ICWG June 2006 MINUTES of WORKING SESSION on STANDARD CANDLES Date : 15 June 2006 Participants : 1. Frank Haberl (Chair) 2. Marcus Kirsch 3. Leo Metcalfe 4. Jelle Kaastra 5. Andy Read 6. Richard Mushotsky 7. Paul Pluchinsky 8. Manabu Ishida 9. Hiroshi Nakajima 10. Motohide Kokubun 11. Kiyoshi Hayashida 12. Olivier Godet 13. Jean Cottam 14. Giancarlo Cusumano 15. Rick Rothschild Haberl opened the discussion, asking what defines a standard candle. Does it have to be a constant-flux source? Examples of constant sources would be : White Dwarfs, Isolated Neutron Stars, Supernova Remnants and Galaxy Clusters. The Crab has been the iconic X-ray "standard candle", but is far too bright even for current, let alone future, missions. It was asked how important it is to establish absolute flux standards in the X-ray. Examples were recalled from the web-based discussion which took place prior to this workshop: H. Marshall had noted that Neutron Star radii could be estimated with a few percent accuracy if absolute fluxes are known to better than 5%. Estimates of the total mass of galaxy clusters are related to the spectral normalisation. For SNRs the masses in various elements depend on the absolute line fluxes. CELESTIAL SOURCES ----------------- A list of sources sufficiently well known to form a core set of candidate standard candles was compiled: White Dwarfs : HZ43, Sirius B, GD153 (very soft) Neutron Stars : RXJ1856, PSR0656 (*) SNRs : 1E0102, N132D, CAS-A SNRs (non-therm.) : Crab, G21.5, 3C58(5'x8') PSR1509(*), 0540-69(*) Clusters : To be defined: bright, small, simple. *) :==> Source added during later "Forum" discussion. Apart from absolute flux calibration the purpose of appropriate targets can also be to calibrate and monitor energy scale, energy resolution and (CCD) re-distribution. Regular monitoring observations of a suitable subsample of standard candles was highly recommended. The various missions represented at the discussion identified their usage of these sources as follows: XMM : RXJ1856 1E0102 Crab N132D GD153 Chandra : RXJ1856 1E0102 CAS A G21.5 Suzaku : RXJ1856 1E0102 CAS A Crab RXTE : CAS A Crab Swift : RXJ1856 1E0102 CAS A Crab INTEGRAL : CAS A Crab "LABORATORY" ABSOLUTE STANDARDS ------------------------------- Considering the resort, in other energy/wavelength ranges, to laboratory calibrated reference sources to establish an absolute calibration of selected celestial sources, the meeting discussed possibilities for such appraches in the high energy regime. The paper: Noergaard-Nielsen 1994 A&A, 285, 705 "Excalibur - absolute calibration of the X-ray spectrum of the Crab nebula", was recalled as an example of a dedicated calibration space mission. Such a dedicated mission, in competition with pure-science-oriented small missions, would probably fail to secure funding. Another option mentioned was a rocket flight mission dedicated to establishing absolutely calibrated celestial targets - but this relatively cheap option would hardly be able to calibrate sources in the faint flux range relevant to future high energy missions. Yet another, and perhaps the most feasible option would be to fly a dedicated reference source on another spacecraft and to use it to achieve a transfer of absolute calibration to celestial reference sources. (The US military infrared mission MSX carried small black spheres which it ejected to use as black-body absolute reference sources.) Mushotsky mentioned a newly developed calibration source (by K. Gendreau) which, if carried on-board a future mission, might serve as an absolute calibration reference. OTHER GENERAL DISCUSSION ------------------------ Kaastra remarked that in the choice of reference sources one should avoid spectra with complicated spectral features, which lead to complications of calibration depending on the resolution of the instrument. It was asked whether XMM-Newton had adjusted the Effective Area of the instruments to adapt ("fudge") to observed celestial calibrations, particularly the Crab. Haberl said that XMM-Newton EPIC had not done this. Some second-order corrections (e.g. the details of some instrumental edges) had been made in this way. No global adjustment to adapt to a canonical Crab model had been made. Metcalfe noted that the RGS effective area had recently been adjusted to remove an artifact systematically found in the spectra of a set of BL Lac objects. This adjustment amounted to up to about 10% at the low energy limit of RGS. Kirsch read, from SPIE 5898/3, a list of missions/instruments that had indicated that they HAVE adjusted their calibrations to the Crab. Some discussion took place of the use of stars with emission lines as standard sources. It was decided that these did not fall within the scope of this discussion of standard candles. STANDARD CANDLES and MODELS ---------------------------- It was remarked that if a source is identified as a standard candle then the model parameters for the source must be agreed. At least the range of credible model parameters should be tabulated. Alternatively, the calibration of the "standard candle" source should be specified as a "band of models", defining the range within which the true absolute calibration of the source should reside. Finally, it was remarked that strikingly successful cross-calibration agreement among instruments greatly enhances the credibility of affected scientific conclusions.