EDP Sciences
Free Access
Issue
A&A
Volume 506, Number 3, November II 2009
Page(s) 1563 - 1574
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/200911783
Published online 03 September 2009

A&A 506, 1563-1574 (2009)

First AGILE catalog of high-confidence gamma-ray sources

C. Pittori1 - F. Verrecchia1 - A. W. Chen2,3 - A. Bulgarelli4 - A. Pellizzoni5 - A. Giuliani2,3 - S. Vercellone6 - F. Longo7,8 - M. Tavani9,10,11,3 - P. Giommi1,12 - G. Barbiellini7,8,3 - M. Trifoglio4 - F. Gianotti4 - A. Argan9 - A. Antonelli13 - F. Boffelli14 - P. Caraveo2 - P. W. Cattaneo14 - V. Cocco10 - S. Colafrancesco1,12 - T. Contessi2 - E. Costa9 - S. Cutini1 - F. D'Ammando9,10 - E. Del Monte9 - G. De Paris9 - G. Di Cocco4 - G. Di Persio9 - I. Donnarumma9 - Y. Evangelista9 - G. Fanari1 - M. Feroci9 - A. Ferrari3,15 - M. Fiorini2 - F. Fornari2 - F. Fuschino4 - T. Froysland3,11 - M. Frutti9 - M. Galli16 - D. Gasparrini1 - C. Labanti4 - I. Lapshov9,17 - F. Lazzarotto9 - F. Liello9 - P. Lipari18,19 - E. Mattaini2 - M. Marisaldi4 - M. Mastropietro9,21 - A. Mauri4 - F. Mauri14 - S. Mereghetti2 - E. Morelli4 - E. Moretti7,8 - A. Morselli11 - L. Pacciani9 - F. Perotti2 - G. Piano9,10,11 - P. Picozza10,11 - M. Pilia22,2,5 - C. Pontoni3,8 - G. Porrovecchio9 - B. Preger1 - M. Prest8,22 - R. Primavera1 - G. Pucella9 - M. Rapisarda20 - A. Rappoldi14 - E. Rossi4 - A. Rubini9 - S. Sabatini10 - P. Santolamazza1 - E. Scalise9 - P. Soffitta9 - S. Stellato1 - E. Striani10 - F. Tamburelli1 - A. Traci4 - A. Trois9 - E. Vallazza8 - V. Vittorini9,3 - A. Zambra2,3 - D. Zanello18,19 - L. Salotti12

1 - ASI Science Data Center, ESRIN, 00044 Frascati (RM), Italy
2 - INAF-IASF Milano, via E. Bassini 15, 20133 Milano, Italy
3 - Consorzio Interuniversitario Fisica Spaziale (CIFS), villa Gualino - v.le Settimio Severo 63, 10133 Torino, Italy
4 - INAF-IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
5 - Osservatorio Astronomico di Cagliari, loc. Poggio dei Pini, strada 54, 09012, Capoterra (CA), Italy
6 - INAF-IASF Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
7 - Dip. Fisica, Università di Trieste, via A. Valerio 2, 34127 Trieste, Italy
8 - INFN Trieste, Padriciano 99, 34012 Trieste, Italy
9 - INAF-IASF Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy
10 - Dipartimento di Fisica, Università Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
11 - INFN Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
12 - Agenzia Spaziale Italiana, viale Liegi 26, 00198 Roma, Italy
13 - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
14 - INFN Pavia, via Bassi 6, 27100 Pavia, Italy
15 - Dipartimento di Fisica, Università di Torino, via P. Giuria 15, 10126 Torino, Italy
16 - ENEA Bologna, via don Fiammelli 2, 40128 Bologna, Italy
17 - IKI, Moscow, Russia
18 - INFN Roma 1, p.le Aldo Moro 2, 00185 Roma, Italy
19 - Dip. Fisica, Università La Sapienza, p.le Aldo Moro 2, 00185 Roma, Italy
20 - ENEA Frascati, via Enrico Fermi 45, 00044 Frascati (RM), Italy
21 - CNR, IMIP, Area Ricerca Montelibretti (RM), Italy
22 - Dip. Fisica, Università dell'Insubria, via Valleggio 11, 22100 Como, Italy

Received 4 February 2009 / Accepted 3 August 2009

Abstract
We present the first catalog of high-confidence $\gamma$-ray sources detected by the AGILE satellite during observations performed from July 9, 2007 to June 30, 2008. Cataloged sources were detected by merging all the available data over the entire time period. AGILE, launched in April 2007, is an ASI mission devoted to $\gamma$-ray observations in the 30 MeV-50 GeV energy range, with simultaneous X-ray imaging capability in the 18-60 keV band. This catalog is based on Gamma-Ray Imaging Detector (GRID) data for energies greater than 100 MeV. For the first AGILE catalog, we adopted a conservative analysis, with a high-quality event filter optimized to select $\gamma$-ray events within the central zone of the instrument field of view (radius of 40$^{\circ}$). This is a significance-limited (4$\sigma$) catalog, and it is not a complete flux-limited sample due to the non-uniform first-year AGILE sky coverage. The catalog includes 47 sources, 21 of which are associated with confirmed or candidate pulsars, 13 with blazars (7 FSRQ, 4 BL Lacs, 2 unknown type), 2 with HMXRBs, 2 with SNRs, 1 with a colliding-wind binary system, and 8 with unidentified sources.

