This page concerns the XMM-Newton Blank Sky files and related software
available for use with XMM-Newton EPIC data.
Blank sky files were constructed using a superposition of pointed
observations that have been processed with SAS version 9.0.0.
Please refer to the paper by Carter and Read
464, p1155, 2007
) for further information on the creation of these
- Sep-2013: discontinuation of the tailored blank field event list service
- Aug-2011: Modified skycast script available
- Aug-2010: Blank sky files constructed using observations processed using SAS 9.0.0, up to revolution 1789
- Oct-2008: Major reworking of website to incoporate new file delivery system
XMM-Newton Blank Sky event files
This page was produced as a result of work within the
XMM-Newton EPIC Background Working Group
When performing detailed XMM-Newton EPIC analysis, a good knowledge of
the background is required. Sometimes it may be possible to extract the
background from a region close to the particular source one is
interested in (using a so-called 'local' background). For a large or
extended source however, one may have to extract the background far
from the target source (the source may in fact be so extended, that no
local background is visible within the field of view). Here, a number
of effects can cause the extracted local (off-axis) background to be
highly inappropriate in analysing the (normally on-axis located) target
source, such as the effective area of the mirrors with off-axis angle,
instrumental fluorescence and the spectral response which can depend on
the position on the detector. These off-axis effects are corrected in
the XMM-Newton EPIC calibration.
The files available via these pages are intended to be used in case of
difficulty extracting a suitable background region from a user's
observation. A guide to these files and their use can be found below.
The creation of individually tailored blank sky files, previously offered by
the X-ray and Observational Astronomy Group of the University of Leicester, UK,
is now no longer possible.
You may find that a blank sky file has already been created that is suitable for
your use. Please see the list of selection criteria used in previous
submissions in this
These files have been provided by Jenny
A. Carter (Department of Physics and Astronomy, University of Leicester,
Creation of the Blank Sky files
The component Blank Sky files that make up the final superimposed
Blank Sky files were created from event files processed
using SAS 9.0.0. Each individual component file that becomes part of a
specific instrument-filter-mode set has undergone source removal and
flare-screening. The source removal extraction radius is based on the
resolved point source flux. A source extraction radius of 45
arcseconds was used for sources with a 2XMM PPS source list EPIC flux
above a threshold of 6.0e-13 ergs cm-2 s-1 and 35 arcseconds for below
this threshold. We have also implemented an improved removal of known
bad pixels for the EPIC-pn.
The flare screening is based on the 2XMM PPS background time series
files. For MOS they are created for full-field events above 14 keV,
and with the selection expression (PATTERN==0) && #XMMEA_22
&& ((FLAG & 0x762ba000) == 0), whereas for PN, they are
created for full-field, bright-source-excluded all-pattern events
between 7 keV and 15 keV. The flare removal that has been used here in
the creation of the Blank Sky files (thresholds: 2 counts/s for MOS
and 60 counts/s for PN) was based on the very conservative limits
suggested by the EPIC instrument teams. A user may therefore wish to
undertake some additional flare screening steps when using these
The blank sky event lists were generated with the SAS version made public
in 2009. Since then, several changes have been introduced in the MOS data
reduction pipeline, as well as in the camera calibration. However, these
changes are expected to have a minimum impact on the scientific quality of
the blank field event lists. The main limiting factor is the accuracy of
the energy scale. Strong instrumental lines in the background can yield
significant residuals in the background-subtracted spectrum if calibrated
at the wrong energy (or if the energy scale of the source plus background
spectrum is inaccurately calibrated). However, reduction with SASv9.0 and
associated calibration files already achieved a nominal accuracy of the
energy scale over the whole detector field-of-view
Still, we recommend users to exercise astrophysical common sense when
interpreting features in blank field background-subtracted spectra close to
strong background lines (see, e.g.
, the discussion in Sect. 8 of
Unfilled and ghosted Blank Sky files
There are two types of background event file; unfilled and ghosted.
