The Palermo Swift-BAT hard X-ray catalogue

G. Cusumano; V. La Parola; A. Segreto; V. Mangano; C. Ferrigno; A. Maselli; P. Romano; T. Mineo; B. Sbarufatti; S. Campana; G. Chincarini; P. Giommi; N. Masetti; A. Moretti; G. Tagliaferri

doi:10.1051/0004-6361/200811184

Free Access

Issue		A&A Volume 510, February 2010


Article Number		A48
Number of page(s)		20
Section		Catalogs and data
DOI		https://doi.org/10.1051/0004-6361/200811184
Published online		05 February 2010

A&A 510, A48 (2010)

II. Results after 39 months of sky survey

G. Cusumano¹ - V. La Parola¹ - A. Segreto¹ - V. Mangano¹ - C. Ferrigno^1,2,3 - A. Maselli¹ - P. Romano¹ - T. Mineo¹ - B. Sbarufatti¹ - S. Campana⁴ - G. Chincarini^5,4 - P. Giommi⁶ - N. Masetti⁷ - A. Moretti⁴ - G. Tagliaferri⁴

1 - INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
2 - Institut für Astronomie und Astrophysik Tübingen (IAAT), Germany
3 - ISDC Data Centre for Astrophysics, Chemin d'Écogia 16, 1290 Versoix, Switzerland
4 - INAF - Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
5 - Università degli studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milan, Italy
6 - ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
7 - INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy

Received 17 October 2008 / Accepted 23 June 2009

Abstract
Aims. We present the Palermo Swift-BAT hard X-ray catalogue obtained from the analysis of data acquired during the first 39 months of the Swift mission.
Methods. We developed a dedicated software to perform the data reduction, mosaicking, and source detection of the BAT survey data. We analyzed the BAT dataset in three energy bands (14-150 keV, 14-30 keV, 14-70 keV), obtaining a list of 962 detections above a significance threshold of 4.8 standard deviations. The identification of the source counterparts was pursued using three strategies: cross-correlation with published hard X-ray catalogues, analysis of field observations of soft X-ray instruments, and cross-correlation with SIMBAD databases.
Results. The survey covers 90% of the sky down to a flux limit of 2.5 $\times$ 10^-11 erg cm^-2 s^-1 and 50% of the sky down to a flux limit of 1.8 $\times$ 10^-11 erg cm^-2 s^-1 in the 14-150 keV band. We derived a catalogue of 754 identified sources, of which ${\sim}69$ % are extragalactic, ${\sim}27$ % are Galactic objects, and ${\sim}4$ % are already known X-ray or gamma ray emitters, whose nature has yet to be determined. The integrated flux of the extragalactic sample is ${\sim}1\%$ of the cosmic X-ray background in the 14-150 keV range.

Key words: X-rays: general - catalogs - surveys

1 Introduction

The study of Galactic and extragalactic sources at energies greater than 10 keV is fundamental to both the investigation of non thermal emission processes and to the study of source populations that are not detectable in the soft X-ray energy band because their emission is strongly absorbed by a thick column of gas or dust. Another major aim of deep and sensitive surveys in the hard X-ray domain is to resolve the diffuse X-ray background (CXB) and identify which class of sources represents the most significant contribution: while the CXB at energies lower than 10 keV has been almost entirely resolved (80-90%, Moretti et al. 2003; Worsley et al. 2006; Brandt & Hasinger 2005; Worsley et al. 2005), only $\sim$ 1.5% of the CXB at higher energies can be associated with resolved sources (Ajello et al. 2008b).

Until now, the observation of the hard X-ray sky has not been performed with imaging grazing incidence telescopes because the reflectivity above 10 keV rapidly declines because of the steep decrease in the critical angle with energy. The first surveys in the hard X-ray domain were performed with detectors equipped with collimator-limited field of view: UHURU (2-20 keV; Forman et al. 1978) and HEAO1 (0.2 keV-10 MeV; Wood et al. 1984). Later, sky images for energies greater than 10 keV were produced using coded mask detectors (e.g., Fenimore & Cannon 1978; Skinner et al. 1987a): in these detectors the entrance window of the telescope is partially masked and the ``shadows'' of the cosmic sources are projected onto a position-sensitive detector. Dedicated algorithms are then used to reconstruct the position and intensity of the sources in the field of view and, therefore, reproduce the image of the observed sky. In the last two decades, space observatories equipped with this type of telescopes have surveyed the sky reporting detections of numerous sources emitting in the hard X-ray domain: Spacelab/XRT (Skinner et al. 1987b), MIR/KVANT/TTM (Sunyaev et al. 1991), GRANAT/ART-P (Pavlinsky et al. 1994,1992), GRANAT/SIGMA (Cordier et al. 1991; Sunyaev et al. 1991), and BeppoSAX/WFC (Jager et al. 1997). Today, the IBIS-ISGRI camera (Lebrun et al. 2003; Ubertini et al. 2003) on the INTEGRAL observatory (Winkler et al. 2003) with its field of view of $8^{\circ}$ $\times$ $8^{\circ}$ (fully coded) is carrying out a hard X-ray survey focussing mostly on the Galactic plane in the 20-150 keV energy band with a sensitivity higher than previous observatories. The main results of this survey and the relevant source catalogues are reported in several papers (e.g. Sazonov et al. 2007; Bird et al. 2007,2004; Krivonos et al. 2007; Bird et al. 2006; Churazov et al. 2007; Bassani et al. 2006; Krivonos et al. 2005).

