Vol 614 ◽  
pp. A29 ◽  
Author(s):  
J. Greiner ◽  
J. Bolmer ◽  
M. Wieringa ◽  
A. J. van der Horst ◽  
D. Petry ◽  
...  
Keyword(s):  
Large Amplitude ◽  
Near Infrared ◽  
Gamma Ray ◽  
Physical Parameters ◽  
Constant Density ◽  
Late Time ◽  
Slow Cooling ◽  
Data Set ◽  
X Ray ◽  
Radio Band

Context. Deriving physical parameters from gamma-ray burst (GRB) afterglow observations remains a challenge, even 20 years after the discovery of afterglows. The main reason for the lack of progress is that the peak of the synchrotron emission is in the sub-mm range, thus requiring radio observations in conjunction with X-ray/optical/near-infrared data in order to measure the corresponding spectral slopes and consequently remove the ambiguity with respect to slow vs. fast cooling and the ordering of the characteristic frequencies. Aims. We have embarked on a multifrequency, multi-epoch observing campaign to obtain sufficient data for a given GRB that allows us to test the simplest version of the fireball afterglow model. Methods. We observed GRB 151027B, the 1000th Swift-detected GRB, with GROND in the optical–near-IR, ALMA in the sub-millimeter, ATCA in the radio band; we combined this with public Swift/XRT X-ray data. Results. While some observations at crucial times only return upper limits or surprising features, the fireball model is narrowly constrained by our data set, and allows us to draw a consistent picture with a fully determined parameter set. Surprisingly, we find rapid, large-amplitude flux density variations in the radio band which are extreme not only for GRBs, but generally for any radio source. We interpret them as scintillation effects, though their extreme nature requires the scattering screen to be at a much smaller distance than usually assumed, multiple screens, or a combination of the two. Conclusions. The data are consistent with the simplest fireball scenario for a blast wave moving into a constant-density medium, and slow-cooling electrons. All fireball parameters are constrained at or better than a factor of 2, except for the density and the fraction of the energy in the magnetic field which has a factor of 10 uncertainty in both directions.

scholarly journals New Gamma-Ray Bursts Found in the Archival Data from the IBIS/ISGRI Telescope of the INTEGRAL Observatory

2019 ◽  
Vol 45 (10) ◽  
pp. 635-654 ◽  
Author(s):  
I. V. Chelovekov ◽  
S. A. Grebenev ◽  
A. S. Pozanenko ◽  
P. Yu. Minaev
Keyword(s):  
Gamma Ray ◽  
Gamma Ray Bursts ◽  
Archival Data

scholarly journals The ‘Big Dipper’: the nature of the extreme variability of the AGN SDSS J2232−0806

2019 ◽  
Vol 485 (2) ◽  
pp. 2573-2588 ◽  
Author(s):  
Daniel Kynoch ◽  
Martin J Ward ◽  
Andy Lawrence ◽  
Alastair G Bruce ◽  
Hermine Landt ◽  
...  
Keyword(s):  
Large Amplitude ◽  
Active Galactic Nucleus ◽  
Accretion Disc ◽  
Archival Data ◽  
Active Nucleus ◽  
Data Set ◽  
X Ray ◽  
Spectroscopic Monitoring ◽  
Extreme Variability ◽  
Subsequent Rise

Abstract SDSS J2232−0806 (the ‘Big Dipper’) has been identified as a ‘slow-blue nuclear hypervariable’: a galaxy with no previously known active nucleus, blue colours, and large-amplitude brightness evolution occurring on a time-scale of years. Subsequent observations have shown that this source does indeed contain an active galactic nucleus (AGN). Our optical photometric and spectroscopic monitoring campaign has recorded one major dimming event (and subsequent rise) over a period of around 4 yr; there is also evidence of previous events consistent with this in archival data recorded over the last 20 yr. Here we report an analysis of the 11 optical spectra obtained to date and we assemble a multiwavelength data set including infrared, ultraviolet, and X-ray observations. We find that an intrinsic change in the luminosity is the most favoured explanation of the observations, based on a comparison of continuum and line variability and the apparent lagged response of the hot dust. This source, along with several other recently discovered ‘changing-look’ objects, demonstrate that AGNs can exhibit large-amplitude luminosity changes on time-scales much shorter than those predicted by standard thin accretion disc models.

UAS radiation hot-spot detection and refinement

2020 ◽  
Author(s):  
Kieran Wood ◽  
Dean Connor ◽  
Sevda Groen ◽  
Dave Smith ◽  
Sam White ◽  
...  
Keyword(s):  
Radiation Pattern ◽  
Gamma Ray ◽  
Hot Spot ◽  
Ground Truth ◽  
Point Sources ◽  
Exclusion Zone ◽  
Real World Data ◽  
Data Set ◽  
Ground Sample ◽  
Spectrometer Data

<p>Unoccupied Aerial Systems (UAS) are ideal tools for responding to nuclear incidents where large outdoor areas have become contaminated with a radiological hazard. They are advantageous because rapid response radiation surveys can be conducted while the human operator remains at a safe distance and avoids direct contamination of the platform. During fieldwork within the Chernobyl Exclusion Zone (Ukraine), an airborne platform was equipped with a GNSS enabled gamma spectrometer and used to survey an area surrounding a known highly contaminated building (a ‘hot-spot’), resulting in a radiation intensity map. The detected radiation pattern, however, was ‘blurred’ since the intensity recorded at any point counted nadir emissions, but also emissions from all sources within line-of-sight; The ‘hot-spot’ had an influence far outside its ground footprint. Methods exist to correct for errors introduced by varying terrain altitude, however, they do not remove the unwanted blurring. Hence, small point sources appear as broad regions of contamination which is entirely an artefact of the measurement process. The effect is further accentuated with increasing height above ground hence understanding and correcting for this phenomenon is particularly relevant to data collected using UAS. Here, we present a novel algorithm to refine the detected pattern to more accurately recover the ground-truth.</p><p>A forward model of the system is created which describes the relationship between the unknown ground-truth and the aerial measurements. Gamma ray emissions from a point source obey the inverse square law of spatial dilution and have an exponential attenuation in air. To model both effects, geometric information of the scene is required and is provided by the geotagged spectrometer data and photogrammetrically processed DEMs of the surveyed terrain. The resulting model is hyper-cube of linear equations, where every aerial measurement point is assumed to be influenced by every ground sample point. By finding the inverse solution of this system, the ground-truth radiation pattern is estimated in more detail. The Kaczmarz method is advantageous because a large system of equations can be broken down into smaller sub-routines and solved iteratively. A caveat is that the solution might settle to false positive. The refinement algorithm will be presented with simulated results, controlled laboratory experiments using robotic arms and sealed radioactive sources, and finally applied to a real-world data set collected in the Chernobyl Exclusion Zone.</p>

scholarly journals DISCOVERY OF NINE GAMMA-RAY PULSARS INFERMILARGE AREA TELESCOPE DATA USING A NEW BLIND SEARCH METHOD

2011 ◽  
Vol 744 (2) ◽  
pp. 105 ◽  
Author(s):  
H. J. Pletsch ◽  
L. Guillemot ◽  
B. Allen ◽  
M. Kramer ◽  
C. Aulbert ◽  
...  
Keyword(s):  
Gamma Ray ◽  
Search Method ◽  
Blind Search
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