The Palermo BAT survey
Surveys in the hard X-ray domain
Imaging techniques based on focusing the X-ray radiation by grazing reflection
from mirrors becomes ineffective above ~10 keV due to technological
constraints. At higher energies, coded-mask techniques are an efficient
alternative method, that allows for medium resolution (arcmin) imaging.
After the early hard X-ray surveys, performed with detectors equipped
with collimator-limited field of view
, 2-20 keV
0.2 keV - 10 MeV), several space observatories
have been equipped with coded-mask telescopes:
ART-P and SIGMA on board GRANAT
Coded-mask imagers are now working on board
(the IBIS-ISGRI telescope),
(the Burst Alert Telescope, BAT),
In such telescopes, the radiation coming from the sky sources is spatially modulated
by a mask consisting of an array of opaque and transparent elements and recorded
by a position sensitive detector.
In order to avoid ambiguities in the reconstruction of the sky image, the mask pattern
is designed in such a way that the radiation from each sky direction casts a unique
shadow on the detector.
In contrast to focusing imaging systems, where source photons are concentrated on a small
detector area, in a coded-mask instrument the photons emitted from a
cosmic source are spread over the whole detector area and the recorded
shadowgram is the additive contribution from all the sources
inside the field of view (FOV) and from the instrumental background.
By knowing the orientation of the satellite in space and by analyzing the data
from the detector it is possible, through image reconstruction algorithms, to
evaluate the position and the intensity of all cosmic sources and
of the background level, thus reproducing the image of the observed sky.
Several algorithms are used to reconstruct the sky observed with coded-mask
instruments. The most widely used method for image
reconstruction is the
cross-correlation of the recorded data pattern with the mask pattern.
The BAT telescope
The Burst Alert Telescope (BAT) on board the
observatory, with its large field of view (1.4 steradian
half coded) and large detector area (a factor 2 greater than ISGRI)
offers the opportunity for a large increase of the sample of objects that
contribute to the luminosity of the sky in the hard X-rays allowing for a
substantial improvement of our knowledge of the AGN population and of the cosmic
hard X-ray background.
is a coded-mask detector sensitive in the 15-150 keV band
used as a GRB detector on board Swift.
While waiting for new GRBs, it continuously collects spectral
and imaging information in hard X-rays and accumulates data in survey
mode covering a fraction between 50 and 80% of the sky every day.
Data are immediately made available to the scientific community
in the public Swift data archive
together with all housekeeping and spacecraft attitude information.
The Swift-BAT entrance window is a 2.7 m2 coded-aperture mask
of 5x5 mm2 elements placed 1 m above the detector plane, with an on-axis point
spread function (PSF) of 22 arcmin FWHM.
The mask is designed with a pseudo-random pattern that has the advantage (with
respect to cyclic patterns like e.g. the one used for ISGRI) of being quite
insensitive to the possible turning off of a limited number of detector
elements and to the presence of detector gaps; moreover the reconstructed
images do not suffer from the presence of ghost sources caused by the
periodic design of the cyclic pattern. On the other hand, it is characterized by a PSF with
an extended plateau that spans over the whole image and induces cross-contaminations
among sources in the same FOV.
The BAT detector plane consists of 5200 cm2 array of 32,768 4x4 mm2
CdZnTe elements organized in 128 elements sub-arrays (modules) separated by
thin gaps and mounted in 16 mechanical structures (blocks).
The BAT survey data are collected in the form of Detector Plane Histograms (DPH).
A DPH consists of a three-dimensional array where an 80 channel spectrum is
accumulated for each detector pixel over the integration period.
Typical DPH integration time is five minutes, but longer
integration times are sometimes found (e.g. when telemetry reduction
is needed) as well as shorter integration times since survey mode
is always interrupted when the spacecraft
begins a slew to a new target or when entering the South Atlantic
The Palermo BAT survey project
In order to exploit the BAT survey archive, a dedicated software, the
BatImager, was developed.
The BatImager code
The BatImager integrates each single DPH
in a selected energy range, producing the corresponding detector plane image
(DPI). A preliminary cleaning of the disabled and noisy pixels is performed,
and the DPI is cross-correlated with the mask pattern, in order to identify and
subtract bright sources (with S/N >8). Then the
background is modelled on a large scale and subtracted. A further search for
bad pixels is performed, obtaining 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, through a time/energy dependent
efficiency map, built 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 the
bad pixels, is processed again, with all the contributions from both the
background and the bright sources identified in the previous steps
computed simultaneously, in order 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. This projection provides an equal-area pixelization
on a sphere and allows for 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.