Key words: gamma rays: observations - catalogs

1 Introduction

AGILE (Astrorivelatore Gamma ad Immagini LEggero) (Tavani et al. 2008, 2009a) is a mission of the Italian Space Agency (ASI) devoted to $\gamma$-ray astrophysics in the 30 MeV-50 GeV and 18-60 keV energy ranges. AGILE was successfully launched on April 23, 2007 in a $\sim$550 km equatorial orbit with low inclination angle, $\sim$2.5$^{\circ}$. High-energy $\gamma$-ray astrophysics is entering a new challenging phase of discovery. During the 1970's and 1980's, the SAS-2 (Fichtel et al. 1975) and COS-B (Bennet et al. 1977; Bignami et al. 1975) space missions discovered the very first cosmic $\gamma$-ray sources around 100 MeV, but our knowledge of high-energy cosmic $\gamma$-ray emission and phenomena up to now was mainly based on the remarkable results obtained by the EGRET instrument, onboard the Compton Gamma-Ray Observatory (CGRO) (Thompson et al. 1993). Nearly 300 $\gamma$-ray sources above 100  MeV were detected by EGRET (Hartman et al. 1999) during the period from April 22, 1991 to October 3, 1995; however, only a small fraction of them ($\sim$30%) have currently been identified. Many sources are variable or transient on short timescales, and our understanding of many high energy phenomena is still preliminary.

\begin{figure}
\par\includegraphics[width=16.5cm,clip]{11783fg1.ps}
\end{figure} Figure 1:

Total AGILE-GRID exposure sky map in Aitoff projection and Galactic coordinates, for energies >100  MeV in units of cm2 s, accumulated during the period July 9, 2007-June 30, 2008 (with the F4 event filter). The regions of deeper exposures (whiter in the color scale) are a consequence of the AGILE specific pointings at the Galactic plane, combined with the effect of Earth occultation.

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AGILE is the first $\gamma$-ray mission operating in space after almost ten years since the end of EGRET operations. AGILE was the only mission entirely dedicated to high-energy astrophysics above 30  MeV during the period April 2007-June 2008. It is currently operating together with the Fermi Gamma-Ray Space Telescope (formerly GLAST), launched on June 11, 2008 (Michelson 2008; Atwood et al. 2009). The highly innovative AGILE instrument is the first of a new generation of high-energy space missions based on solid-state silicon technology, expected to substantially advance our knowledge in several research areas including the study of active galactic nuclei, gamma-ray bursts, pulsars, unidentified $\gamma$-ray sources, Galactic compact objects, supernova remnants, etc.

The AGILE Payload detector consists of the silicon tracker (ST) (Barbiellini et al. 2001; Prest et al. 2003), the X-ray detector SuperAGILE (Feroci et al. 2007), the CsI(Tl) Mini-Calorimeter (MCAL) (Labanti et al. 2006), and an anticoincidence system (ACS) (Perotti et al. 2006). The combination of ST, MCAL, and ACS forms the Gamma-Ray Imaging Detector (GRID). Accurate timing, positional and attitude information is provided by the precise positioning system and the two star sensors units. The silicon tracker, based on the process of photon conversion into electron-positron pairs, is the core of the AGILE-GRID. It consists of a total of 12 trays, the first 10 of which are capable of converting $\gamma$-rays by a Tungsten layer tracked by silicon microstrip detectors providing the two orthogonal coordinates for each element (point) along the track. AGILE-GRID event processing is operated by onboard trigger logic algorithms (Argan et al. 2004) and by on-ground event filtering. In this paper we use the on-ground GRID event filter called ``F4'' (Giuliani et al. 2006).

During its first year in orbit, AGILE surveyed the $\gamma$-ray sky and detected many galactic and extragalactic sources. The AGILE Commissioning phase ended on July 9, 2007, and the following science verification phase lasted about four months, up to November 30, 2007. On December 1, 2007 the baseline nominal observations and pointing plan of Cycle-1 started with the guest observer program. In this paper we present the first catalog of high-confidence $\gamma$-ray sources detected by AGILE including data from July 9, 2007 to June 30, 2008, thus covering science verification phase data and the first seven months of the Cycle-1. Cataloged sources are detected by merging all the available data over the entire time period.