The unfilled event files result from the stacking of event files that
have been subjected to various filtering steps, which includes the
removal of sources. Therefore proper consideration of the exposure
maps is required when using the final event file that applies to a set
of combined observations. However, we have developed a method to fill
in the source regions that are extracted from each individual
observation by sampling events close to the extracted regions and
filling the vacated region of the event file, randomising just the
spatial coordinates. Adjustments are made for region crossovers and
chips edges. This results in smooth (ghosted) event file images and
exposure maps. Both types of event file are available here, with
corresponding vignetted and non-vignetted exposure maps.
On the left is shown an image created from a pn events file with sources
removed, and on the right, the image of the events file after the event
The creation of individually
tailored blank sky files has been discontinued. You can find a list of
blank sky event lists for a certain combination of sky coordinates,
Galactic column density and instrumental configurations
Each Blank Sky file was constructed from several different event files,
spanning a large range in time over the mission. Date-related and positional-related keywords,
for example DATE-OBS, have therefore been removed. A few SAS tasks
require date and time stamp keywords. It is recommended that these
keywords be added by the user as required. Missing date and time
keywords could be e.g. set to that of the user's source spectrum. This
can be achieved for example using the FTOOLS viewer/editor fv:.
- fparkey '2004-07-12T12:30:00.0' blank_sky_event_lists.ds+0 DATE-OBS add=yes
- fparkey '2004-07-12T12:30:00.0' blank_sky_event_lists.ds+1 DATE-OBS add=yes
Users should be aware of a few issues when using ghosted Blank Sky
- Due to the method of creation of the ghosted blank
sky files, some events with particular flags may be positioned
within inappropriate areas of an image, e.g. near a chip gap.
How to use EPIC background blank field files.
This thread briefly explains how to make use of blank sky event lists to
create background spectra.
Scaling of Blank Sky exposure maps.
The Blank Sky exposure maps were created in the energy range 200 eV to
10000 eV. However, a user may wish to work in a different energy
range. We can recommend the following method to scale the Blank Sky
exposure maps for use in any required energy range:
- From the user's dataset create an exposure map in the required
band and also create a similar exposure map in the Blank Sky band with
pimin set to 200 and pimax set to 10000.
- Next, calculate the ratio map from these two maps (using, for
example the FTOOLS task farith), i.e. divide the exposure map
in required band by the exposure map in the original 200 - 10000 eV
band. It is recommended that you pre-add (FTOOLS, fcarith) a
tiny value, e.g. 1e-20, to the 200-10000 eV map to avoid division by
- Multiply the Blank Sky exposure map by this ratio map to scale the
appropriate Blank Sky exposure map to the required energy range.
All related software can be obtained from here
Script to cast an XMM-Newton EPIC background dataset (or indeed any
EPIC event dataset) onto the sky, at the position given by an input
template event dataset (e.g. the event file you are interested in
producing a background for).
Projection of Sky maps: A script to rebin and reproject exposure maps
(or any DETX/DETY image of any binning) onto the sky to the spatial
scale and sky position of a user-input image. This is useful when
wanting to work in sky coordinates and to a different spatial scale
than e.g. the 4 arcsecond scale used in the production of the exposure
maps. This script is a wrapper for a fortran executable and is
dependent on the user's operating system.
A script that calls the IDL procedure ghostit_ind.pro to fill in the
regions that are extracted from each individual observation
(e.g. after using the SAS task regions), by sampling events close to
the extracted regions and filling the vacated region of the event
file, randomising just the spatial coordinates as used in the creation
of the Blank Sky ghosted event files. Both the script ghostholes_ind
and the IDL procedure ghostit_ind.pro must be downloaded. An EPIC
source list file, as created by the SAS task regions and a calibration
index file are required to use this script.
Blank Sky file properties
Various properties of the Blank Sky files are described on the web
page relating to files prior to October 2008, which can be found
Link to pre October 2008 Blank Sky page
The previous page, including access to the old Blank Sky files can be found