The Burst Alert Telescope (BAT; Barthelmy et al. 2005) onboard the Swift observatory (Gehrels et al. 2004), because of its large field of view ( $100^{\circ}$ $\times$ $60^{\circ}$ half coded) and large detector area (a factor of 2 greater than ISGRI), offers the opportunity to significantly increase the number of detections contributing to the luminosity of the sky in the hard X-rays allowing a substantial improvement of our knowledge of the AGN and of the cosmic hard X-ray background. The first results on the BAT survey have been presented in Markwardt et al. (2005), Ajello et al. (2008b,a), and Tueller et al. (2008). The latter presents a catalogue of sources detected in the first 9 months of the BAT survey data, identifying 154 extragalactic sources (129 at $\vert b\vert>15^{\circ}$ ).

To take full advantage of the BAT survey archive, we developed the dedicated software B ATI MAGER (Segreto et al. 2010), which is independent from the software developed by the Swift-BAT team. In this paper, we present the results obtained from the analysis of 39 months of BAT sky survey. The paper is organized as follows: in Sect. 2, we describe the BAT telescope; in Sect. 3, we describe the data set and screening criteria; in Sect. 4, we present a brief description of the code used for the analysis and illustrate our analysis strategy. In Sect. 5, we describe the survey properties. The catalogue construction and the results are reported in Sect. 6. The last section summarizes our results. The spectral properties of our extragalactic sample will be discussed in a forthcoming paper (La Parola et al. 2010, in preparation).

The cosmology adopted in this work assumes H₀=70 km s^-1 Mpc^-1, k=0, $\Omega_{\rm m}=0.3$ , and $\Lambda_0=0.7$ . Quoted errors are at $1\sigma$ confidence level, unless stated otherwise.

2 The BAT telescope

The BAT, one of the three instruments onboard the Swift observatory, is a coded aperture imaging camera consisting of a 5200 cm² array of 4 $\times$ 4 mm² CdZnTe elements mounted on a plane 1 m behind a 2.7 m² coded aperture mask of 5 $\times$ 5 mm² elements distributed with a pseudo-random pattern. The telescope, which operates in the 14-150 keV energy range, has a large field of view (1.4 steradian half coded), and a point spread function (PSF) of 17 arcmin, and is devoted mainly to the monitoring of a large fraction of the sky for the occurrence of gamma ray bursts (GRBs). The BAT can measure their position with the accuracy (1-4 arcmin) that is necessary to slew the spacecraft towards a GRB position and bring the burst location inside the field of view of the narrow field instruments in a couple of minutes. While waiting for new GRBs, it continuously collects spectral and imaging information in survey mode, covering a fraction of between 50% and 80% of the sky every day. The data are immediately made available to the scientific community through the public Swift data archive .

3 Survey data set and screening criteria

We analyzed the first 39 months of the BAT survey data archive, from 2004 December to the end of 2008 February. The BAT survey data are in the form of detector plane histograms (DPH). These are three dimensional arrays (two spatial dimensions, one spectral dimension) that collect count-rate data in (typically) 5-m time bins for 80 energy channels.

The data were retrieved from the Swift public archive and screened out from bad quality files, excluding those files where the spacescraft attitude was unstable (i.e., with a significant variation in the pointing coordinates). The resulting dataset was pre-analyzed (see Sect. 4), to produce preliminary Detector Plane Images (DPI, obtained integrating the DPH along the spectral dimension) from where the bright sources (S/N > 8) and background were subtracted; very noisy DPHs, i.e., with a standard deviation significantly larger than the average value where subtracted. The list of bright sources detected in each DPH was used to identify and discard the files suffering from inaccurate position reconstruction. After cross-correlating the position of these sources with the ISGRI catalogue, the GRB positions, and the newly discovered Swift sources documented in literature (Tueller et al. 2008; Markwardt et al. 2005; Ajello et al. 2008a), we discarded the files where:

the bright sources in the BAT field of view are detected more than 10 arcmin from their counterpart position (because of a star tracker loss of lock);
the reconstructed image of at least one bright source has a strongly elongated shape (maybe due to an unrecognized slew).

After the screening based on these criteria, the usable archive has a total nominal exposure time of 72.7 Ms, corresponding to 91.2% of the total survey exposure time during the period under investigation.

4 Methodology

To perform a systematic and efficient search for new hard X-ray sources, we developed the B ATI MAGER, a dedicated software that produces an all-sky mosaic directly from a list of BAT data files. A complete and detailed description of the software and its performance is presented in Segreto et al. (2010). Here we report only the details of the procedure which are relevant to this work.