This is a significance-limited catalog that includes only sources above 4$\sigma$extracted from the sample of AGILE detections obtained with a conservative data analysis, as described in the following sections. The catalog sensitivity is nonuniform, reflecting the inhomogeneous first-year AGILE sky coverage. The first-year exposure (see Fig. 1) was focused mainly towards the Galactic plane, mostly in the Carina-Crux and in the Cygnus regions. The average effective AGILE-GRID exposure time across the sky, for the chosen F4 event filter, is $\langle T_{\exp}\rangle \simeq 0.8 \times 10^6$ s, with peak values of ${\sim} 7 \times 10^6$ s. For a given statistical significance, the limiting point-source flux varies with position, owing to the diffuse $\gamma$-ray emission that represents a nonuniform background over which the pointlike sources are seen. The Galactic diffuse continuum $\gamma$-ray emission dominates other components and has a wide distribution with most emission coming from the Galactic plane. We detect limiting fluxes of about $2 \times 10^{-7}$ ph cm-2 s-1 with ${\sim} 4 \sigma$ statistical significance at galactic latitudes $\vert b\vert>10^{\circ}$.

The outline of the paper is as follows. In Sect. 2 we briefly discuss the AGILE-GRID response characteristics and in Sect. 3 describe the AGILE diffuse $\gamma$-ray model used in the data analysis. In Sect. 4 we describe the satellite pointing strategy and data flow. We then present in Sect. 5 the data reduction and analysis method used to build the first AGILE catalog. Our results and the list of detected high-confidence $\gamma$-ray sources are shown in Sect. 6. Finally in Sect. 7 we discuss our results and make some concluding remarks.

2 AGILE-GRID response characteristics

The AGILE-GRID inflight calibration during the first year and a half of observations has been recently completed and details of the instrument response characteristics will be given in Tavani et al. (2009b). The results are consistent with prelaunch simulations and instrument tests (Tavani et al. 2009a).

The energy-dependent in-flight GRID instrument point spread function (PSF) has a full-width at half-maximum (FWHM) of approximately 3.5$^{\circ}$ at 100 MeV, and gradually improves at higher energies. AGILE PSF is better than that of EGRET by a factor of $\sim$2 above 400 MeV. The GRID effective area, as determined by inflight calibrations, reaches 500 $\rm cm^2$ at several hundred MeV, depending on the GRID event filter used. The conservative event filter F4, chosen for our analysis, applies tight event selection cuts to eliminate a higher fraction of possible particle background counts. This filter is optimized to select $\gamma$-ray events within the central field of view (FoV) zone ( ${\sim} 40^{\circ}$ radius) at the expense of the effective area. In the energy range 200-400 MeV and at ${\sim} 30^{\circ}$ off-axis, the average effective area for the F4 filter is $\langle A_{\rm eff}\rangle_{(F4)} \sim 300$ cm2. It can be parametrized as a function of the off-axis angle $\theta$, in the range $\in[0 \div \theta_M]$, as

\begin{displaymath}\langle A_{\rm eff}(\theta)\rangle_{(F4)}= A_0~\left[1-\left(\frac{\theta}{\theta_M}\right)^{\alpha_1}\right]^{\alpha_2}
\end{displaymath} (1)

where A0=366 cm2, $\theta_M=64^\circ$, $\alpha_1=3$, and $\alpha_2=2$.

Both AGILE PSF and effective area are characterized by a very good off-axis performance and are calibrated well up to almost $60^\circ$, showing very smooth variations with the angle relative to the instrument axis.

3 AGILE diffuse gamma-ray model

In the data analysis we use the AGILE diffuse emission model (Giuliani et al. 2004,2009a) for diffuse $\gamma$-ray background-count predictions. Diffuse $\gamma$-ray emission includes a combination of two components: (1) diffuse emission from the Galactic interstellar medium and (2) an approximately isotropic extragalactic component, as well as possible contributions from unresolved and faint point sources. Diffuse emission coming from the Galactic plane dominates other components and, as in the EGRET model (Hunter et al. 1997), it is assumed to be produced by the interaction of cosmic rays with the interstellar medium through three physical processes: proton-proton collision, Bremsstrahlung, and inverse Compton emission.

The AGILE diffuse emission model substantially improves the previous EGRET model by using state-of-the-art neutral hydrogen (HI) and CO updated maps to model the matter distribution in the Galaxy. It is based on a 3-D grid with $0.25\times 0.25$square degrees binning in Galactic longitude and latitude, and a 0.2 kpc step in distance along the line of sight. For the distribution of neutral hydrogen, we used the Leiden-Argentine-Bonn (LAB) Survey of Galactic HI (Kalberla et al. 2005). The LAB survey improves previous results especially in terms of sensitivity (by an order of magnitude), velocity range, and resolution. To properly project the velocity-resolved radio data, we used the Galactic rotation curves parameterized by Clemens et al. (1985). The detailed and relatively high-resolution distribution of molecular hydrogen is obtained from the CO observations described in Dame et al. (2001). The CO is assumed to be a tracer of molecular hydrogen, through a known ratio between hydrogen density and CO radio emissivity. Cosmic rays can emit $\gamma$-rays through the inverse Compton mechanism due to their interaction with photons of the cosmological background and of the interstellar radiation field (ISRF). To account for this component, we used the analytical model proposed by Chi & Wolfendale (1991). It describes the ISRF as the result of three main contributions: far infrared (due to dust emission), near infrared, and optical/UV (due to stellar emission). The distribution of cosmic rays (both protons and electrons) in the Galaxy was obtained using the GALPROP cosmic-ray model (Moskalenko et al. 2007; Strong et al. 2004).

