The large fields-of-view (FoVs) typical of X-ray imaging instruments have long been exploited for serendipitous surveys, resulting in far-reaching and fundamental advances in our knowledge of the X-ray universe and indeed the universe as a whole. The unprecedented spatial resolution of Chandra ( 0.5 at field center) combined with its very low background, allows a view of the X-ray sky up to 20 times deeper than previous X-ray surveys as well as highly efficient source identification based on the accurate X-ray positions1. A multi-institution collaboration (including: CXC, MIT, CfA, MMTO, KPNO, CTIO, UofA, SFSU, MSU, SDSS, OSU) is carrying out a serendipitous survey, the Chandra Multi-wavelength Project (ChaMP), using Chandra archival, full FoV imaging data with a wide range of exposure times. We estimate that the ChaMP X-ray sample will total sources in sq. degs. per year and run for 5 years, thus including statistically robust samples of rare source types such as BL Lac objects, quiescent X-ray binaries, and high-redshift clusters.
The aim of the ChaMP survey is to make use of the parts of the images which are not the primary science of the PI project. For instance, we omit fields which have a large galaxy in them or an X-ray bright cluster of galaxies. We are concentrating on fields which have, for example, a point source as the main science objective. In this way, we will assemble our sky sample from data not expected to be intensively studied by the PI's. 14% of the Chandra AO1 fields met the criteria for inclusion in ChaMP. The analysis the team will do on such topics as testing the source detection algorithms, understanding the background, and the relation between the two on a large sky area will be a result from Champ which will benefit the whole Chandra community.
To unambiguously classify the X-ray sources into the various source types - stars, galaxies, clusters, and active galactic nuclei (AGN) - optical fluxes are required. We have designed an optical imaging program to reach optical magnitude limits (20-25) matched to the X-ray flux limit for each field. The combination of three-color optical and X-ray fluxes will allow classification of > 75% of the X-ray sources both within and outside the Galactic Plane. Optical spectroscopy to determine redshifts and verify classification methods will be obtained for a significant, representative subset.
The optically identified ChaMP X-ray selected sample will enable us to address several key questions already posed by a variety of research areas in astronomy while no doubt providing the surprises that usually result from new levels of sensitivity and resolution. The scientific motivation includes:
Complement the small-area, deep Chandra surveys, which will resolve the majority of the soft (0.5-2.5 keV) and hard (2-10 keV) Cosmic X-ray background (CXRB), by covering a larger area to brighter flux limits, providing strong constraints on the intermediate between pre-Chandra and Chandra deep surveys.
Determine the X-ray luminosity function (LF) of AGN and other source types as a function of redshift to fainter flux levels than previously.
Provide a uniform sample of AGN relatively unaffected by the biases due to line-of-sight absorption that are present in existing optical and soft-X-ray surveys. Study AGN Spectral Energy Distributions (SEDs) as a function of redshift, luminosity and AGN type to probe their primary and secondary energy sources.
Identify high redshift (z) clusters of galaxies to constrain cosmological parameters and to determine cluster evolution by comparison with lower redshift samples.
Double the number of known galactic plane cataclysmic variables (CVs), with the most luminous seen at > 10 times the current distance limit. Measure the X-ray luminosity function of accreting binary stars such as CVs and low mass X-ray binaries, (LMXBs).
Study the coronal X-ray emission of main sequence stars from the onset of dynamo activity in A stars to late-M stars, where the cores are fully convective.
The advantages of Chandra are graphically demonstrated in Fig. 13, which shows a Chandra ACIS-S image of the z cluster RXJ003033.2+261819, taken during Orbital Activation and Calibration, as compared with the ROSAT PSPC image of the same area of sky. The cluster, while barely resolved with ROSAT, is clearly an extended source in the Chandra image and the order of magnitude improvement in the X-ray positions is also clear.
The analysis of serendipitous sources in this field, shows 24 detected point sources outside the immediate cluster region (Cappi et al. 2000). A typical source has 20 counts, or erg cm-2 s-1, ROSAT detected 11 sources to 10-14erg cm-2 s-1. In an area surrounding the cluster, the 0.5-2 keV surface density of point sources ( above erg cm-2 s-1) exceeds by a factor that expected based on the ROSAT . This high density is seen despite the relatively high Galactic absorbing column for the field ( ). A non-cluster comparison field shows no such excess. Optical identifications and redshifts are needed to confirm the implied association with the cluster.
The wide availability of optical catalogs and surveys at the bright end of this range allows us to design an efficient identification program using existing material where possible. At fainter optical magnitudes no catalogs are currently available3. The limiting optical V magnitudes required for the ChaMP Cycle 1 list of 74 fields are in the range 20.5 V 24.7 (Fig 15). Optical imaging will require nights per year on a combination of 4 m and 0.9 m telescopes at NOAO (KPNO and CTIO) and 1.5m at SAO (Smithsonian Astrophysical Observatory) using the large area (4 shooter or MOSAIC) detectors. 28 nights were awarded by NOAO and SAO between Dec 1999 and Jun 2000 with 17 completed to date. We have submitted proposals to the NOAO Survey and SAO longterm programs (for the fields with brighter flux limits) to obtain the remainder of the nights assuming a similar area will be covered each year.
Optical and X-ray source properties will be combined in order to classify the sources. At high galactic latitude, SDSS and filters provide the best option for efficient source classification. Fig. 16 illustrates how 3 SDSS filters and X-ray fluxes yield a multicolor space of unprecedented AGN selection efficiency across all redshifts. All AGN are separated from stars following the the color-plane methodology of Newberg et al. (1998). Often, further discrimination will be available from optical and X-ray morphology and from X-ray spectral information.
(hea-www.harvard.edu/CHAMP/) along with the field lists for each Chandra Cycle as they become available. The ChaMP X-ray and optical data and X-ray catalog products will be made publicly available on a field-by-field basis within a year of completion of the data reduction of the full dataset for that field. We are confident that the multi-wavelength dataset so provided will prove a valuable scientific resource for the community.
- Belinda Wilkes, Paul Green
Cappi, M. et al., 2000, ApJ, submitted
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Miyaji, T., Hasinger, G., Schmidt, M. 2000, AA, in press
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Stocke, J. T. et al 1991, ApJS, 76, 813
1The degraded inflight energy resolution of part of the ACIS instrument does not affect the ChaMP survey limits.
2available on the CXC website asc.harvard.edu
3Once the Sloan Digital Sky Survey (SDSS) archive for all northern fields opens ( ), it will cover ChaMP imaging needs to V<23 with the same filters as the remainder of the ChaMP. While these magnitudes will not be temporally as close to Chandra observations as our own imaging, they will reduce our need for telescope time for northern fields, while still ensuring color uniformity with the ChaMP.