4.1 The code

The B ATI MAGER integrates each single DPH in a selected energy range, producing the corresponding DPI. A preliminary cleaning of the disabled and noisy pixels is performed, and the DPI is cross-correlated with the mask pattern, to identify and subtract bright sources (with S/N > 8). The background, modelled on a large scale from the analysis of the shadowgram residuals by performing a Principal Component Analysis (Kendall 1980), is then subtracted. A further search for bad pixels is performed, to obtain the final map of all pixels to be excluded in the following steps. A further correction is applied to take into account differences in the detection efficiency of single detector pixels, by means of a time/energy dependent efficiency map, built by stacking all the processed DPI and equalizing the average residual contribution for each pixel. The original DPI, corrected for the efficiency map and cleaned for bad pixels, is processed again, with all the contributions from the background and the bright sources identified in the previous steps computed simultaneously, to correct for cross-contamination effects. These contributions are subtracted from the DPI, that is then converted into a sky image, using the Healpix projection (Górski et al. 2005). This projection provides an equal-area pixelization on a sphere and allows the generation of an all-sky map, avoiding the distortion introduced by other types of sky projections far from the projection center. This sky map is then corrected for the occultation of Sun, Earth, and Moon. The sky maps produced from each DPI are added together, with the intensity in a given sky direction computed from the contribution from all the sky images, each inversely weighted for its variance in that direction. As described above, the bright sources and background were already subtracted from each single DPI; therefore, this all-sky mosaic contains only the residual sky contribution. To correct for residual systematic effects (e.g., imperfect modelling of the source illumination pattern or of the background distribution), the all-sky S/N map is sampled on a scale significantly larger than the PSF: the local average S/N is subtracted and its measured variance used to normalize the local S/N distribution. Finally, we obtain a S/N map with zero average and unitary variance that can be used to complete a blind source detection.

4.2 Detection strategy

We created all-sky maps in three energy bands: 14-150 keV, 14-70 keV, and 14-30 keV. The source detection in the all-sky map is performed by searching for local excesses in the significance map. The source position and its peak significance are then refined with a fit restricted within a region of a few pixels, where the excess dominates over the noise distribution. Only detections with peak significance greater than 4.8 sigma are included in our list of detected sources. We found that this threshold represents the optimal value maximizing the number of detectable sources, maintaining at the same time an acceptable number of spurious detections: taking into account the total number of pixels in the all sky map and their spatial correlation, the PSF, and the Gaussian distribution of the noise, we expect 15 spurious detections above our threshold in each energy band, because of statistical fluctuations (Segreto et al. 2010). Therefore, the total number of spurious detections will be between 15 and 45 (1.6% to 4.7% of the total number of our detections, see below), the best case occurring if each noise fluctuation above the threshold appears simultaneously in all three bands, the worst case occurring if each fluctuation appears only in one energy band. A few sources ( ${\sim}5\%$ ) detected with a significance slightly lower than our threshold were included in the detection list because their S/N is significantly higher than the negative excess (in modulus) of the local noise distribution.

The resulting detection catalogues (one for each of the three energy bands) were cross-correlated and merged into a single catalogue: when source candidates closer than 10 arcmin were present in the sky maps of different energy bands, they were reported in the merged catalogue as a single source candidate. We obtain a final number of 962 source candidates (detected in at least one of the three energy bands). We assume the most accurate source position to be that corresponding to the energy range with the highest detection significance.

We evaluated the hardness ratio of the detected sources as Rate(30-150 keV)/Rate(14-30 keV) (the hard rate is evaluated as the difference between the count rates in the 14-150 and in the 14-30 energy bands). In Fig. 1, we plot the hardness ratio as a function of the significance of each detected source, showing the energy range where the detection has the highest significance. Repeating the detection process in three energy bands optimizes the S/N for each source, yielding more reliable values of the source position, whose uncertainty scales inversely with the significance. Moreover, a significant subsample of sources was detected in only one of the three energy bands (56 in the 14-150 keV energy band, 38 in the 14-30 keV energy band, 78 in the 14-70 keV energy band) demonstrating that searching in different energy bands maximizes the number of detectable sources.

Figure 2 shows that the distribution of the detected sources (orange squares) versus Galactic latitude flattens for |b| > 5, which we shall hereafter consider to be our operational definition of the Galactic plane.

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{11184_01.ps} \end{figure}$	Figure 1: Hardness ratio (defined as R(30-150)/R(14-30)) of the sources detected with B ATI MAGER as a function of the best detection significance. Different symbols refer to the energy range where each source was detected at the highest S/N. The solid line is the average hardness ratio value.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{11184_02.ps} \end{figure}$	Figure 2: Distribution of the detected sources versus Galactic latitude. Each bin corresponds to a solid angle of ${\sim }0.50$ sr.
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4.3 Identification strategy

The identification of the counterpart to the BAT detections was performed following three different strategies:

A. The position of each of the 962 detected excesses was cross-correlated with the coordinates of the sources included in the INTEGRAL General Reference Catalogue (v. 27), which contains 1652 X-ray emitters, and with the coordinates of the counterpart of the 48 new identifications of BAT sources already published (Ajello et al. 2008b; Tueller et al. 2008; Markwardt et al. 2005; Ajello et al. 2008a) and not included in the above catalogue. We adopted as a counterpart a source within a radius R=8.4 arcmin from the BAT position (4 standard deviations error circle for a source detection at 4.8 standard deviations, Segreto et al. 2010). With this method, we obtained 458 identifications, 295 with $\vert b\vert>5^{\circ}$ . Our choice of the error radius enabled us to maximize the associations and ensure that the number of spurious associations were maintained to a negligible level. The number of spurious identifications due to chance spatial coincidence was evaluated using the following expression:

$\begin{displaymath}% N_{\rm sp}=\frac{N\times A_R}{A}\times{N_{\rm cat}} \end{displaymath}$

(1)

where, A_R is the selected error circle area, A is the total sky area under investigation, and N and $N_{\rm cat}$ are the number of BAT detections and of candidate counterparts in A. The above formula assumes both source distributions to be uniform over the sky. To take into account the higher density of sources on the Galactic plane, we divided the sky into two regions: $\vert b\vert\leq 5^{\circ}$ (the Galactic plane, with N= 190, $N_{\rm cat}=651$ ) and $\vert b\vert>5^{\circ}$ (N=772, $N_{\rm cat}=1049$ ). The number of expected spurious identifications is 2.1 within $\vert b\vert<5^{\circ}$ and 1.3 elsewhere. Since the assumption of uniform distribution could be only a crude approximation, we verified the evaluation of expected spurious associations with an alternative method: we produced a set of 962 coordinate pairs by inverting the position of the detected excesses with respect to the Galactic reference system and cross-correlated these positions with both the INTEGRAL General Reference Catalogue and the published BAT identifications. We obtained 3 spurious associations, in full agreement with the value obtained in Eq. (1).