\begin{figure}
\par\includegraphics[width=15.8cm,clip]{11783fg2.ps}
\end{figure} Figure 2:

Total AGILE-GRID count map in Aitoff projection and Galactic coordinates, for energies >100  MeV in units of ph cm-2 s-1 sr-1accumulated during the period July 9, 2007-June 30, 2008 (with the F4 event filter). The effect of the nonuniform exposure is particularly evident for pointings centered near the Carina-Crux region.

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4 AGILE data flow and Cycle-1 observational program

AGILE satellite raw Telemetry data are down-linked approximately every 100 min to the ASI Malindi ground station in Kenya and transmitted first to the Mission Control Center at Telespazio, Fucino, and subsequently to the AGILE Data Center (ADC) for data reduction, scientific processing, and archiving. The ADC is the scientific component of the AGILE ground segment and is part of the ASI Science Data Center (ASDC) located in Frascati (Italy). The ADC includes scientific personnel from both the ASDC and the AGILE Team. More details on the ADC organization and tasks will be given in Pittori et al. (2009).

The AGILE pointings are subject to strict constraints requiring that the fixed solar panels always be oriented within 3$^{\circ}$ from the Sun direction. AGILE pointings are called Observation Blocks (OBs) and usually consist of predefined long exposures, typically lasting 10-30 days, drifting about 1 degree per day with respect to the initial boresight direction to match solar panels illumination constraints. The large GRID FoV ($\sim$2.5 sr) and the low altitude orbit imply that, for most pointing directions, the Earth (partially) occults the FoV, thus the observing efficiency and exposure for a given source varies depending on its coordinates. To eliminate the Earth-albedo $\gamma$-ray contamination originating from interactions of cosmic rays with the upper atmosphere, a limb-angle cut was applied for all the $\gamma$-ray events with reconstructed directions less than $80^{\circ}$ with respect to the satellite-Earth vector. With this event selection we do not expect systematic effects caused by albedo photon-background fluctuations.

A predefined AGILE baseline pointing plan, aimed at reaching specific scientific goals that maximize the scientific output of the mission, is made public in advance at the AGILE web pages at ASDC[*] to allow for the organization of multi-wavelength campaigns. Part of the AGILE science program is open to guest observers on a competitive basis through Announcements of Opportunity. Guest observers can apply for data which will be collected within the pointing plan. In case of Target of Opportunity (ToO) observations, the baseline plan is interrupted and resumed at the end of the ToO, so that usually a ToO replaces some of the foreseen baseline pointings, and does not shift in time the execution of the remaining planned observations. The ADC web pages provide interactive tables for both the predefined AGILE baseline pointing plan and the actual list of pointings, including previously unforeseen ToOs.

In this paper we analyzed AGILE-GRID data of the 63 Observation Blocks reported in Table 2, covering the period from July 9, 2007 to June 30, 2008. The total $\gamma$-ray exposure and counts maps obtained over the selected period with the F4 filter, in Aitoff projection and Galactic coordinates, are shown in Figs. 1 and 2, respectively.

5 AGILE data reduction and analysis

Raw AGILE telemetry received at ADC is archived and transformed in FITS format through the AGILE Pre-Processing System (TMPPS) (Trifoglio et al. 2008). All GRID data are then routinely processed using the scientific data reduction software tasks developed by the AGILE instrument team and integrated into an automatic pipeline system developed at the ASI Science Data Center. The first step of the pipeline converts the satellite data time into terrestrial time (TT) on a contact-by-contact basis, and performs some preliminary calculations and unit conversions. A second step consists of the $\gamma$-ray event selection. We use an AGILE-GRID specific implementation of the Kalman Filter technique for track identification, event direction, and energy reconstruction (Pittori & Tavani 2002; Giuliani et al. 2006). A quality flag is assigned to each event depending on whether it is recognized as a $\gamma$-ray event, a charged particle, a ``single-track'' event, or an event of uncertain classification. An AGILE auxiliary (LOG) file is created, containing all the spacecraft information relevant to the computation of the effective exposure and livetime. Finally, the event directions in sky coordinates are reconstructed and reported in the AGILE event files (EVT), excluding events flagged as charged background particles. This step produces the Level-2 archive of LOG and EVT files on temporal intervals of few hours. A third step in the pipeline creates quick look (QL) counts, exposure, and diffuse $\gamma$-ray emission maps on different timescales: days, weeks, and daily increments on the OB timescale.

