J/A+A/598/A134 γ-ray signature in WHSP blazars (Arsioli+, 2017)
Searching for γ-ray signature in WHSP blazars. Fermi-LAT detection of 150 excess signal in the 0.3-500 GeV band. Arsioli B., Chang Y.-L. <Astron. Astrophys., 598, A134 (2017)> =2017A&A...598A.134A (SIMBAD/NED BibCode)
ADC_Keywords: Gamma rays ; BL Lac objects Keywords: galaxies: active - BL Lacertae objects: general - radiation mechanisms: non-thermal - gamma rays: diffuse background - gamma rays: general Abstract: A direct search of γ-ray emission centered on multifrequency selected candidates is a valuable complementary approach to the standard search adopted in current γ-ray Fermi-LAT catalogs. Our sources are part of the 2WHSP sample that was assembled with the aim of providing targets for Imaging Atmospheric Cherenkov Telescopes (IACT). A likelihood analysis based on their known position enabled us to detect 150 γ-ray excess signals that have not yet been reported in previous γ-ray catalogs (1FGL, 2FGL, 3FGL). By identifying new sources, we solve a fraction of the extragalactic isotropic γ-ray background (IGRB) composition, improving the description of the γ-ray sky. We perform data reduction with the Fermi Science Tools using positions from 400 high synchrotron peaked (HSP) blazars as seeds of tentative γ-ray sources; none of them have counterparts from previous 1FGL, 2FGL and 3FGL catalogs. Our candidates are part of the 2WHSP sample (currently the largest set of HSP blazars). We focus on HSPs characterized by bright synchrotron component with peak flux νf(ν)≥10-12.1erg/cm2/s, testing the hypothesis of having a γ-ray source in correspondence to the WHSP positions. Our likelihood analysis considers the 0.3-500GeV energy band, integrating over 7.2yr of Fermi-LAT observation and making use of the Pass 8 data release. From the 400 candidates tested, a total of 150 2WHSPs showed excess γ-ray signature: 85 high-significance detections with test statistic (TS)>25, and 65 lower-significance detections with TS between 10 to 25. We assume a power law spectrum in the 0.3-500GeV band and list the spectrum parameters describing all 150 new γ-ray sources. We study the γ-ray photon spectral index distribution, the likelihood of detection according to the synchrotron peak brightness (figure of merit parameter), and plot the measured γ-ray LogN-LogS of HSP blazars, also discussing the portion of the IGRB that has been resolved by the present work. We also report on four cases where we could resolve source confusion and find counterparts for unassociated 3FGL sources with the help of high-energy TS maps together with multifrequency data. The 150 new γ-ray sources are named with the acronym 1BIGB for the first version of the Brazil ICRANet Gamma-ray Blazar catalog, in reference to the cooperation agreement supporting this work. Description: Based on the position of a potential γ-ray source, we downloaded Pass 8 processed events from the public Fermi-database6 that includes all photons recorded in a region of interest (ROI) of 25° radius from the candidate's position, for the whole 7.2yr of observations (MM/DD/YYYY: 08/04/2008 to 11/04/2015). In our analysis we used the Fermi Science Tools (v10r0p5), performing binned analysis to deal with the long integration time. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file tablea1.dat 86 150 The 150 new gamma-ray signatures detected down to TS>10 level tablea2.dat 52 30 *List of 30 2WHSPs detected with the largest significance within our gamma-ray likelihood analysis, all having TS>45 based Pass 8 Fermi-LAT data when integrating over 7.2yr of observations
Note on tablea2.dat: All those cases were re-evaluated based on Pass 7 data, integrating over the first 4yr of mission; in order to reproduce the setup used to prepare the 3FGL catalog (see Sect. 3.4 for the setup description).
