[Paper Review] The ROSAT Deep Survey I. X-ray sources in the Lockman Field

G. Hasinger, R. Burg+4 more|arXiv (Cornell University)|Sep 15, 1997

Astrophysical Phenomena and Observations12 citations

TL;DR

This paper presents the first complete X-ray source catalogue from the ROSAT Deep Survey in the Lockman Field, detecting 50 sources with fluxes above 5.5 × 10⁻¹⁵ erg cm⁻² s⁻¹ using PSPC and HRI data. It derives a log(N)–log(S) function resolving 70–80% of the 0.5–2 keV X-ray background and demonstrates that confusion limits deeper surveys, challenging claims of a new faint X-ray source population.

ABSTRACT

The ROSAT Deep Survey in the Lockman Hole is the most sensitive X-ray survey performed to date, encompassing an exposure time of 207 ksec with the PSPC and a total of 1.32 Msec with the HRI aboard ROSAT. Here we present the complete catalogue of 50 X-ray sources with PSPC fluxes (0.5--2 keV) above $ 5.5 imes 10^{-15} erg cm^{-2} s^{-1}$. The optical identifications are discussed in an accompanying paper (Schmidt et al., 1997). We also derive a new log(N)--log(S) function reaching a source density of $970 \pm 150 deg^{-2}$ at a limiting flux of $10^{-15} erg cm^{-2} s^{-1}$. At this level 70-80% of the 0.5--2 keV X-ray background is resolved into discrete sources. Utilizing extensive simulations of artificial PSPC and HRI fields we discuss in detail the effects of source confusion and incompleteness both on source counts and on optical identifications. Based on these simulations we set conservative limits on flux and on off-axis angles, which guarantee a high reliability of the catalogue. We also present simulations of shallower fields and show that surveys, which are based on PSPC exposures longer than 50 ksec, become severely confusion limited typically a factor of 2 above their $4σ$ detection threshold. This has consequences for recent claims of a possible new source population emerging at the faintest X-ray fluxes. Keywords: surveys -- cosmology: diffuse radiations -- X-rays: galaxies

Motivation & Objective

• To produce a complete, reliable catalogue of X-ray sources in the Lockman Field using ROSAT PSPC and HRI data.
• To determine the log(N)–log(S) function for X-ray sources down to a flux limit of 10⁻¹⁵ erg cm⁻² s⁻¹.
• To assess the effects of source confusion and incompleteness on source counts and optical identifications in deep X-ray surveys.
• To establish conservative flux and off-axis angle limits that ensure high reliability in source detection and identification.
• To evaluate the reliability of claims about a new faint X-ray source population based on deep surveys.

Proposed method

• Conducting a deep X-ray survey using ROSAT's Position Sensitive Proportional Counter (PSPC) and High Resolution Imager (HRI), with total exposure times of 207 ksec and 1.32 Msec, respectively.
• Applying a 4σ detection threshold to identify X-ray sources above a flux limit of 5.5 × 10⁻¹⁵ erg cm⁻² s⁻¹ in the 0.5–2 keV band.
• Performing extensive simulations of artificial PSPC and HRI fields to model source confusion and incompleteness effects.
• Using simulations to derive conservative flux and off-axis angle thresholds that minimize false detections and misidentifications.
• Simulating shallower fields to determine the flux level at which confusion becomes a dominant limitation, typically 2× above the 4σ detection threshold.
• Deriving the log(N)–log(S) function from the detected source counts and comparing it to the observed X-ray background to assess source resolution.

Experimental results

Research questions

• RQ1 What is the complete catalogue of X-ray sources in the Lockman Field detected with ROSAT PSPC and HRI, and what are their fluxes and positions?
• RQ2 To what extent does the detected source population resolve the 0.5–2 keV X-ray background?
• RQ3 How do source confusion and incompleteness affect source counts and optical identifications in deep X-ray surveys?
• RQ4 At what flux level do confusion effects become dominant in deep X-ray surveys, and how does this affect claims of new source populations?
• RQ5 What are the conservative flux and off-axis angle thresholds that ensure high reliability in source detection and identification?

Key findings

• The survey detected 50 X-ray sources with PSPC fluxes above 5.5 × 10⁻¹⁵ erg cm⁻² s⁻¹, forming a complete and reliable catalogue.
• The log(N)–log(S) function reaches a source density of 970 ± 150 deg⁻² at a flux limit of 10⁻¹⁵ erg cm⁻² s⁻¹.
• At this flux level, 70–80% of the 0.5–2 keV X-ray background is resolved into discrete sources.
• Simulations show that surveys with PSPC exposures longer than 50 ksec become confusion-limited approximately a factor of 2 above their 4σ detection threshold.
• The study sets conservative flux and off-axis angle limits that ensure high reliability in source detection and optical identification.
• The results challenge recent claims of a new faint X-ray source population, as confusion effects likely explain apparent excesses at the faintest fluxes.