At the completion of each OB, we run the AGILE standard analysis OB pipeline that removes the data corresponding to repointing slews and occasional losses of fine-pointing attitude. GRID data used in our analysis have been processed with the standard software and in-flight calibrations available at the time of writing[*]. We used the high-quality F4 event filter[*], whose response characteristics were described in Sect. 2.

The standardized and cleaned OB Level-2 archive is the basis for creating guest observers data packets and for the data merging used to build this first catalog.

5.1 Data merging from the OB archive

To merge the data from different observing periods over the whole sky, we produced sets of FITS images in the ARC projection (Calabretta & Greisen 2002) in Galactic coordinates, with a radius of $40^{\circ}$ and a bin size of $0.25^{\circ} \times 0.25^{\circ}$, oriented with the north Galactic pole facing upward. The centers of the maps were chosen according to the HEALPix (Hierarchical Equal Area isoLatitude Pixelization) algorithm (Górski et al. 2005) with $N_{\rm side} = 4$, for the coverage of the full sky with 192 maps, whose centers are at a constant latitude. The HEALPix algorithm produces a subdivision of a spherical surface in which each pixel covers the same surface area as every other pixel. However, the HEALPix projection in FITS (Calabretta & Roukema 2007) is not used here. Only the property of the HEALPix grid that the pixel centers occur on a discrete number of rings of constant latitude is used to represent all-sky $\gamma$-ray data binned in sky coordinates. The circular sky areas defined by a centroid and a radius constructed around the 192 HEALPix points are hereafter called ``rings''.

For each 12-h period, we produced maps of counts and exposure in the full energy band E=100 MeV-50 GeV in rings yielding at least 20 min of effective exposure time within $30^{\circ}$ from each HEALPix point[*]. The 12-h maps covering the whole sky were then summed over the entire one-year data span and analyzed with two independent source detection algorithms as described in the next section.

5.2 Source detection method

The AGILE source detection method is based on a maximum likelihood (ML) analysis to derive the best parameter estimates of source significance, $\gamma$-ray flux, and source location for each candidate source (Chen et al. 2009). The ML statistical technique, already used in the past in the analysis of $\gamma$-ray data (Mattox et al. 1996), compares measured counts in each pixel with the predicted counts derived from the diffuse $\gamma$-ray model to find statistically significant excesses consistent with the instrument point spread function. In the analysis we use the AGILE diffuse $\gamma$-ray model described in Sect. 3 for diffuse background (gas) map generation.

\begin{figure}
\par\includegraphics[angle=90,width=16cm,clip]{11783fg3.ps}
\end{figure} Figure 3:

The First AGILE-GRID Catalog of high-confidence sources, plotted in in Aitoff projection and galactic sky coordinates. Symbol sizes are proportional to source flux values, and symbol colors indicate different source classes.

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The likelihood ratio test is then used to compare the null (diffuse background-only) hypothesis with the possible presence of point-source components. According to Wilks' theorem (Wilks 1938), the point source ``test statistic'' (TS), defined as

\begin{displaymath}TS = -2~(\ln L_0 - \ln L_1)
\end{displaymath} (2)

is expected to behave as $\chi_1^2$ in the null hypothesis, plus terms of order O(N-1/2), where N is the number of counts. In practice for a number of AGILE counts N>20, the significance of a source detection at a certain position is given by a number of standard deviations $\sigma$ equal to $\sqrt{TS}$.

Our method for source detection consists of three steps:

1)
preliminary automatic detection of counts map excesses and ML analysis on the resulting fixed positions. This step was performed with two independent detection strategies (A and B described below);
2)
selection of high-confidence detections, according to the criteria described below;
3)
refined analysis with a ML multi-source task to optimize source locations and flux estimates.
In step 1) two independent automatic source detection strategies over each ring count map were used:
A)
identification of possible source locations using the standard Ximage[*] software for astronomical imaging (Giommi et al.1992), adapted to $\gamma$-ray data analysis, and then single source ML analysis (with the AG_srctest_fixed task of the AGILE scientific pipeline). The Ximage detection algorithm locates point sources using a sliding-cell method so that positions and fluxes of each detected source are evaluated in a box maximizing the signal-to-noise ratio;
B)
identification of possible source locations using a dedicated algorithm developed by the AGILE Team called SPOT based on a wavelet filtering technique adapted to $\gamma$-ray data (Bulgarelli et al. 2008), followed by a multi-source ML analysis (with the task AG_srclist) used iteratively.
The AGILE SPOT algorithm used as method B) is a two-step procedure that extracts the excesses from counts maps and builds a list of candidate gamma-ray objects that are then analyzed by a likelihood method. To determine count excesses over the background, the SPOT algorithm analyzes the binned count maps with a smoothing of 1 degree, considers the bins with the largest number of counts, and adds to them the neighboring bins, thereby increasing the connected region, as described in (Di Stefano & Bulgarelli 1999). The process ends when another connected region is merged with the first growing region. At that point, the merging step is reversed and two distinct connected regions are obtained. The centroids of all regions obtained in this way identify the positions of the gamma-ray candidate sources to be analyzed by a multi-source ML. Method A), which uses a single source likelihood analysis, typically optimizes detections of isolated $\gamma$-ray sources in extragalactic sky regions, whereas method B) is more efficient in complex regions such as on the Galactic plane, where multiple source contributions may contaminate the result. In both cases we use an analysis radius of  $10^{\circ}$ and a single power-law source model with spectral photon index $\alpha$. In our analysis we adopted a standard value of $\alpha = -2.1$, except for the Vela ( $\alpha_{\rm Vela}= -1.69$) and Geminga ( $\alpha_{\rm Gem}= -1.66$) pulsars. This assumption is motivated by the known Crab-like spectral properties of the majority of EGRET sources and by the relatively small statistical significance of several AGILE sources, limiting our spectral analysis capability with chosen data sample. We postpone a detailed spectral analysis of the sources appearing in this catalog to forthcoming publications.