B. We searched for observations from Swift/XRT containing the remaining (504) unidentified excesses in their field. We found Swift/XRT observations for 186 BAT source candidates. Source detection inside these X-ray images was performed using XIMAGE (v4.4). When a source was detected inside a 6.3 arcmin error circle (99.7% confidence level for a source detection at 4.8 standard deviations, Segreto et al. 2010), we first measured its hardness ratio in the 0.3-10 keV range (with 3 keV as a common boundary of the two ratio bands) and its count rate above 3 keV. We identified a source as the counterpart of a BAT detection, if at least one of the above conditions was satisfied: hardness ratio >0.5, count rate above 3 keV >5 $\times$ 10^-3 c s^-1. In seven cases where two candidates, satisfying at least one of the threshold conditions, were found inside the BAT error circle, we selected the counterpart to be the closest source to the BAT position. With this method, we identified 170 source counterparts. To evaluate the number of expected spurious identifications, we collected a large sample of XRT observations of GRB fields, using only late follow-ups (where the GRB afterglow has faded) with the same exposure time distribution as the XRT pointings of the BAT sources. We searched for sources within a 6.3 arcmin error circle centered on the nominal pointing position in each of these fields, excluding any GRB residual afterglow, and satisfying at least one of the above threshold conditions. We detected 7 sources, therefore, the number of expected spurious identifications is consistent with the number of multiple XRT detections inside the BAT error circle. We also searched for field observations with other X-ray instruments (XMM-Newton, Chandra, BeppoSAX), finding 25 identifications, out of 30 pointings. Given the low number of available fields, the number of expected spurious identifications within this sample is irrelevant.

C. For the remaining unidentified sky map excesses (309), we searched for spatial coincidence inside an error circle of 4.2 arcmin radius ( $90\%$ confidence level for a source detection at 4.8 standard deviations, Segreto et al. 2010) with sources included in the SIMBAD catalogues. The size of the search radius was fixed to 4.2 arcmin to ensure a negligible number of spurious identifications (see below). We restricted our search to the following SIMBAD object classes: cataclysmic variable (CV), high mass X-ray binaries (HXB), low mass X-ray binaries (LXB), Seyfert 1 (Sy1), Seyfert 2 (Sy2), Blazar and BL Lac (Bla, BLL), and LINERs (LIN), for a total of 22 425 objects in the SIMBAD database. This strategy allowed us to identify 92 detections, only one source being at low Galactic latitude ( $\vert b\vert<5^{\circ}$ ). The number of expected spurious identifications was evaluated with the two methods described by strategy A. According to Eq. (1), we expect 0.03 spurious identifications within $\vert b\vert<5^{\circ}$ (20 BAT detections and 391 Simbad sources in the classes of interest) and 2.7 elsewhere (289 BAT detections and 22 034 Simbad sources); using the set of 309 coordinate pairs obtained inverting with respect to the Galactic center the positions of the sources in our sample, we find 3 spurious associations, consistent with the first method. The cross-correlation between unidentified sky excess and the SIMBAD catalogue of QSOs was treated separately because the coincidence error circle of 4.2 arcmin radius results in a high number of spurious associations (9 out of 17 associations). A radius of 2 arcmin allowed us to identify 9 sources as QSOs, and to optimize the ratio of the total number of associations to the expected number of spurious associations ( ${\sim}2$ ).

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{11184_03.ps} \end{figure}$

Figure 3:

Offset between the BAT position and the counterpart position as a function of the detection significance. A few values are far from the overall distribution: those marked with a star (sources number 535, 564, 565, 570, 571, 574, 584 and 586 in Table 2) are in crowded field and the reconstructed sky position suffers from the contamination of the PSF of the nearest sources; the one marked with a circle is an extended source (Coma Cluster). The solid line represents the fit to the data (excluding the few outliers) with a power law.

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In Fig. 3, we report the offsets of each BAT source with respect to its identified counterpart as a function of the detection significance (S/N). The offset versus the detection significance can be modeled with a power-law plus a constant. The best-fit equation that we obtained was:

$\begin{displaymath}% \textup{ offset}{\rm (arcmin)}\! =\! (9.1\pm 1.6) \times (S/N)^{-0.93\pm 0.09} \!+\! (0.21\pm 0.03). \end{displaymath}$

(2)

The constant in Eq. (2) represents the systematic error due to a residual boresight misalignment. At the detection threshold of 4.8 standard deviations, the average offset is ${\sim}2.6$ arcmin.

Figure 4 shows the distribution of the identified sources for each identification strategy as a function of the offset between the BAT position and the counterpart position. The peak of the distribution is at a lower offset for strategy A because the sample of the sources identified with this strategy contains the brightest objects. The peak of the distribution relevant to strategy B is at a lower offset than the distribution of strategy C because the XRT follow-up observations were performed on the more significant still unidentified source candidates.

