This page describes the general Blank Sky event files and associated
exposure maps that were created prior to October 2008. This page has
been superseded by the page found
here.
A user may now wish to request a tailor-made Blank Sky event file by
submitting a request using the form found
here.
General XMM-Newton background 'blank sky' events files for the 3 EPIC
instruments in their different instrument mode/filter combinations have
been constructed using a superposition of many pointed observations of
pipeline product data from the 2XMM reprocessing (
Second XMM-Newton Serendipitous Source Catalogue)
and have been processed with SAS 6.5.
Exposure maps in the different instrument/mode/filter combinations have
also been produced. Refer to the related paper by Carter and Read
(
A&A 464, p1155, 2007)
for further information. On these web pages, details can be found on
how to obtain these background products together with related software.
Contents:
Latest Updates
- Nov-2008: Restructuring of this page. This page refers to Blank Sky files prior to October 2008.
- Sep-2008: Advice added to website regarding scaling of exposure maps and the use of refilled event files
- Nov-2007: New script BGSelector added to replace SelectRADec, with much more functionality
- May-2007: Additional exposure maps provided, created using flag (FLAG & Ox2fb0008)==0
- May-2007: Corrections to unfilled vignetted exposure maps
- May-2007: Removal of some component observations from blank sky files, adjustments to BlankSky.tgz
- May-2007: TCRVL6 and TCRVL7 keywords added to event files
- Feb-2007: TLMINn and TLMAXn keywords added to event headers
- Dec-2006: Advice regarding use of files with SAS tasks and the addition of keywords
- Aug-2006: GTI extensions revised for all event files. Modified BlankSky.tgz
- Jun-2006: TSTART and TSTOP keywords added to EXPOSURE extensions of unfilled event files
- May-2006: Initial web site
XMM-Newton blank sky event files
This page was produced as a result of work within the
XMM-Newton EPIC Background Working Group.
It follows on from work completed by A. Read, whilst at the University
of Birmingham, the results of which can be found within the associated
previous background web pages.
The current work presented on these pages includes the analysis of many
more pointed observations and the addition of new tools in comparison
with the work previously presented.
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.
Available XMM-Newton background files
The pre-October 2008 background event files and relevant exposure
maps can be obtained
here.
There are background event files for MOS1 and MOS2 in full-frame mode
and PN in both full-frame and extended full frame mode. Each filter
mode combination event file is available. Filters can be thin, medium
or thick. Therefore there are twelve different instrument-filter-mode
combinations. For each event file there are two types of exposure map:
vignetted and non-vignetted. Note that the vignetted maps apply to the
energy range 0.2-10 keV.
The creation of these files and background flare screening
The blank sky files were created from sets of files available from the
2XMM reanalysis. Each individual component file that becomes part of a
specific instrument-filter-mode set has undergone source removal and
flare-screening. 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 files.
Refilled and unfilled event files
There are two types of background event files: unfilled and refilled.
The unfilled event files result from the stacking of pipeline product
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 event file images and exposure
maps. Both types of event file are available here, with corresponding
vignetted and non-vignetted exposure maps. The naming convention
described below indicates which type of event file is available for
download.
|
|
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
filling procedure
|
Location on the sky dependence
In some cases, due to very large file sizes of the final stacked event
files, it was necessary to consider individual observations for
stacking based on their location on the sky. These files were split into
'near-hemispheres', based on the galatic centre and galactic
anti-centre. This is shown in the file naming convention shown below by
the "location" tag.
Details of the observation sets
The processing of the observations that contribute to the final files
involved the manipulation of data from approximately 200 observations.
The table below details the total exposure time that each final
instrument-filter-mode combination event file incorporates, after all
cleaning has been completed. Users will need to scale the event files
accordingly when applying these sets to their observations.
File |
Final total time (s) |
Number of files used |
MOS1 thin full frame | 2141130 | 111 |
MOS1 medium full frame | 1526510 | 60 |
MOS1 thick full frame | 250323 | 11 |
MOS2 thin full frame | 2034300 | 109 |
MOS2 medium full frame | 1618360 | 62 |
MOS2 thick full frame | 287884 | 12 |
PN thin full frame | 960633 | 41 |
PN medium full frame | 1066180 | 32 |
PN thick full frame | 184756 | 9 |
PN thin extended full frame | 825204 | 62 |
PN medium extended full frame | 435142 | 27 |
PN thick extended full frame | 58664 | 3 |
The naming scheme is detailed below.
Available Files
Event files
The pre-October 2008 files are found
here.