We populate two databases with all the results obtained with the automatic methods A) and B), and in step 2) we cross-correlate the two subsamples extracted from the database with the following conditions:

-
distance of the candidate source location from the center of the ring field of view has to be less than or equal to $30^{\circ}$, in order to perform the $10^{\circ}$ data analysis within the confidence region of the chosen F4 filter algorithm ( ${\le} 40^{\circ}$).
-
in each database we create a subsample by associating all the detections within a radius of 90' to a single entry;
-
for sources appearing in different ring areas, we select only detections with minimal distance from the center of the ring FoV;
-
we select detections with $\sqrt{(TS)} > 4$, which corresponds to a statistical significance of about 4$\sigma$.
We obtain
-
81 source candidates with source detection method A);
-
77 source candidates with source detection method B).
An initial cross-correlation radius of 90' between the two dataset was used to select high-confidence galactic and extragalactic source candidates.

Then in step 3), a manual refined analysis was performed with a multi-source likelihood analysis task, AG_multi, to confirm the detection and derive optimized source parameters. Special care should be used in particular on the galactic plane region ( $\vert b\vert< 10^{\circ}$) to deal with possible source confusion and flux contamination. In the analysis of complex regions, positioning results obtained with detection methods A) and B) were also compared with a third standard peak detection software, SExtractor (Bertin & Arnauts 1996), adapted to $\gamma$-ray data, using a wavelet filtering and deblending algorithm. We define ``high-confidence'' as those detections that pass all the requirements described in this section.

6 First AGILE catalog of high-confidence gamma-ray sources

The resulting list of validated sources, detected by using AGILE-GRID data from July 9, 2007 to June 30, 2008 with the method and criteria described in Sect. 5, includes 47 high-confidence sources. The sources of this first AGILE catalog are plotted in Fig. 3 in galactic sky coordinates, classified in Table 1, including both confirmed and possible associations, and listed in Table 3.

Table 1:   Census of the 47 First AGILE high-confidence gamma-ray sources.

In Table 1, for ``confirmed'' counterparts is meant $\gamma$-ray sources for which there are peer-reviewed publications demonstrating high-confidence association with refined analysis methods. Associations for uncertain sources were selected using cross-correlations with various updated public catalogs of $\gamma$, hard X-ray, and radio sources either specific mission or specific source classes, such as:

Summing up, the first AGILE catalog includes 21 confirmed or candidate pulsars, 13 blazars (7 FSRQ, 4 BL Lacs, 2 unknown type), 2 HMXRBs, 2 SNRs, 1 colliding-wind binary system (CWB), and 8 unidentified sources.

Table 2:   AGILE pointings in the period 9 July 2007-30 June 2008, corresponding to the 63 Observation Blocks (OB) considered in our analysis. Acronyms used in table are: ToO = Target of Opportunity pointing, SA = SuperAGILE special pointing.

Table 3:   First AGILE high-confidence gamma-ray sources list.

In Table 3 we report the values of the following relevant source parameters:

  • AGILE source name;
  • source position both in celestial and galactic sky coordinates: RA, Dec (J2000), and LII, BII;
  • position error (95%), defined as the 2-D error circle radius at 95% confidence level, statistical error only[*];
  • the $\sqrt{TS}$ values of the significance of the detection as determined from the refined ML analysis;
  • the mean value of the F4 exposure map in units of 108 cm2 s, relative to the sky area (Ring) used for each source analysis;
  • the source flux above 100 MeV and its $1\sigma$ statistical error[*] in units of 10-8 ph cm-2 s-1. This is the average source flux value over the entire time period;
  • source classification;
  • counterpart name for confirmed sources;
  • possible counterparts in the AGILE error radius and other names, both for ``confirmed'' and ``uncertain'' counterparts.