All the identifications obtained with the three strategies (754) were merged into the final catalogue reported in Table 2 (see Sect. 6), where a flag indicates the identification method for each source. Figure 2 shows the distribution of all identified sources (black diamonds) as a function of the Galactic latitude.

A set of 208 detections could not be associated with a counterpart. These source candidates have a detection significance of between 4.8 and 14 standard deviations and a flux in the 14-150 keV band of between 6.7 $\times$ 10^-12 and 2.7 $\times$ 10^-11 erg cm^-2 s^-1. Thirty-three sources out of 208 are detected in all the three enegy bands, and 63 in two energy bands. The unidentified detections are distributed quite uniformly across the sky (Fig. 2, green circles), 190 sources out of 208 being located above the Galactic plane ( $\vert b\vert>5^{\circ}$ ).

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{11184_04.ps} \end{figure}$	Figure 4: Distribution of the identified sources for each identification strategy (Sect. 4.3) as a function of the offset between the BAT position and the counterpart position.
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5 Sky coverage and limiting flux

Figure 5 shows the sky coverage, defined as the fraction of the sky covered by the survey as a function of the detection limiting flux. The limiting flux for a given sky direction is calculated by multiplying the local image noise by a fixed detection threshold of 5 standard deviations. This threshold, higher than the one adopted for source detection (Sect. 4.2), was used to compare the BAT sky coverage with those produced with the INTEGRAL data survey. The large BAT field of view, the large geometrical area, and the Swift pointing distribution, which covers the sky randomly and uniformly according to the appearance of GRBs, allowed an unprecedented sensitive and quite uniform sky coverage to be obtained. The 39 month BAT survey covers 90% of the sky down to a flux limit of 2.5 $\times$ 10^-11 erg cm^-2 s^-1 (1.1 mCrab), and 50% of the sky down to 1.8 $\times$ 10^-11 erg cm^-2 s^-1 (0.8 mCrab). In the same figure, the BAT sky coverage is compared with that of INTEGRAL/ISGRI after 44 months of observation (Krivonos et al. 2007).

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{11184_05.ps} \end{figure}$	Figure 5: Fraction of the sky covered by the Swift-BAT and INTEGRAL-ISGRI surveys vs. limiting flux.
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$\begin{figure} \par\includegraphics[width=17cm,clip]{11184_06-NEW.eps} \end{figure}$	Figure 6: Map of the limiting flux (in mCrab) of the 39-months BAT-survey data in the 14-150 keV band, projected in Galactic coordinates, with the ecliptic coordinates grid superimposed (the thick lines represents the ecliptica axes). The scale on the colorbar is in mCrab.
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Figure 6 shows the limiting flux map in Galactic Aitoff projection, with the ecliptic coordinates grid superimposed. The minimum detection limiting flux is not fully uniform on the sky: the Galactic center and the ecliptic plane are characterized by a poorer sensitivity because of high contamination from intense Galactic sources and to the observing constraints on the Swift spacecraft. The highest flux sensitivity is achieved close to the ecliptic poles, where a detection flux limit of about 1.1 $\times$ 10^-11 erg cm^-2 s^-1 is reached ( ${\sim}0.5$ mCrab).

6 The 39-month catalogue

The complete catalogue of the sources identified in the first 39 months of BAT survey data is reported in Table 2. The table contains the following information:

Palermo BAT Catalogue (PBC) name of the source (Col. 2), compiled from the BAT coordinates with the precision of 0.1 arcmin on RA.
Counterpart identification (Col. 3) and source type (Col. 4) coded according to the nomenclature used in SIMBAD.
RA and Dec of the BAT source in decimal degrees (Cols. 5, 6).
Error radius (Col. 7), offset with respect to the counterpart position (Col. 8) and significance (Col. 9), obtained for the energy band with the highest significance (a flag in Col. 14 indicates the energy range with the maximum significance).
Flux in the widest band of detection, averaged over the entire survey period (Col. 10). For most of the sources, this is 14-150 keV. In the other cases, a flag in Col. 14 indicates the appropriate band. To convert count rates into fluxes, we derived a conversion factor for each of the three bands using the corresponding Crab count rate and the Crab spectrum used for BAT calibration purposes, as reported in the BAT calibration status report.
Hardness ratio defined as Rate[30-150 keV]/Rate [14-30 keV], where the hard rate is evaluated as the difference between the count rates in the 14-150 and in the 14-30 energy bands (Col. 11).
Redshift of the extragalactic sources (Col. 12), from the SIMBAD database (or NED, for the few cases that were not reported in SIMBAD).
Log of the rest-frame luminosity in the 14-150 keV band for extragalactic objects (Col. 13), calculated using the luminosity distance for sources with redshift >0.01, and using the distance reported in the Nearby Galaxies Catalogue (NBG, Tully 1988) or NED, for the few cases not reported in the NBG catalogue, for sources with redshift <0.01.
Flag column (Col. 14) with information about: energy band with the highest significance (A), energy band used for the calculation of the flux (B), flag for already known hard X-ray sources (C), position with respect to the Galactic plane ( $\vert b\vert<5^{\circ}$ , D), and strategy used for the identification (E, see Sect. 4.3).