In order 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 unitary and variance that can be used for a
blind source detection.
Click here for a block diagram of the
The 1st Palermo BAT catalog
The 2nd Palermo BAT catalogue
We have performed a complete analysis of 54 months of BAT survey data in the
15-30, 15-70, 15-150 keV bands (Cusumano et al. 2010, A&A 524, A64). The data
sample we used covers 50% of the sky down to a flux limit in the 15-150 keV band
of 1.0E-11 erg cm-2 s-1 and 9.2E-12 erg cm-2
s-1 for |b|<10° and |b|>10° respectively.
The minimum detection limiting flux is not fully uniform on the sky.
The map below shows the limiting flux in Galactic Aitoff projection, with the
ecliptic coordinates grid superimposed. The Galactic centre 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
~6.2 × 10-12 erg cm-2 s-1 is reached; the
lowest flux sensitivity is in the
region of the Galactic centre with a detection flux limit of ~3 × 10-11
erg cm-2 s-1.
Map of the 54-months survey limiting flux in the 15-150 keV band in galactic Aitoff projection,
with the ecliptic coordinates grid superimposed. The scale on the colorbar is in
erg cm-2 s-1.
The catalogue obtained by cross-correlating and merging the lists of
excesses detected in the three energy bands contains 1256 source candidates.
For each of them, we have searched for counterparts at lower energies using two
different strategies. First we have analysed archival soft X-ray observations
covering the position of the BAT excesses, applying count rate thresholds to
select the most likely counterparts. With this strategy, we have
been able to associate 920 BAT excesses with a single softer counterpart; for
8 BAT excesses, we found two possible counterparts. The BAT excesses lacking
any association after strategy A were cross-correlated with a list of possible
counterparts compiled by merging several source lists (X-ray binaries,
cataclysmic variables, supernova remnants, pulsars, cluster of galaxies,
different classes of active galaxies, already known soft X-ray and gamma-ray
sources). This second strategy enabled us to associate 151 BAT
sources with counterparts (18 with a double association, 2 with a triple
association). The final catalogue contains 1079 BAT sources with at least
one associated counterpart and 177 unassociated sources (~14%).
Map of the sources detected in the BAT survey data with BatImager.
denote different object classes, as detailed in the legend. The size of the
symbol is proportional to the source flux in the 15-150 keV band.
The plot below shows the distribution of the catalog sources
among the different object classes. The whole sample
consist of ~15.2%
Galactic sources, ~49.3% extragalactic sources, ~12% sources with a
counterpart at softer energies whose nature has not yet been determined and
23.5% sources with no associated counterpart
The remaining unassociated sources detected with the BatImager in the
54-months all-sky map are uniformly distributed in
An identification campaign of these source candidates with
Swift-XRT through the fill-in target program is in progress.
The 2nd Palermo BAT survey catalogue and the related products are available
The BATsurvey Team members in Palermo:
Giancarlo Cusumano |
Valentina La Parola ||
Alessandro Maselli |
Alberto Segreto |
Fabio D'Anna |
Giacomo Fazio |
Giancarlo.Cusumano at iasf-palermo.inaf.it
Catalog refereed papers
The Palermo Swift-BAT hard X-ray catalogue. III. Results after 54 months of sky
Cusumano G., La Parola V., Segreto A., Ferrigno C., Maselli A., Sbarufatti B.,
Romano P., Chincarini G., Giommi P., Masetti N., Moretti A., Parisi P.,
2010 A&A 524 A64
The Palermo Swift-BAT Hard X-ray Catalogue. II. Results After 39 Months of Sky Survey
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;
2010 A&A 510 A48
The Palermo Swift-BAT hard X-ray catalogue. I. Methodology
Segreto A., Cusumano G., Ferrigno C., La Parola, V., Mangano V., Mineo T.,
2010 A&A 510 A47
More refereed papers
Evidence for a resonant cyclotron line in IGR J16493-4348 from the Swift-BAT
hard X-ray survey
D'Ai A., Cusumano G., La Parola V., Segreto A., di Salvo T., Iaria R.,
Robba N. R.
2011 A&A 532 73
On the nature of the absorber in IRAS 09104+4109: the X-ray and mid-infrared
Vignali C., Piconcelli E., Lanzuisi G., Feltre A., Feruglio C., Maiolino R.,
Fiore F., Fritz J., La Parola V., Mignoli M., Pozzi, F.
2011 MNRAS 416 2068
Suzaku and SWIFT-BAT observations of a newly discovered Compton-thick AGN
Severgnini P., Caccianiga A., Della Ceca R., Braito V., Vignali C., La Parola
V., Moretti A.
