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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.