Event files take the following file-naming convention:
"instrument"."filter"."mode".EVLI"type""location".FIT
The following table explains the meaning of the symbols for instrument, filter, mode and type.
Symbol |
Use |
Meaning |
PN | instrument | pn |
M1 | instrument | MOS1 |
M2 | instrument | MOS2 |
T | filter | thin |
M | filter | medium |
K | filter | thick |
FF | mode | full frame |
EF | mode | extended full frame |
U | type | unfilled |
R | type | refilled |
A | location | galactic anti-centre |
C | location | galatic centre |
P | location | not position specific |
Example:
M1.T.FF.EVLIUP.FIT
This file would be the unfilled events file for MOS1, thin filter with
full frame mode, and includes events that can come from any location on
the sky.
Exposure Maps
Individual exposure maps that apply to the event files are available. They take the following file-naming convention:
"instrument"."filter"."mode".EXM"vig""type""location".FIT
where:
- "instrument", "mode", "filter", "type" and "location" are the same as described in the Events file section above.
- "vig" indicates whether the exposure maps accounts for vignetting (V if a vignetted exposure map, N if not).
Example:
PN.K.EF.EXMNRP.FIT
This file would be the non-vignetted refilled exposure map for PN,
thick filter with extended full frame mode and applies to the event
files where events can come from any location on the sky.
There are additional exposure maps available that were created with the
additional expression (FLAG & Ox2fb0008)==0. These files can be found in
AF_FILES sub-directory
of the file download area page, and have the suffix 'af' added to the filename.
Software available relating to background files
All related software can be obtained from
here.
Scripts
Scripts also applicable to the Blank Sky files as prior to October 2008
as well as the new style files can be found
here.
An additional script is available for use with the file as prior to
October 2008 and is detailed below.
BGSelector
This script replaces and supersedes the script SelectRADec. This
script can be used to select events from a Blank Sky event file, using
a set of selection criteria as given by the user. The user can select
on events within a given radius from a set of coordinates, either
equatorial (J2000) or galactic, by revolution number, exposure time,
XMM-Newton time or nH column density values. A final event file and
exposure map is produced along with a text file summarising the
results of the selection. This script requires the download of a
compressed tar file, BlankSky.tgz which contains a set of individual
exposure map files (in the sub directory Exposures), the script
BGexpmerge, the python coordinate conversion routine
(galcoordstoradec.py) and the master list of observations
(Mastertable.txt). Please note that the Mastertable.txt file was
updated in November 2007. This file should be downloaded, unzipped and
untarred accordingly.
Using these background event files
Previous guidance
A brief introduction to using these files is given at the
Birmingham site.
Properties of the (pre-October 2008) Background Files
Variations in spectra with count rate
|
Variation of spectra with count rate. Black = high, red = medium and green = low count rate |
The user should be aware that spectra produced from the files presented
here show slight variations with count rate, where count rate is
classified as being high, medium or low. In the future a tool may be
supplied on these pages to select count rates that are specifically
appropriate for a user's data.
Variations in spectra over the sky
|
Variation
of spectra with location: 30 degrees centred about a certain location.
Black = galactic centre, red = galactic anti-centre, green = north
galactic pole, blue = south galactic pole. Note the higher count rate
of the galactic centre due to higher levels of soft x-ray emission
|
Regions of the sky have been selected around four coordinate centres:
the galactic centre, galactic anti-centre, the
south galactic
pole and
the north galactic
pole. These regions were selected using the script
BGSelector
as described above. Spectra were produced for each of the resulting
subset event files. These are shown plotted here. Note that the galactic
centre shows a higher count rate at between approximately 0.3 keV to 3
keV due to increased soft x-ray emission in the galactic plane and
centre.
Blank Sky count rates
For each of the blank sky files (based on the instrument-filter-mode
classification) count rates have been calculated in the standard SSC
energy bands (0.2-0.5 keV, 0.5-1 keV, 1-2 keV, 2-4.5 keV, 4.5-12 keV)
both for in and out of field of view (FOV) areas for both events with
pattern 0, and events with patterns equal to or less than 12 for MOS
and events with patterns equal to or less than 4 for PN. These count
rate plots, giving an estimate of the to be expected EPIC background
in 'low background' periods, are available
here.
Internal BG
By looking at one of the refilled event files for the PN between the
energies of 7.8 keV and 8.2 keV, one can see the instrumental copper
'hole' which has only slightly been affected by the ghosting procedure
as described above (
refilled and unfilled event
files). This effect, therefore, is only of minimal significance.
|
Image made from a PN event file showing the Cu line between 7.8 keV and 8.2 keV
|