The XID Core Programme

The core programme aims at identifying spectroscopically significant samples of XMM sources at several flux limits and Galactic latitudes

Optimal energy band for source selection

The optimal energy band to be used to define the sample of sources should:  After considerable thought and simulation we adopted the 0.5 - 4.5 keV as the band to be used for the XID programme. We also decided that the X-ray images to use for the core programme should be such that the detection limit is considerably below the faintest source used.
This ensures that we will have X-ray colour and extent information and the possibility of studying variability. In particular, X-ray colour information is very important to find absorbed AGN (see Della Ceca et al 1999).

In order to avoid severe vignetting and confusion due to an excessive degradation of the PSF, we resort to X-ray sources found within a radius of 12' from the optical axis. The area covered per pointing for survey purposes is therefore ~ 0.13 deg2.

High Galactic Latitude

The high galactic latitude core programme consists of 1000 sources each in three different flux classes (>10-15, >10-14, >10-13, erg cm-2 s-1) given in the above passband. The relevance of these surveys is highlighted in graph 3 against existing and planned surveys.

    For the logN-logS function prediction we used the Hasinger et al (1998) source counts in the 0.5-2 keV band and the prediction from Cagnoni et al (1997) (based on the Comastri et al 1995 model) for the 2-10 keV band, both approximately converted to the 0.5-4.5 keV band. We also compute the AGN content of the fields in the various samples by carrying out simulations of the Comastri et al (1995) AGN. Note that the Medium Sample counts, and to a much larger extent the Faint Sample counts, are extrapolated as opposed to measured.

Sample Flux limit
erg cm-2 s-1
N (> S) 
Faint (F) 10-15 2200 24-25 262 260
Medium (M) 10-14 340 21-23 36 19
Bright (B) 10-13 10 17-21 1.1 0.4

Faint sample

 This class needs specific XMM pointings (about 100-400 ks), which will have to be made for the purpose of deep surveys.
In the XMM PV-Cal and GT programme, a few groups have already put in deep pointings fulfilling this purpose (e.g. Giacconi deep field, MSSL field, Lockman Hole).
At least in part the Subaru deep survey (which will be carried out in GT for the SSC) will fulfill some of the goals, although the average XMM exposure time of 50 ks, per field will not be sensitive enough.
To fight source confusion teaming up with Chandra deep surveys is a good strategy, which has already partially happened since two of the XMM deep fields (Giacconi field and Lockman Hole) will also be observed with Chandra.
The optical follow-up of the deep XMM fields definitely needs 8m-class telescopes and the respective teams to have access to these (e.g. Keck, VLT, Gemini, Subaru, later LBT, GTC).

Figure 3: A compendium of hard x-ray surveys.

Medium sample

About 30-50 fields with an exposure time close to the median exposure of all XMM pointings are necessary for this class. This programme is very similar in scope to the RIXOS project, but with a considerably larger source density.
The RIXOS experience shows that it is advisable to select fields which have a significantly lower sensitivity threshold than the flux limit aimed for. This way the Eddington bias is minimized, the source positions errors are minimized and all sources in the survey sample have significant X-ray information (spectra, time variability, extent).

Unlike RIXOS, however, the availability of modern optical instrumentation will make the bulk of this project a relatively easy task.
Wide-field imaging instruments are available which will cover the XMM fields in one go. In order to take advantage of the facilities that will eventually be available in both hemispheres, the target fields will be distributed over the whole sky.

The spectroscopic follow-up can ideally be initiated with existing wide-field multifibre instruments (e.g. on the AAT and the WHT). For emission-line objects these reach about ~22m in a 2 hour exposure. We will probably need 2 different exposures per field in order to cope with close candidate counterparts. Assuming 3-4 hours per field in total including setup, we need a total of about 20-25 nights on 4m-class telescopes.

Those counterparts which turn out fainter than the fibre limit will have to be followed up later, e.g. with individual exposures or multislit exposures from 4-8m class telescopes. For distant cluster candidates it will also be interesting to make integral-field spectroscopy.