2011 A&A 525 38
The populations of hard X-ray and Gamma-ray sources: a correlation study and new
Maselli A., Cusumano G., Massaro E., Segreto A., La Parola V., Tramacere A.,
2011 A&A 531 153
The Swift-BAT survey reveals the orbital period of three high-mass X-ray binaries
D'Ai A., La Parola V., Cusumano G., Segreto A., Romano P., Vercellone S.,
Robba N. R.
2011 A&A 529 30
The broad-band X-ray spectrum of the Seyfert 1 galaxy, MCG+8-11-11
Bianchi S., de Angelis I., Matt G., La Parola V., de Rosa A., Grandi P.,
Jimenez Bailon E., Piconcelli E.
2010 A&A 522 64
Galaxy Clusters in the Swift/BAT Era. II. 10 More Clusters Detected Above 15 keV
Ajello M., Rebusco P., Cappelluti N., Reimer O., Bohringer H., La Parola V.,
2010 ApJ 725 1688
The blazar content in the Swift-BAT hard X-ray sky
Maselli A., Cusumano G., Massaro E., La Parola V., Segreto A., Sbarufatti B.
2010 A&A 520 47
The Swift-BAT hard X-ray sky monitoring unveils the orbital period of the
HMXB IGR J16493-4348
Cusumano G., La Parola V., Romano P., Segreto A., Vercellone S., Chincarini G.
2010 MNRAS 406 16
Detection of an orbital period in the supergiant high-mass X-ray binary
IGR J16465-4507 with Swift-BAT
La Parola V., Cusumano G., Romano P., Segreto A., Vercellone S., Chincarini G.
2010 MNRAS 405 66
A Strong Excess in the 20-100 keV Emission of NGC1365
Risaliti G., Braito V., La Parola V., Bianchi S., Elvis M., Fabbiano G.,
Maiolino R., Matt G., Reeves J., Salvati M., Wang J.
2009 ApJ 705 1
A search for heavily obscured AGN in the 2nd Palermo BAT catalogue
The Palermo Swift-BAT Hard X-ray Catalogue: Results after 66 months of sky
Swift-BAT all-sky monitoring: transient phenomena with timescales from days to months.
Swift-BAT: the hard-X-ray sky after 64 months of survey
Hard X-ray monitoring of HMXB with Swift-BAT: a systematic search to reveal
long term periodicities
The Palermo Swift-BAT Hard X-ray Catalogue: Results After 39 Months of Sky Survey
Heavily obscured AGN in the local Universe
The Blazar content in the Swift-BAT hard X-ray sky
The Palermo Swift-BAT Hard X-ray Catalogue: Results after 54 months of sky
La Parola V., Cusumano G., Segreto A., Maselli A., Sbarufatti B., Ferrigno C.,
Chincarini G., Tagliaferri G., Campana S., Moretti A., Giommi P., Masetti N.
AIP Conference Proceedings, Volume 1248, pp. 459-460
The Palermo Swift-BAT Hard X-ray Catalogue: Results After 54 Months of Sky
Cusumano G., La Parola V., Segreto A., Romano P., Sbarufatti B., Ferrigno C.,
Maselli A., Campana S., Moretti A., Chincarini G., Tagliaferri G., Giommi P.,
HEAD meeting #11, #35.08; Bulletin of the American Astronomical Society,
Vol. 41, p.715
The Palermo Swift-BAT Hard X-ray Catalogue. Results After 39 Months of Sky
New hard X-ray emitters discovered by BAT
G. Cusumano, V. La Parola, C. Ferrigno, V. Mangano, A. Segreto, P.
Romano, S. Campana, G. Chincarini, G. Tagliaferri, P. Giommi;
Proceedings of Science, Integral08, 134
Detection of ISGRI sources in 3 years of BAT all sky survey
Hard X-ray search for unidentified EGRET sources in the BAT survey archive
V. Mangano, A. Segreto, G. Cusumano, V. La Parola, C. Ferrigno, P. Romano,
S. Campana, G. Chincarini, G. Tagliaferri, P. Giommi;
Proceedings of Science, Integral08, 133
Search for Roma-BZCAT Blazars in the Palermo BAT Survey archive
The Palermo BAT-survey project and first application to AGNs
Swift at IASFPa
Swift NASA Official Site
ASDC Multi-Mission Interactive Archive
INTEGRAL General Reference Catalogue
Last Modified: February 10, 2012
Edited by Valentina La Parola