[Paper Review] A Deep XMM-Newton Serendipitous Survey of a middle-latitude area. II. New deeper X-ray and optical observations

G. Novara, N. La Palombara+5 more|arXiv (Cornell University)|Apr 24, 2009

Astrophysical Phenomena and Observations26 references3 citations

TL;DR

This study presents a deep XMM-Newton survey of the intermediate-galactic-latitude field around 1E1207.4−5209, combining 350 ks of X-ray observations with new optical data from the ESO/MPG 2.2m telescope to identify 144 serendipitous X-ray sources. It finds that the X-ray source population is dominated by extra-galactic sources in the hard band (2–10 keV), but includes a significant galactic component in the soft band (0.5–2 keV), with 68% classified as AGNs and 20% as stars based on X-ray/optical flux ratios and spectral analysis.

ABSTRACT

The radio-quiet neutron star 1E1207.4-5209 has been the target of several XMM-Newton observations, with a total exposure of ~350 ks. The source is located at intermediate galactic latitude (b~10 degrees), i.e. in a sky region with an extremely interesting mix of both galactic and extra-galactic X-ray sources. The aim of our work is to investigate the properties of both the intermediate-latitude galactic and extra-galactic X-ray source populations in the 1E1207.4-5209 field. We performed a coherent analysis of the whole XMM-Newton observation data set to build a catalogue of serendipitous X-ray sources detected with high confidence and to derive information on the source flux, spectra, and time variability. In addition, we performed a complete multi-band (UBVRI) optical coverage of the field with the Wide Field Imager (WFI) of the ESO/MPG 2.2m telescope (La Silla) to search for candidate optical counterparts to the X-ray sources, down to a V-band limiting magnitude of ~24.5. We detected a total of 144 serendipitous X-ray sources. Thanks to the refined X-ray positions and to the WFI observations, we found candidate optical counterparts for most of the X-ray sources in our compilation. For most of the brightest ones we proposed a likely classification based on both the X-ray spectra and the optical colours. Our results indicate that at intermediate galactic latitude the X-ray source population is dominated by the extra-galactic component, but with a significant contribution from the galactic component in the soft energy band, below 2 keV.

Motivation & Objective

• To investigate the X-ray source population at intermediate galactic latitude (b ≈ 10°), where both galactic and extra-galactic sources coexist.
• To improve source detection and classification by combining deeper XMM-Newton X-ray data with new multi-band optical observations.
• To refine the log N–log S distribution and assess the contribution of galactic versus extra-galactic sources in different energy bands.
• To identify optical counterparts for X-ray sources down to V ≈ 24.5 and classify them using X-ray spectra and optical colors.

Proposed method

• Combined all XMM-Newton observations of the 1E1207.4−5209 field, totaling ~350 ks of exposure time, to improve source detection and flux measurement.
• Performed a coherent analysis of the EPIC-MOS data to produce a high-confidence catalogue of 144 serendipitous X-ray sources.
• Conducted new UBVRI optical imaging with the Wide Field Imager (WFI) on the ESO/MPG 2.2m telescope to reach a limiting magnitude of V ≈ 24.5.
• Cross-matched X-ray sources with optical counterparts using refined X-ray positions and applied color-based classification using UBVRI photometry.
• Used X-ray hardness ratios and X-ray-to-optical flux ratios (f_X/f_opt) to classify sources as AGNs, stars, or galaxies.
• Performed spectral and variability analysis on the 40 brightest sources, identifying 10 transient sources and two showing short-timescale variability.

Experimental results

Research questions

• RQ1 What is the relative contribution of galactic and extra-galactic X-ray sources in the soft (0.5–2 keV) and hard (2–10 keV) energy bands at intermediate galactic latitude?
• RQ2 How does the log N–log S distribution in this field compare to those at high galactic latitudes and in the Galactic plane?
• RQ3 What is the reliability of classifying X-ray sources as AGNs, stars, or galaxies using combined X-ray spectral and optical color information?
• RQ4 Which X-ray sources show significant variability, and what does this imply about their nature?
• RQ5 What is the redshift of the Seyfert-2 galaxy XMMU J121029.0−522148, and how does it compare to previous estimates?

Key findings

• A total of 144 serendipitous X-ray sources were detected, with 114 detected in the soft band (0.5–2 keV) and 87 in the hard band (2–10 keV), reaching flux limits of ~10⁻¹⁵ erg cm⁻² s⁻¹ and 4×10⁻¹⁵ erg cm⁻² s⁻¹, respectively.
• The log N–log S distribution in the soft band shows a clear flattening at low fluxes (~4×10⁻¹⁵ erg cm⁻² s⁻¹), indicating a significant contribution from galactic sources, while the hard band distribution is consistent with high-latitude and Galactic plane surveys, indicating dominance by extra-galactic sources.
• Sixty-eight sources (65%) were classified as AGNs based on X-ray spectra and optical colors, with 27 of the brightest sources confirmed as AGNs and 7 as stars.
• Twenty-one sources (20%) were classified as stars, primarily based on X-ray-to-optical flux ratios below -1.5, while 13 sources (13%) remained unclassified due to conflicting or unconstrained data.
• Ten of the 40 brightest sources showed large flux variations between 2002 and 2005 observations, indicating transient behavior, and two showed short-timescale variability on ~0.1 and ~10 ks timescales.
• The Seyfert-2 galaxy XMMU J121029.0−522148 was re-estimated to have a best-fit redshift of z = 0.042, higher than the previously reported z = 0.032.