6.1 Statistical properties of the catalogue

Table 1 describes the distribution of the 754 sources in our catalogue among different object classes: ${\sim}69$ % of the catalogue consists of extragalactic objects, ${\sim}27$ % are Galactic objects, and ${\sim}4$ % are already known X-ray or gamma-ray emitters whose nature is still to be determined. Figure 7 shows the distribution of all the sources in our catalogue, colour-coded according to the object class, where the size of the symbol is proportional to the 14-150 keV flux (for those sources not detected in the 14-150 keV band, the flux in the widest band of detection has been extrapolated to the 14-150 keV range using the BAT Crab spectrum).

Table 1: Classification of the known sources detected in the BAT survey.

We compared this distribution with the third ISGRI catalogue (Bird et al. 2007). The results are plotted in Fig. 8. We measured a dramatic improvement in the detection of extragalactic objects, both in the nearby Universe (normal galaxies, LINERs) and at greater distances (Seyfert galaxies, QSO, clusters of galaxies). As expected from the sky coverage achieved by the BAT survey data (Fig. 5), most of our identified sources have a flux below 1 $\times$ 10^-10 erg s^-1 cm^-2 and are located outside the Galactic plane. We also detected many Galactic sources that are not included in the ISGRI catalogue, most of which are cataclysmic variables and X-ray binaries. This can be explained in part by the different pointing strategy of the two instruments. However, Fig. 2 shows that, although most of our newly identified sources (red triangles) are above the Galactic plane, where the ISGRI exposure is low, we also detect a few sources on the Galactic plane most of which we identify as X-ray binaries (1E 1743.1-2852, GRO 1750-27, SAX J1810.8-2609, and XTE J1856+053). We verified that their detections are caused by a transient intense emission observed in the large FoV of BAT.

$\begin{figure} \par\includegraphics[width=17cm,clip]{11184_07.ps} \end{figure}$	Figure 7: Map of the sources that we detect in the BAT survey data (Galactic coordinates). Different colors denote different object classes, as detailed in the legend. The size of the symbol is proportional to the source flux in the 14-150 keV band.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11184_08.ps}\vspace*{2mm} \includegraphics[width=8cm,clip]{11184_09.ps} \end{figure}$	Figure 8: Comparison between the sources in our catalogue and those reported in the third ISGRI catalogue (Bird et al. 2007). Top: galactic sources. Bottom: extragalactic sources.
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$\begin{figure} \par\includegraphics[width=8.5cm,clip]{11184_10.ps} \end{figure}$	Figure 9: Redshift distribution of the extragalactic sources in the BAT survey catalogue for different classes of extragalactic sources.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{11184_11.ps} \end{figure}$	Figure 10: $\langle V/V_{\rm Max} \rangle$ versus significance for our sample of extragalactic sources. The solid line is the expected value (0.5), the dashed line is the average value for $S/N>4.5\sigma$ , and the shaded area covers the $1\sigma$ error for the average value.
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We detected emission from 18 clusters of galaxies. We verified that for 17 of them the spectral distribution in the 14-150 keV band is consistent with the tail of a thermal emission with kT $\sim$ 10 keV, without evidence of hard non-thermal emission. Only for Abell 2142 did we find evidence of a power law component that could be related to the AGN content of the cluster.

6.2 The extragalactic subsample

The catalogue contains 519 extragalactic objects. Figure 9 shows the redshift distribution of our sample for the main classes of extragalactic objects. Most of the emission-line AGNs are located at z<0.1, but we also detected a few Seyfert 1 galaxies at higher redshift (up to ${\sim}0.29$ ). Seyfert 2 galaxies are detected up to $z\sim 0.4$ . Blazars are detected up to $z\sim 3.7$ , and QSOs are detected up to $z\sim 2.4$ .

We verified the completeness of our sample of 366 emission-line galaxies (i.e., the significance limit down to which we include in the sample all objects above a given flux limit) using the $V/V_{\rm Max}$ test (Schmidt 1968; Huchra & Sargent 1973). This method was developed to test the evolution of complete samples of objects, but can also be used to test the completeness of non-evolving samples. For each source, V is the volume enclosed by the object distance, while $V_{\rm Max}$ is the volume corresponding to the maximum distance where the object could still be revealed in the survey (and thus depends on the limiting flux in the direction of the object). In the case of no evolution, the expected value of $<V/V_{\rm Max}>$ , averaged over the entire sample, is 0.5. We assumed the hypothesis of no evolution and uniform distribution in the local Universe. For each source in the sample and for each significance level tested for completeness ( $\sigma_{\rm T}$ ), we computed the quantity $V/V_{\rm Max}$ as $[F/\sigma_{\rm T}\Delta F)]^{-3/2}$ , where F is the flux of the source and $\Delta F$ is its 1 standard deviation uncertainty. The quantity $<V/V_{\rm Max}>$ was obtained by averaging $V/V_{\rm Max}$ over the number N of all sources in the sample detected with a significance higher than $\sigma_{\rm T}$ , and its error is 1/12N. Figure 10 shows the results of this test: the distribution becomes constant at $S/N\gtrsim 4.5\sigma$ , with a mean value of $0.497 \pm0.007$ , consistent with the expected value of 0.5. Thus, we can confidently assume that our sample is complete to our adopted significance threshold of $4.8\sigma$ .

Table 2: BAT survey 39 months catalogue.

