The Palermo Swift-BAT Hard X-Ray Catalogue
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Astronomy & Astrophysics manuscript no. 15249 c ESO 2018 October 24, 2018 The Palermo Swift-BAT hard X-ray catalogue III. Results after 54 months of sky survey G. Cusumano1, V. La Parola1, A. Segreto1, C. Ferrigno1,2,3, A. Maselli1, B. Sbarufatti1, P. Romano1, G. Chincarini4,5, P. Giommi6, N. Masetti7, A. Moretti5, P. Parisi7,8, G. Tagliaferri5 1 INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, Via U. La Malfa 153, I-90146 Palermo, Italy 2 Institut f¨ur Astronomie und Astrophysik T¨ubingen (IAAT) 3 ISDC Data Centre for Astrophysics, Chemin d’Ecogia´ 16, CH-1290 Versoix, Switzerland 4 Universit`adegli studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, I-20126 Milan, Italy 5 INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, 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, I-40129 Bologna, Italy 8 Dipartimento di Astronomia, Universit`adi Bologna, Via Ranzani 1, I-40127 Bologna, Italy ABSTRACT Aims. We present the Second Palermo Swift-BAT hard X-ray catalogue obtained by analysing data acquired in the first 54 months of the Swift mission. Methods. Using our software dedicated to the analysis of data from coded mask telescopes, we analysed the BAT survey data in three energy bands (15–30 keV, 15–70 keV, 15–150 keV), obtaining a list of 1256 detections above a significance threshold of 4.8 standard deviations. The identification of the source counterparts is pursued using two strategies: the analysis of field observations of soft X-ray instruments and cross-correlation of our catalogue with source databases. Results. The survey covers 50% of the sky to a 15–150 keV flux limit of 1.0 × 10−11 erg cm−2 s−1 and 9.2 × 10−12 erg cm−2 s−1 for |b| < 10◦ and |b| > 10◦, respectively. The Second Palermo Swift-BAT hard X-ray catalogue includes 1079 (∼ 86%) hard X-ray sources with an associated counterpart (26 with a double association and 2 with a triple association) and 177 BAT excesses (∼ 14%) that still lack a counterpart. The distribution of the BAT sources among the different object classes consists of ∼ 19% Galactic sources, ∼ 57% extragalactic sources, and ∼ 10% sources with a counterpart at softer energies whose nature has not yet been determined. About half of the BAT associated sources lack a counterpart in the ROSAT catalogues. This suggests that either moderate or strong absorption may be preventing their detection in the ROSAT energy band. The comparison of our BAT catalogue with the Fermi Large Area Telescope First Source Catalogue identifies 59 BAT/Fermi correspondences: 48 blazars, 3 Seyfert galaxies, 1 interacting galaxy, 3 high mass X-ray binaries, and 4 pulsars/supernova remnants. This small number of correspondences indicates that different populations make the sky shine in these two different energy bands. Key words. X-rays: general - Catalog - Surveys 1. Introduction 2010) contains a list of 754 hard X-ray sources with an associ- ated counterpart detected in the first 39 months of the Swift mis- The Burst Alert Telescope (BAT; Barthelmy et al. 2005) on- sion. Among them, ∼ 69% are extragalactic, ∼ 27% are Galactic board the Swift observatory (Gehrels et al. 2004) is a coded- objects, ∼ 4% are already known X-ray or γ-ray emitters whose aperture imaging camera operating in the 15–150 keV energy nature has not yet been determined. range with a large field of view (1.4 steradian half coded) and a point spread function (PSF) of 17 arcmin (full width half max- imum). The telescope is mainly devoted to the monitoring of arXiv:1009.0522v2 [astro-ph.HE] 7 Sep 2010 In this paper, we provide the Second Palermo Swift-BAT hard a large fraction of the sky for the occurrence of gamma ray X-ray catalogue obtained from the analysis of the data relative bursts (GRBs). While waiting for new GRBs, BAT continuously to the first 54 months of the Swift mission and including 1256 collects spectral and imaging information about the sky, cover- BAT high-energy sources. The paper is organised as follows: in ing a fraction of between 50% and 80% of the sky every day, Sect. 2, we describe the screening and the analysis of the BAT providing the opportunity for a substantial gain in our knowl- survey data and the global survey properties; in Sect. 3, we illus- edge of the Galactic and extragalactic sky in the hard X–ray trate our analysis strategy; in Sect. 4, we describe the counterpart domain and increasing the sample of objects that contribute to association strategy; the 54-month catalogue and its properties the luminosity in this energy range. The first results of the BAT are described in Sects. 