A variability study of the AGILE first catalog of γ-ray sources on 2.3 years of AGILE pointed observations
F. Verrecchia, C. Pittori, A. Bulgarelli, A. W. Chen, M. Tavani, F. Lucarelli, P. Giommi
aa r X i v : . [ a s t r o - ph . H E ] N ov A variability study of the AGILE first catalog of γ -ray sources on 2.3years of AGILE pointed observations F. Verrecchia , , C. Pittori , , A. Bulgarelli , A.W. Chen , M. Tavani , F. Lucarelli , , P. Giommi , onbehalf of the AGILE Collaboration ASI Science Data Center, ESRIN, I-00044 Frascati (RM), Italy Consorzio Interuniversitario Fisica Spaziale, villa Gualino - v.le S. Severo 63, I-10133 Torino, Italy; INAF-OAR, Astronomical Observatory of Rome, Monte Porzio Catone, Italy; INAF-IASF Bologna, via Gobetti 101, I-40129 Bologna, Italy; INAF-IASF Milano, via E. Bassini 15, I-20133 Milano, Italy; and INAF-IASF Roma, via del Fosso del Cavaliere 100, I-00133 Roma, Italy;
AGILE pointed observations performed from July 9, 2007 to October 30, 2009 cover a very large time interval,with a γ –ray dataset useful to perform studies of medium to high brightness galactic sources in the 30 MeV– 50 GeV energy range. We present a study of the 1AGL galactic sources in the E >
100 MeV band, over thecomplete Agile pointed Observation Blocks (OBs) archive. The first AGILE Gamma-Ray Imaging Detector(GRID) catalog included a sample of 47 sources (1AGL; [19]), detected with a conservative analysis over thefirst year of operations dataset. In the analysis here reported we used data obtained with an improved full Fieldof View (FOV) event filter, on a much larger (about 27.5 months) observation dataset, analyzing the mergingof all data and each OB separately. The data processing resulted in an improved source list as compared to the1AGL one, particularly in complex regions of the galactic plane. We present here some results on the revised1AGL galactic sources and on the variability of some of them.
1. Introduction
AGILE (Astrorivelatore Gamma ad Immagini LEg-gero) ([22]) is a mission of the Italian Space Agencydedicated to γ –ray astrophysics in the 30 MeV – 50GeV energy band, and simultaneously to hard X–rayin the 18 – 60 KeV band. AGILE, in orbit since April23 2007, has been the first instrument of a new gen-eration of high-energy space missions based on thesolid-state silicon technology, permitting to advanceour knowledge on many source classes from activegalactic nuclei, to pulsars (PSRs), unidentified γ –raysources, Galactic compact objects and supernova rem-nants. On June 11, 2008 the Fermi Gamma-Ray SpaceTelescope ([17],[4]) was launched, and is currently op-erating together with AGILE.The AGILE Payload detector consists of the silicontracker (ST; [5], [20]), the X–ray detector SuperAG-ILE ([9]), the CsI(Tl) Mini-Calorimeter (MCAL; [14])and an anti-coincidence system (ACS; [18]). The com-bination of ST, MCAL, and ACS forms the Gamma-Ray Imaging Detector (GRID). GRID is sensitive tophoton energies in 30 MeV – 50 GeV energy band,and thanks to silicon technology has a wide FOV (2.5sr in pointing) and accurate timing (a few µ s) and po-sitional information (15 ′ location accuracy for > σ detection), with an angular resolution of 3.5 ◦ at 100MeV and 1 ◦ above 1 GeV.AGILE spacecraft operated in fixed-pointingmode since October 2009 (completing 101 pointingsor Observation Blocks, OBs), when the attitudecontrol system had to be reconfigured into “spinningoperation mode”. Currently the instrument pointingdirection scans the sky with an angular velocity of about 0.8 ◦ /s, accessing about 80% of it each day. TheAGILE satellite raw data, down-linked about every100 minutes, are transmitted from Mission ControlCenter at Telespazio, Fucino, to the AGILE DataCenter (ADC), part of the ASI Science Data Center(ASDC) located in Frascati (Italy). The ADC hasthe duties of data reduction, scientific processing andarchiving and finally to distribute standard Level-2data to Guest Observers (GOs) or, when data becomepublic, to all the scientific community (see ADC webpage http://agile.asdc.asi.it).The significance-limited (4 sigma) sample of 471AGL sources were detected in the E >
100 MeV bandwith a conservative analysis of the inhomogeneousfirst-year sky coverage dataset. We present here thefirst results of a variability study of a sample of 62sources (Fig. 1) analyzing separately each OB in the2.3 years AGILE pointing mode dataset. The sam-ple was obtained with a revision of the 1AGL sampleon the maps obtained from the whole dataset (“deep”maps from now on).