6.3 log(N) - log(S) distribution

The $\log(N)-\log(S)$ distribution was evaluated by summing the contributions of all the detected sources firmly identified with extragalactic objects (Table 2) and all the unidentified detections. We selected only sources with $\vert b\vert>5^{\circ}$ : Fig. 2 (orange squares) shows that the detection distribution is uniform above this Galactic latitude limit. The cumulative distribution is weighted by the area in which these sources could have been detected. The following formula was applied:

$\begin{eqnarray*}N({>}S)=\sum_{S_i>S} \frac{1}{\Omega_i}, \end{eqnarray*}$

where N is the total number of detected sources with fluxes greater than S, S_i is the flux of the ith source and $\Omega_i$ is the sky coverage associated with the flux S_i (Fig. 5).

To avoid the presence of systematic errors in the determination of the $\log(N)-\log(S)$ caused by spurious source detections and to the large relative uncertainty in the sky coverage at the lower end of the flux scale, we limited the construction of the $\log(N)-\log(S)$ to fluxes greater than ${\sim}1.5$ $\times$ 10^-11 erg cm^-2 s^-1. The resulting $\log(N)-\log(S)$ distribution contains 330 sources (14 unidentified) and covers a flux range up to 3 $\times$ 10^-10 erg s^-1 cm^-2.

We applied a linear least-square fit to determine the slope of the $\log(N)-\log(S)$ distribution assuming a power law in the form N(>S)=K $\times$ $(S/S_0)^{-\alpha}$ , where S₀ is assumed to be 1 $\times$ 10^-11 erg cm^-2 s^-1. The fit infers a value of $\alpha=1.56$ $\pm$ 0.06 and a normalization of 570 $\pm$ 24 sources with flux greater than 10^-11 erg cm^-2 s^-1, corresponding to a density of $(1.38\pm 0.06)$ $\times$ 10^-2 deg^-2. The single power-law model is found to provide an acceptable description of the data ( $\chi^2 = 0.65$ ; 31 d.o.f.) with a slope consistent with a Euclidean distribution.

The presence of spurious detections in the sample of unidentified sources could introduce a systematic effect in both the slope and the normalization of the $\log(N)-\log(S)$ . We expect between 15 and 45 spurious detections to be caused by statistical fluctuations (see Sect. 4.2), which correspond to a percentage between ${\sim}7$ and ${\sim}22$ % in the sample of the ${\sim}208$ unidentified sources. This implies that 1-3 unidentified sources among those used in the fit of the $\log(N)-\log(S)$ could be spurious. We checked that their contribution does not introduce any significant systematic errors in the best-fit values.

The integrated flux is ${\sim}4.5$ $\times$ 10^-13 erg cm^-2 s^-1 deg^-2 corresponding to $\sim$ 1.4% of the intensity of the X-ray background in the 14-170 keV energy band as measured by HEAO-1 (Gruber et al. 1999).

We compared this $\log(N)-\log(S)$ law with the one derived from INTEGRAL data (Krivonos et al. 2007) in the 17-60 keV band. To convert our $\log(N)-\log(S)$ into the 17-60 keV band, we used the Crab spectral parameters derived by the INTEGRAL analysis (Laurent et al. 2003). We determined a slope of $\alpha=1.62$ $\pm$ 0.08 and a normalization of 240 $\pm$ 12 sources with flux higher than 1 mCrab, corresponding to a density of $(5.8\pm 0.3)$ $\times$ 10^-3 deg^-2. These parameters are in full agreement with those reported by Krivonos et al. (2007).

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{11184_12.ps} \end{figure}$	Figure 11: $\log(N)-\log(S)$ distribution for the BAT extragalactic sources.
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7 Conclusions

We have analyzed the BAT hard X-ray survey data of the first 39 months of the Swift mission. To complete this analysis we developed a dedicated software (Segreto et al. 2010) that performs data reduction, background subtraction, mosaicking, and source detection for the BAT survey data. This software is completely independent from that developed by the Swift-BAT team. It is a single tool that provides all the products relevant to the BAT survey sources (e.g., images, spectra, and light curves).

The large BAT field of view, the large geometrical area, and the Swift pointing strategy have allowed us to obtain an unprecedented, very sensitive, and quite uniform sky coverage that has provided a significant increase in sources detected in the hard X-ray sky. The survey flux limit is 2.5 $\times$ 10^-11 erg cm^-2 s^-1 (1.1 mCrab) for 90% of the sky and 1.8 $\times$ 10^-11 erg cm^-2 s^-1 (0.8 mCrab) for 50% of the sky.

We have derived a catalogue of 754 identified sources detected above a significance threshold of 4.8 standard deviations. The association of these sources with their counterparts has been performed in three alternative strategies: cross-correlation with the INTEGRAL General Reference Catalogue and with previously published BAT catalogues (Tueller et al. 2008; Markwardt et al. 2005; Ajello et al. 2008a); analysis of soft X-ray field observations with Swift-XRT, XMM-Newton, Chandra, BeppoSAX; and cross-correlation with the SIMBAD catalogues of Seyfert galaxies, QSOs, LINERs, Blazars, cataclysmic variables, and X-ray binaries. The expected total number of spurious identifications is negligible. A set of 208 detections have not yet been associated with a counterpart. These candidate sources will be the subject of a follow-up campaign with Swift-XRT in the near future.