5 and 6. Then, in Sect. 7 we summarise survey were presented in Markwardt et al. (2005), Ajello et al. our results. (2008a), Ajello et al. (2008b), Tueller et al. (2008), Tueller et al. (2010), Cusumano et al. (2010), and Maselli et al. (2010). The First Palermo Swift-BAT hard X-ray catalogue (Cusumano et al. The cosmology adopted in this work assumes H0 = 70 km −1 −1 s Mpc ,k=0, Ωm = 0.3, and Λ0 = 0.7. Quoted errors are at Send offprint requests to: G. Cusumano, [email protected] 1σ confidence level, unless otherwise specified. 2 G. Cusumano et al.: The 54-month Palermo Swift-BAT Hard X-ray Catalogue 2. The BAT survey data 100 The results presented in this paper were obtained by analising 54 months the first 54 months of BAT survey data, from 2004 December 44 months 34 months to the end of 2009 May. The data were retrieved from the Swift 80 public archive1 in the form of detector plane histograms (DPH): 24 months |b|<10 degrees three-dimensional arrays (two spatial dimensions, one spectral 14 months dimension) that collect count-rate data in 5-minute time bins for 60 α=−0.49 +/− 0.02 80 energy channels. ) 2*10-11 -1 To process the survey data, we developed and applied a code s 40 -2 that performs screening, mosaicking, and source detection on Sky Fraction (%) data from coded mask instruments. This code is described in de- 10-11 tail in Segreto et al. (2010). To screen out poor quality files from 20 Flux (erg cm 9*10-12 the data set, we rejected the DPHs: 10 20 30 40 50 Time (Months) – with unstable spacecraft attitude (i.e., with a significant vari- 0 ation in pointing coordinates). 1·10-11 2·10-11 3·10-11 4·10-11 15-150 keV Limiting Flux (erg cm-2 s-1) – produced near the SAA and characterized by a count rate much higher than the average value. ff 100 – a ected by inaccurate position reconstruction. This was ver- 54 months ified through a pre-analysis procedure where the position of 44 months the sources detected in single DPHs was checked against a 34 months 80 list of hard X-ray sourcesand transients(see Cusumano et al. 24 months |b|>10 degrees 2010 for details). 14 months – that were very noisy, i.e., with a standard deviation in the 60 α=−0.49 +/− 0.01 count rate (subtracted of both bright sources and back- 2*10-11 ) ground) significantly larger (a factor of 2) than that expected -1 s 40 -2 from statistics. Sky Fraction (%) After the screening based on these criteria, the usable archive 10-11 20 Flux (erg cm -12 has a total nominal exposure time of ∼100 Ms, corresponding to 9*10 ∼ 92% of the total survey exposure time during the period under 10 20 30 40 50 Time (Months) investigation. 0 1·10-11 2·10-11 3·10-11 4·10-11 Figure 1 shows the sky coverage, defined as the fraction of -2 -1 the sky covered by the survey as a function of the 15–150 keV 15-150 keV Limiting Flux (erg cm s ) detection limiting flux, at different survey epochs starting from Fig. 1. Fraction of the sky (top: |b| < 10◦, bottom: |b| > 10◦) the beginning of the mission. The limiting flux of a given sky covered by the BAT survey as a function of the 15–150 keV de- direction is calculated by multiplying the local image noise by tection limiting flux for a detection threshold of 4.8 standard de- a detection threshold of 4.8 standard deviations. We derived the viations. Different colours refer to different survey epochs. The sky fraction for two sky regions (top panel: |b| < 10◦, bottom insets show the limiting flux achieved for 50% of the sky as a panel: |b| > 10◦). The 54-month BAT survey covers 50% of the function of time; the best fit is a power law consistent with t−0.5. sky to a flux limit of 1.0 × 10−11 erg cm−2 s−1 and 9.2 × 10−12 erg cm−2 s−1 for |b| < 10◦ and |b| > 10◦, respectively. The insets in Fig. 1 show the limiting flux achieved for 50% of the sky as a spherical grid projection (G´orski et al. 2005) with a pixel size of function of the cumulative observing time of the screened BAT ≈2.5 arcmin radius. survey data; the data are modelled well with a power law (N×tα, For each of these energy ranges, we derived a signal-to-noise where N=(7.5±0.3)×10−11 erg cm−2 s−1and α = −0.49±0.02for ratio (S/N) map as the ratio of the mosaic intensity to the asso- |b| < 10◦ and N=(6.4±0.2)×10−11 erg cm−2 s−1and α = −0.49± ciated statistical error.