6.1 Notes on individual sources

As described in Sect. 5, pointlike $\gamma$-ray sources parameters reported in this paper are determined by a likelihood analysis of the $10^{\circ}$ field surrounding the candidate sources. The analysis depends on the local Galactic diffuse emission, the $\gamma$-ray photon statistics, the instrument PSF, the response matrix as a function of energy and off-axis angle, and on the background filtering. Particular care is required to carry out the analysis in regions of the Galactic plane that are characterized by a relatively high and structured flux of the diffuse Galactic emission, as well as in regions harboring nearby $\gamma$-ray sources leading to possible source confusion. For such regions we insert the label (C), for ``Confused'', in the Confirmed Counterpart column of Table 3. These are significant AGILE detections, which however have flux and location parameters that may be affected (within the statistical+systematic errors) by other nearby sources. In the following we briefly comment on some specific AGILE detections.  

 

1AGL J0006+7311. This AGILE source, positionally coincident with the EGRET $\gamma$-ray source 3EG J0010+7309 located in supernova remnant CTA 1, is associated with the first radio quiet pulsar recently discovered through its $\gamma$-ray pulsations by the Fermi Gamma-Ray Space Telescope (Abdo et al. 2008). This new class of young pulsar sources may be possibly associated with most unidentified Galactic $\gamma$-ray sources in star-forming regions and SNRs. Search for pulsations in $\gamma$-ray AGILE data is currently under way. At the border of the AGILE error box, there is also the blazar source BZQ J0019+7327.

1AGL J0535+2205 and 1AGL J0634+1748 (Crab and Geminga). These two well known strong $\gamma$-ray pulsars, together with the Vela pulsar, were used for in-flight AGILE calibrations. We report the flux values obtained during calibration subperiods. These values agree with pulsed flux values reported in (Pellizzoni et al. 2009). We note, however, that we observed higher flux values, over $1\sigma$ from the reported mean flux, for both sources when merging all the data, including shorter (1 day) integration periods during 2007. This point is under investigation.

1AGL J0617+2236. This AGILE detection provides an improved positioning compared to the 3EG J0617+2238 error box. This source is positionally coincident with the SNR IC443 (Tavani et al. 2009c). The AGILE error box also contains the PSR J0614+2229.

1AGL J0657+4554 and 1AGL J0714+3340. These two high-latitude (|b|>10 deg) AGILE sources, associated with blazars of unknown type in the BZCAT, have no EGRET counterparts probably owing to flux variability.

1AGL J0835-4509 (Vela pulsar). As the most luminous steady source in the $\gamma$-ray sky, Vela has been extensively used for in-flight AGILE-GRID calibrations. With the F4 filter version and the strict criteria used to build this first catalog, the resulting effective exposure is quite low (only about $0.81
\times 10^8$ cm2 s on source over the entire period).

1AGL J1022-5822. This source lies in the complex Carina region, and multiple source contributions are possible.

1AGL J1043-5931. This source (not detected by EGRET) is close to 1AGL J1043-5749 in the Carina region. Our refined analysis leads to the association of this $\gamma$-ray source with the colliding wind binary Eta Carinae (Tavani et al. 2009d).

1AGL J1104+3754 and 1AGL J1222+2851. The effective exposure on these sources is low, just about 2 effective days, but it includes a ToO period on the source W Comae.

1AGL J1412-6149 and 1AGL J1419-6055. This source lies in the complex Crux region, and multiple source contributions are possible.

1AGL J1511-0908. The total effective exposure on this source is very low, just about 2 effective days, but it includes a ToO period on the associated source PKS 1510-089.

1AGL J1736-3235, 1AGL J1746-3017. These sources are in the complex region around $10^{\circ}$ from the Galactic center, and multiple source contributions are possible. We emphasize the relatively small exposure of the Galactic center region achieved until June 30, 2008, which does not allow a deeper analysis of the complex $\gamma$-ray emission from the center of our Galaxy.

1AGL J1801-2317. This source is spatially coincident with the TeV source HESS J1801-233. Remarkably, they both appear to be associated with the northeastern section of the SNR W28 shell (Giuliani et al. 2009b).

