Background Analysis

This page gives information on the background analysis of all XMM-Newton instruments (EPIC, RGS, OM) in order that a proper data reduction may be undertaken.
 

EPIC

Introduction

The XMM-Newton observatory provides unrivalled capabilities for detecting low surface brightness emission features from extended and diffuse galactic and extragalactic sources, by virtue of the large field of view of the X-ray telescopes and the high throughput yielded by the heavily nested telescope mirrors. In order to exploit the excellent EPIC data from extended objects, the EPIC background, now known to be higher than estimated pre-launch, needs to be understood thoroughly.

There are several different components to the EPIC background:

1. Photons:
   - The astrophysical background, dominated by thermal emission at lower energies (E<1 keV) and a power law at higher energies (primarily from unresolved cosmological sources). This background varies over the sky at lower energies.
   - Solar wind charge exchange.
   - Single reflections from outside the field of view, out-of-time events etc.
2. Particles:
   - Soft proton flares with spectral variations from flare to flare. For weak sources the only option is to select quiet time periods from the data stream for analysis.
   - Internal (cosmic-ray induced) background, created directly by particles penetrating the CCDs and indirectly by the fluorescence of satellite material to which the detectors are exposed.
3. Electronic Noise:
   - Bright pixels, columns etc., readout noise etc.

A table summarizing the temporal, spectral and spatial properties of these EPIC background components is available here.

There have been various attempts to describe/model the EPIC Background in the past. This page provides an overview on all sources of background analysis and modelling, to our knowledge, pointing out the recommended mainstream for background treatment by the EPIC consortium.

In 2005 the XMM-Newton EPIC Background working group was founded as a steering and supervising committee to provide the user with clear information on the EPIC Background and (SAS)-Tools to treat the EPIC Background correctly for various scenarios. Although the group stopped its main activities in 2012 the SOC is still responsible for maintaining some of its products.

The progress of the XMM-Newton EPIC Background working group during its 7 years of existence can be found here.

Products

• XMM-Newton Extended Source Analysis Software package, XMM-ESAS
  As of SAS version 9.0, the XMM-ESAS package is integrated in SAS. XMM-ESAS allows the user to model the quiescent particle background for both spectral and spatial analysis of EPIC pn and EPIC MOS observations.
   
• XMM-Newton 'blank sky' background event files
  XMM-Newton EPIC blank sky user facility released in August 2010 following a processing of the XMM-Newton archive (up to revolution 1789) using SAS 9.0.
  The creation of individually tailored blank sky files has been discontinued.
   
• Filter Wheel Closed data
  Updated in July 2022 the stacked collections of Filter Wheel Closed (FWC) data are available for the MOS and pn cameras. The full repository has been analysed with SASv20.
   
• Exospheric solar wind charge exchange affected observations
  Lines-of-sight to XMM-Newton targets sometimes traverse regions of X-ray emission in the vicinity of the Earth. This emission results from a charge transfer process between ions in the solar wind and neutral gas (primarily hydrogen) close to the Earth, and can exhibit temporal signatures that make it possible to identify affected observations.
   
• Further EPIC Background Scripts
  • Estimation of the residual Soft Proton flare contamination
  • Background correction for faint extended EPIC PN emission
  • Specific scripts to be used with Blank Sky event files

Other Useful Information

The following sources of information (including historical collections of background blank sky fields) are also available:

1. Presentation: "Using NDSLIN in EPIC-pn as a proxy for the QPB", I. de la Calle, XMM-Newton SOC (2019)
2. Paper: "Radial temperature profiles for a large sample of galaxy clusters observed with XMM-Newton", A. Leccardi, S. Molendi, A&A 486, L359 (2008)
3. Paper: "The EPIC-MOS particle-induced background spectra", Kuntz, K. D. & Snowden, S. L., A&A 478, 575 (2008)
4. Paper: "The XMM-Newton EPIC background and the production of background blank sky event files", J. A. Carter & A. M. Read, A&A 464 (2007)
   Web site: XMM-Newton EPIC 'Blank Sky' Background
5. Paper: "The XMM-Newton EPIC background: Production of background maps and event files", A.M. Read & T.J. Ponman, A&A 409, 395 (2003)
   Web site: Related EPIC background event files, maps, software, analysis techniques etc.
6. Paper: "XMM-Newton EPIC background modelling for extended sources", J. Nevalainen, M. Markevitch & D. Lumb, ApJ 629, 172 (2005)
   Web site: Supporting data, background event files etc.
7. Paper: "X-ray background measurements with XMM-Newton EPIC", D. Lumb, R.S. Warwick, M. Page & A. De Luca, A&A 389, 93 (2002)
8. Paper: "The EPIC/MOS view of the 2-8 keV Cosmic X-ray Background Spectrum", A. De Luca & S. Molendi, A&A 419, 837 (2004)
9. Paper: "XMM-Newton Data Processing for Faint Diffuse Emission: Proton Flares, Exposure Maps and Report on EPIC MOS1 Bright CCDs Contamination", J. Pradas & J. Kerp, A&A 443, 721 (2005)
10. Paper: "Identifying XMM-Newton observations affected by solar wind charge exchange - Part II", J.A. Carter, S. Sembay & A.M. Read, A&A 527, 115 (2011)
    Web site: Table of temporally-variable SWCX-affected observations
11. Paper: "A systematic analysis of the XMM-Newton background: I. Dataset and extraction procedures", Marelli et al., Experimental Astronomy 44, 297 (2017)
12. Paper: "A systematic analysis of the XMM-Newton background: II. Properties of the in-Field-Of-View excess component", Salvetti et al., Experimental Astronomy 44, 309 (2017)
13. Paper: "A systematic analysis of the XMM-Newton background: III. Impact of the magnetospheric environment", Ghizzardi et al., Experimental Astronomy 44, 273 (2017)
14. Paper: "A systematic analysis of the XMM-Newton background: IV. Origin of the unfocused and focused components", Gastaldello et al., Experimental Astronomy 44, 321 (2017)
15. ESAS Related threads
    Web site: Creation of EPIC background subtracted, exposure corrected images
    Web site: Creation of EPIC merged background subtracted and exposure corrected images
    Web site: Creation of EPIC spectral analysis files for a cluster radial profile
16. Blank Sky event files related thread
    Web site: How to use EPIC background blank field files

RGS

The RGS background is dominated by soft protons entering through the telescope mirrors. The shape of the spectrum of the background is clearly correlated with its intensity. Therefore, the background spectrum of a given observation is determined entirely by the time-dependent background within the observation, and can be computed as a linear combination of several 'templates' of different intensities (see a detailed description of the method in XMM-SOC-CAL-TN-0058.

An accurate determination of the RGS background is particularly important for extended sources, but the use of a model background, with far better signal-to-noise ratio than the background extracted from the the observation itself, also improves the detectability of weak sources and weak spectral features.

The RGS background model is computed by the SAS task rgsbkgmodel.
 

OM

1. Sky Background Calculations for the Optical Monitor, T.S. Poole, 2005 (pdf)
2. OM background measurements, N. Loiseau and XMM-Newton INSCONS, 2005. (pdf)