The extragalactic sources represents ${\sim}69$ % of our catalogue (519 objects), ${\sim}27$ % are Galactic objects, and ${\sim}4$ % are already known X-ray or gamma-ray emitters, whose nature is still to be determined. Compared with the 3rd ISGRI catalogue (Bird et al. 2007), we identify 176 more Seyfert galaxies, 26 more normal galaxies, 13 more galaxy clusters, 13 more QSO, 57 more Blazars, and 5 more LINERs. The redshift limit for the detected emission-line AGNs is ${\sim}0.4$ , with 31 objects with z>0.1. Blazars and QSOs are detected up to $z\sim 3.7$ and $z\sim 2.4$ , respectively. Among the Galactic sources, we significantly increase the number of cataclysmic variables detected in the hard X-ray band (29 new objects). We also detect 22 X-ray binaries that are not included in the ISGRI catalogue, even though the total number of X-ray binaries that we detect is lower than the sample included in the ISGRI catalogue.

Based on the extragalactic sources sample and on the achieved sky coverage, we have evaluated the $\log(N)-\log(S)$ distribution for fluxes higher than 1.5 $\times$ 10^-11 erg cm^-2 s^-1. The slope 1.55 $\pm$ 0.06 is consistent with a Euclidean distribution. We estimate that the total number of extragalactic sources at $\vert b\vert>5^{\circ}$ and flux greater than 1.0 $\times$ 10^-11 erg cm^-2 s^-1 is ${\sim}566$ . Converting this $\log(N)-\log(S)$ into the 17-60 keV band, our results are in full agreement with those reported by Krivonos et al. (2007) for the INTEGRAL survey. The integrated flux of this extragalactic sample is ${\sim}1.4\%$ of the cosmic X-ray background in the 14-150 keV range (Gruber et al. 1999; Frontera et al. 2007; Churazov et al. 2007; Ajello et al. 2008c).

Forthcoming papers will focus on the detection of transient sources, spectral properties of the extragalactic sample, and updates of the catalogue.

Acknowledgements

G.C. acknowledges B. Sacco and M. Ajello for useful discussions that helped to improve this paper, and the referee W. Voges for his helpful comments and suggestions. This research has made use of NASA's Astrophysics Data System Bibliographic Services, of the SIMBAD database, operated at CDS, Strasbourg, France, as well as of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This work was supported by contract ASI/INAF I/011/07/0.

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Footnotes

... survey: Table 2 is also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/510/A48
... team: http://heasarc.gsfc.nasa.gov/docs/swift/analysis/
... archive: http://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/swift.pl
... Catalogue: http://isdc.unige.ch/?Data+catalogs
... report: http://swift.gsfc.nasa.gov/docs/swift/analysis/bat_digest.html#calstatus

All Tables

Table 1: Classification of the known sources detected in the BAT survey.

Table 2: BAT survey 39 months catalogue.

All Figures

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{11184_01.ps} \end{figure}$	Figure 1: Hardness ratio (defined as R(30-150)/R(14-30)) of the sources detected with B ATI MAGER as a function of the best detection significance. Different symbols refer to the energy range where each source was detected at the highest S/N. The solid line is the average hardness ratio value.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{11184_02.ps} \end{figure}$	Figure 2: Distribution of the detected sources versus Galactic latitude. Each bin corresponds to a solid angle of ${\sim }0.50$ sr.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{11184_03.ps} \end{figure}$	Figure 3: Offset between the BAT position and the counterpart position as a function of the detection significance. A few values are far from the overall distribution: those marked with a star (sources number 535, 564, 565, 570, 571, 574, 584 and 586 in Table 2) are in crowded field and the reconstructed sky position suffers from the contamination of the PSF of the nearest sources; the one marked with a circle is an extended source (Coma Cluster). The solid line represents the fit to the data (excluding the few outliers) with a power law.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{11184_04.ps} \end{figure}$	Figure 4: Distribution of the identified sources for each identification strategy (Sect. 4.3) as a function of the offset between the BAT position and the counterpart position.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{11184_05.ps} \end{figure}$	Figure 5: Fraction of the sky covered by the Swift-BAT and INTEGRAL-ISGRI surveys vs. limiting flux.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[width=17cm,clip]{11184_06-NEW.eps} \end{figure}$	Figure 6: Map of the limiting flux (in mCrab) of the 39-months BAT-survey data in the 14-150 keV band, projected in Galactic coordinates, with the ecliptic coordinates grid superimposed (the thick lines represents the ecliptica axes). The scale on the colorbar is in mCrab.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[width=17cm,clip]{11184_07.ps} \end{figure}$	Figure 7: Map of the sources that we detect in the BAT survey data (Galactic coordinates). Different colors denote different object classes, as detailed in the legend. The size of the symbol is proportional to the source flux in the 14-150 keV band.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{11184_08.ps}\vspace*{2mm} \includegraphics[width=8cm,clip]{11184_09.ps} \end{figure}$	Figure 8: Comparison between the sources in our catalogue and those reported in the third ISGRI catalogue (Bird et al. 2007). Top: galactic sources. Bottom: extragalactic sources.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{11184_10.ps} \end{figure}$	Figure 9: Redshift distribution of the extragalactic sources in the BAT survey catalogue for different classes of extragalactic sources.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{11184_11.ps} \end{figure}$	Figure 10: $\langle V/V_{\rm Max} \rangle$ versus significance for our sample of extragalactic sources. The solid line is the expected value (0.5), the dashed line is the average value for $S/N>4.5\sigma$ , and the shaded area covers the $1\sigma$ error for the average value.
Open with DEXTER
In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{11184_12.ps} \end{figure}$	Figure 11: $\log(N)-\log(S)$ distribution for the BAT extragalactic sources.
Open with DEXTER
In the text

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