7 Conclusions

The AGILE Cycle-1 pointing plan covered the whole sky focusing mainly toward the Galactic plane. The AGILE first catalog includes only high-significance sources characterized by a prominent mean $\gamma$-ray flux above 100 MeV when integrated over the total exposure period 2007 July-2008 June. With our one-year long integration, only sources with ``steady'' flux values above ${\sim} 20 \times 10^{-8}$ ph cm-2 s-1are detected over $4\sigma$. Source detections during flaring state and determination of peak fluxes are not included in this catalog. This should be taken into account when comparing with the results of the third EGRET catalog, which includes detections over $4\sigma$in each of the EGRET viewing periods during its effective 6-year lifetime. An analysis of $\gamma$-ray detection by the AGILE-GRID on short timescales (several weeks, 1-week, days) is beyond the scope of this catalog, so will be published elsewehere. The AGILE-GRID spatial resolution reached with long exposures is substantially better than that of EGRET, and the total exposure accumulated by AGILE in several sky regions, particularly near the Galactic plane, is comparable to the one obtained by EGRET in 6 years effective time. It is then interesting to compare the relatively high-flux sources detected by AGILE with the equivalent sources of the third EGRET catalog. Many bright $\gamma$-ray sources detected by EGRET are confirmed by AGILE, which provides comparable or better positioning. AGILE in the first catalog detected five sources that were not present in the 3EG catalog: 3 blazars and 2 candidate pulsars. As expected from statistical detection effects and source variability, some of the prominent (with flux range $40{-}100 \times 10^{-8}$ ph cm-2 s-1) 3EG sources in the Galactic plane are not detected by AGILE with mean flux values at a significance level that is sufficient to be included in this first catalog.

It is also important to note that the AGILE-GRID exposure in the selected period has been accumulated mostly in the Carina-Crux and in the Cygnus regions, with relatively low exposure at the Galactic center. This explains the relatively small number of sources in the Galactic center region.

Finally, taking into account that the AGILE first catalog is not a complete flux-limited sample and is affected by selection effects due to the assumed fixed value (-2.1) of the unknown source spectral indices, we observe that with a limiting flux of about $2 \times 10^{-7}$ ph cm-2 s-1, the number and rate of $\gamma$-ray Blazars observed by AGILE (13 Blazars: 7 FSRQs and 5 BL Lacs) is roughly consistent with expectations from the EGRET log N-log S (Mücke & Pohl 2000; Özel & Thompson 1996). A detailed study over a complete AGILE AGN sample will be performed in the future.

A variability study of the sources of this first catalog on different timescales will appear in Verrecchia et al. (2009).

Acknowledgements
The AGILE Mission is funded by the Italian Space Agency (ASI) with scientific and programmatic participation by the Italian Institute of Astrophysics (INAF) and the Italian Institute of Nuclear Physics (INFN). We acknowledge funds from ASI grant I/089/06/2.

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Footnotes

... ASDC[*]
http://agile.asdc.asi.it
... writing[*]
Software build version: BUILD GRID_STD_16 and BUILD GRID_SCI_15.2.
... filter[*]
New filter algorithms highly efficient and optimized over a wider FoV have been developed and will be distributed by ADC during Cycle-2.
... point[*]
We describe here our choice of parameters for map generation. To reduce the particle background contamination, only events tagged as confirmed $\gamma$-ray events were selected (filtercode = 5). The South Atlantic Anomaly data were excluded (phasecode = 18) and all the $\gamma$-ray events whose reconstructed directions with respect to the satellite-Earth vector is smaller than $80^{\circ}$ (albrad = 80) were also rejected, to eliminate the Earth albedo contamination.
... Ximage[*]
Ximage (Giommi et al. 1992) is part of the NASA's Heasarc Xanadu standard software package for multi-mission X-ray astronomy.
... only[*]
The AGILE Team recommends adding a systematic error of ${\pm} 0.1^{\circ}$ linearly to this value.
... error[*]
The AGILE Team recommends adding a systematic error of $10\%$ to the flux statistical error.

All Tables

Table 1:   Census of the 47 First AGILE high-confidence gamma-ray sources.

Table 2:   AGILE pointings in the period 9 July 2007-30 June 2008, corresponding to the 63 Observation Blocks (OB) considered in our analysis. Acronyms used in table are: ToO = Target of Opportunity pointing, SA = SuperAGILE special pointing.

Table 3:   First AGILE high-confidence gamma-ray sources list.

All Figures

  \begin{figure}
\par\includegraphics[width=16.5cm,clip]{11783fg1.ps}
\end{figure} Figure 1:

Total AGILE-GRID exposure sky map in Aitoff projection and Galactic coordinates, for energies >100  MeV in units of cm2 s, accumulated during the period July 9, 2007-June 30, 2008 (with the F4 event filter). The regions of deeper exposures (whiter in the color scale) are a consequence of the AGILE specific pointings at the Galactic plane, combined with the effect of Earth occultation.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=15.8cm,clip]{11783fg2.ps}
\end{figure} Figure 2:

Total AGILE-GRID count map in Aitoff projection and Galactic coordinates, for energies >100  MeV in units of ph cm-2 s-1 sr-1accumulated during the period July 9, 2007-June 30, 2008 (with the F4 event filter). The effect of the nonuniform exposure is particularly evident for pointings centered near the Carina-Crux region.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[angle=90,width=16cm,clip]{11783fg3.ps}
\end{figure} Figure 3:

The First AGILE-GRID Catalog of high-confidence sources, plotted in in Aitoff projection and galactic sky coordinates. Symbol sizes are proportional to source flux values, and symbol colors indicate different source classes.

Open with DEXTER
In the text


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