Bright sample

This sample is the classical "jump and run" sample, the identification mode used for the Einstein Extended Medium Sensitivity Survey (EMSS) and the ROSAT Bright Survey (RBS).
In the case of XMM, the error boxes will be so good and the median counterparts so bright, that only about 1.5 spectra per counterpart are necessary. Spectroscopy can be done with 2m class telescopes.
Our experience with theRBS is that under good conditions up to 20 objects can be identified per night.
Out of 1000 objects to be identified in this sample, about 30% may already have catalogue identifications (stars, type I AGN, Abell clusters etc.). We will therefore need about 35 good nights on 2m class telescopes to do most of the job. However, this will clearly be a task spanning several years as the sample of 1000 sources will not be completed before 2 years of operations at a rate of 500 new fields per year.

For the roughly 20 high-redshift luminous clusters expected in this sample, we would need multi-slit/integral field spectroscopy in a 4m class telescope, but this will happen in future runs.

Galactic plane

The plan of the galactic core programme is to identify a sample of 1000 sources spanning a range in galactic latitude (| b | 0° _ 20°), longitude and total galactic absorption, down to a sensitivity limit of ~7× 10-15 erg cm-2 s-1 (0.1 - 10 keV). Such a medium faint sample can be accumulated in about 20 medium deep exposures, already available from the PV and GT programme only.

 A large variety of sources populate the galactic plane, ranging from supersoft binaries to very absorbed 6-s anomalous pulsars or Be/X-ray binaries. This fact, combined with the need to properly calibrate the statistical identification process, implies that we place relatively wide limits on the energy range in which source significance is tested. This in particular means the inclusion of the very soft band (0.1-0.5,keV), representing a small difference between the extragalactic and the galactic parts of the programme.

We also plan to build a galactic sample of comparable size, but at flux limits 10 times higher. The accumulation of this sample will take much longer. Of course, a large fraction of these bright sources may be readily identified from the literature or archival material, thus their identification will not require observing time at the beginning of the XID programme.

In the soft band of XMM (0.5-2~keV), the low galactic latitude landscape is not expected to be very different from that seen by ROSAT.
A large fraction of the extragalactic background sources is shielded by galactic absorption, and active stars dominate in number in most directions.
The second numerically important population may be cataclysmic variables. The situation in the hard X-ray band of XMM (2-10~keV) is quite different.
Most active coronae do not radiate much in hard X-rays whereas the higher energy allows X-rays from remote CVs, X-ray binaries and AGNs to shine through the galactic fog. In the hard X-ray band, the dominant population, even at low latitudes, is likely to be AGNs.

The expected galactic log N-log S curves are shown in graph 4. Using these curves as guidelines, we anticipate that the sample of 1000 medium faint low galactic sources will contain about 300 stars, 600 AGN and may be up to 100 CVs (see Watson, 1999, for a more detailed discussion of the likely CV source numbers).
Simple simulations suggest that using 4m class telescopes the total stellar population should be identified, while about 50-70% of the CVs and less than 10% of the AGN will have an optical identification.

The stellar and cataclysmic binary galactic samples combined with those accumulated in the high galactic fields should be large enough to constrain the most important population parameters such as scale height and space density.
So far the scale height of young stars is poorly known because of the difficulties encountered to properly select young main sequence late type stars (F-M). On the other hand efficient selection of such stars using X-ray data will allow to overcome the problem.
By comparison with predictions of stellar X-ray population models it will be possible to address the question of the thermalisation of this young population as well as the shape of the age vs. velocity dispersion relation associated to a process known as 'disk heating'.

Figure 4: Expected logN - logS curve of stars, CVs and AGNs.

Depending on the outcome of this campaign, the XID galactic core programme may be extended using this time X-ray / optical selection in order to enhance the detection rate of a particular class of galactic objects (e.g. stars).
The high-galactic latitude XID programme will provide a quantitative estimate of the extragalactic "contamination" in the galactic plane and vice versa will the low-galactic latitude part of the programme help to quantify the fraction of galactic X-ray sources at high galactic latitudes.

The strategy of the galactic core programme is to concentrate as much as possible on the identification of the galactic population.
Fibre spectroscopy is an efficient way to obtain spectra of candidates for an entire XMM field. Pre-selection of candidates based on optical colours (and perhaps X-ray or optical/UV variability) together with the two-pass scheme should yield a good identification success rate for stars and CVs.

Last modified 10th March 2000
Text based on AXIS proposal by X.Barcons and the SSC consortium.
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