2. OB source detection procedure
The standard analysis OB pipeline at the ADCwas executed at the end of an OB to remove datacorresponding to slews and occasional losses of fine-pointing attitude, and to build the official OB dataarchive. Moreover scientific maps in E >
100 MeV en-ergy band with a size of 60 ◦ × ◦ , binned at 0 . ◦ ,were created selecting confirmed events and exclud-ing albedo contaminated time intervals. The “point- eConf C110509 Figure 1: The Aitoff plot of the 62 distinct source positions detected in a revision of the original 47 1AGL ones, on allthe pointed observations data (colors and symbol sizes are proportional to significance, blue the lowest and orange thehighest). On the right the AGILE-GRID 2.3 years all-sky exposure map in Aitoff projection from all the pointedobservations. ing mode” OB archive is composed of 101 OB cov-ering the wide timespan with non uniform exposures(ranging from 1d to 45dd). All the OB were recentlyreprocessed with the last software release. The proce-dure developed for this analysis on the whole archivein E >
100 MeV band, is based on source detectionat fixed preselected positions using the AGILE Maxi-mum Likelihood (ML). It consists of two main steps:I) preliminary revision of the 1AGL source list, basedon updated deep maps and on more recent publishedAGILE results; II) execution of a ML multi-sourcetask on each OB data in a specific pipeline, keep-ing reference source positions fixed to those previouslydetermined. In the analysis no automatic source de-tection process is implemented. A check leaving theposition free is implemented to verify the goodness ofcandidate transient sources. Both steps were repeatedfor 5 main iterations to improve the multi-source de-tection in complex regions (see section 2.2).
The detection method used is the ML. The signifi-cance (number of sigmas) of a source detection is givenby the square root of the test statistic TS, defined as-2 times the log of the likelihood ratio, and expectedto behave as χ ([15], Chen et al. in preparation).The likelihood ratio test is built considering for thebackground only hypothesis the AGILE diffuse γ –raymodel ([11], Giuliani et al. in preparation). We usedthe task “AG multi2” included in the AGILE softwarefor the ML “multi-source” analysis. The first AGILE catalog ([19]) was built from dataof the first year (July 2007 – June 2008). The dataanalysis, based on a conservative event filter ([16]),and the non uniform sensitivity due to inhomoge-neous first-year AGILE sky coverage, limited the re-sults in complex galactic regions. A preliminary revi-sion of 1AGL sources in regions such as the Carina,Cygnus, Crux and Galactic Center fields, was realizedon updated merged maps from data up to October2009 (see Fig. 1), using data from a new event fil-ter (FM3.119; Bulgarelli et al in preparation). In thisrevision new candidate source were considered, andsome with 3 < √ T S ≤
3. Detection selection and variabilityanalysis
The results from the last execution (the fifth iter-ation) of the ML multi-source task on sources in thereference list, were filtered taking into account off-axis eConf C110509
011 Fermi Symposium, Roma., May. 9-12 Figure 2: On the left a simple case of position refinement, the high latitude 1AGL J1846+6714. The new smaller errorcircle is indicated inside the 1AGL one. Right, a difficult position refinement case, the Carina region . New positionsare indicated with small squares, with sizes not proportional to errors, while the 1AGL error circles are reported. F l u x ( x − ph c m − s − ) Crab, OB Processing s q r t ( T S ) O ff − a x . ang . ( deg ) TIME (MJD) 5.435×10 Figure 3: The Crab E >
100 light curve (in 10 − ph cm − s − ; upper panel) and the plot of √ T S and theoff-axis angle (in degrees) vs time (in MJD). The 1AGLflux value and the 1 sigma error levels are shown asmagenta and black dashed lines. The 2007 flare episodeis the most significant detection, at a flux of 619 ± − s − . angle and significance. Our goal was to investigatethe variability within the OB dataset for establishedsources on deep maps, and possibly search for flareepisodes for some low significance source. As finalselection only those sources having at least one de-tection at significance higher than 3 σ were accepted,for all sources having high significance ( > =4) on deepmaps, or otherwise with at least one detection above4 σ . Then all detections with p ( T S ) ≥ γ –ray flux variability among all OBs was tested according to themethod developed by [16] and recently reported in ananalysis of 1AGL J2022+4032 data ([8]). The vari-ability index V=-log(Q) is evaluated after the calcu-lation of the weighted mean flux and its error and soalso the relative χ . Q is the probability of having avalue of χ ≥ χ observed for a source with constant flux.Sources with V < ≤ V < ≥ χ . Both indices were computed also adding a system-atic component of 10% to flux errors (V sys , χ sys ), totake into account the not well known particle back-ground systematic, expected to vary among differentintegrations.
4. Conclusions and future developments
Applying our selection criteria, we obtained a sam-ple of 1267 detection of 62 distinct sources (Fig. 1;previous processing preliminary results were presentedin [29]). For all sources a revision of single detec-tions is on-going as a verification and improvement ofthe multi-source ML analysis and taking into accountmore recent calibrations. The non uniform exposuresamong the OBs, from 1 to 45 days, together with thepointing strategy, put strong constrains to the capac-ity of “resolving” complex regions in a large numberof OBs and also to the variability analysis.The light curve of the most bright sources with sta-ble flux, such as Geminga and Vela PSR, have beenpreliminarily checked. A particular case is the one ofthe Crab, which has been recently reported to havea “non stable” γ –ray emission ([25], [26], [3], [30]).In Fig. 3 is shown the Crab light curve showing the eConf C110509 October 2007 flare episode, discussed in [26], at anhigher flux compared to the 2010 flare. We obtainedso a V sys of 0.4 for Geminga and 3.36 for Crab.As an example of long-term monitoring light curvesin the E >
100 MeV band obtained in this analysisat the OB timescale, we show below (Fig.5 and 6)the results for the two known galactic sources 1AGLJ1836+5923, associated to the LAT PSR J1836+5925,and 1AGL J0242+611 associated with the HMXB LSI+61303.The use of V sys and χ sys gave compatible resultsfor most of the sources. The source class which in-cluded more variable sources were the bright Blazarsaccording to both parameters, the most variable onebeing 3C 454.3. The complete refined results on theOB timescale will be presented in Verrecchia et al.2011. Acknowledgements.
We acknowledge financial con-tribution from the agreement ASI-INAF I/009/10/0.
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