See also: J/A+A/598/A17 : The 2WHSP catalog (Chang+, 2017) Byte-by-byte Description of file: tablea1.dat
Bytes Format Units Label Explanations
1- 5 A5 --- --- [1BIGB] 7- 22 A16 --- 1BIGB Source name (JHHMMSS.d+DDMMSS) (1) 23- 25 A3 --- n_1BIGB [abc ] Note on 1BIGB (2) 27- 35 F9.5 deg RAdeg Right ascension (J2000) 37- 45 F9.5 deg DEdeg Declination (J2000) 47 A1 --- l_z Lower limit flag on z (3) 48- 52 F5.3 --- z Redshift from literature (4) 53 A1 --- u_z [?] Uncertainty flag on z 55- 58 F4.2 --- GAMMA Spectral photon index (5) 60- 63 F4.2 --- e_GAMMA rms uncertainty on GAMMA 65- 68 F4.2 10-13ph/cm2/s/MeV N0 N0 parameter (5) 70- 73 F4.2 10-13ph/cm2/s/MeV e_N0 rms uncertainty on N0 75- 79 F5.1 --- TS Test statistic 81- 84 F4.2 10-10ph/cm2/s Flux Photon counts calculated by integrating Eq. (1) along the energy range 1-100 GeV (Flux1-100GEv) 86 A1 --- Note [*] * indicates sources with re-evaluated parameters in tablea2
Note (1): Sources are named with the acronym 1BIGB (for the first version of "Brazil ICRANet Gamma-ray Blazar" catalog) with coordinates corresponding to those of 2WHSP seeds used for the likelihood analysis. Note (2): Notes as follows: a = source also reported in the 2FHL catalog (Ackermann et al. 2016, Cat. J/ApJ/810/14) b = source reported as possible counterparts of photon clustering detected by Fermi-LAT at E>10GeV (Campana et al. 2015Ap&SS.360...65C) c = Source 2WHSPJ194356.2+211821 is the single case located at |b|<10° (Also to improve the completeness of the final sample, known HSP sources at |b|<10° were incorporate in the 2WHSP catalog.) Note (3): all lower-limits shown here were derived in Arsioli et al., Cat. J/A+A/579/A34; Chang et al., Cat. J/A+A/598/A17. Note (4): from Shaw et al. 2013, Cat. J/ApJ/764/135b; Pita et al., 2014A&A...565A..12P; Furniss et al., 2013ApJ...768L..31F; Danforth et al., 2010ApJ...720..976D; Shaw et al., 2013AJ....146..127S; Masetti et al., 2013A&A...559A..58M; Sbarufatti et al., 2005AJ....129..559S; Massaro et al., 2015Ap&SS.357...75M, Cat. VII/274) Sources with currently absent redshift were given "0" value. Note (5): The gamma-ray model parameters from Fermi Science Tools assume a power law to describe the spectrum within the studied energy range 0.3-500GeV. The parameters N0 and GAMMA are direct outputs from the likelihood analysis over 7.2yr of Fermi-LAT data in the 0.3-500GeV band; those results consider the pivot energy fixed as E0=1GeV. The spectrum of each gamma-ray candidate is always modeled as a power law: dN/dE=N0(E/E0)-GAMMA (equation (1)), where E0 is a scale parameter (also known as pivot energy), N0 is the pre-factor (flux density at the pivot energy E0), and GAMMA is the photon spectral index for the energy range under consideration.
Byte-by-byte Description of file: tablea2.dat
Bytes Format Units Label Explanations
1- 5 A5 --- --- [2WHSP] 6- 21 A16 --- 2WHSP 2WHSP source name (JHHMMSS.s+DDMMSS) 22 A1 --- n_2WHSP [*] Note on 2WHSP (1) 24- 27 F4.2 --- GAMMA Spectral photon index (2) 29- 32 F4.2 --- e_GAMMA rms uncertainty on GAMMA 34- 37 F4.2 10-13ph/cm2/s/MeV N0 pre-factor parameter N0 (2) 39- 42 F4.2 10-13ph/cm2/s/MeV e_N0 rms uncertainty on N0 44- 47 F4.1 --- TS4yr Test statistic based on Pass 7 data, integrating over the first 4yr of mission 49- 52 F4.2 10-10ph/cm2/s Flux Integral flux in the 1-100GeV energy band
Note (1): *: for two sources the power-law fitting does not converge well given their significance is too low. Note (2): Fitting parameter assuming a power-law model to describe the gamma-ray spectrum in the 0.3-300GeV band (we integrated up to 300GeV following Fermi-LAT team recommendations to work with Pass 7).
History: From electronic version of the journal
(End) Patricia Vannier [CDS] 24-May-2017
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