XMM-Newton talks again loud and clear

23 October 2008
XMM-Newton, ESA’s X-ray observatory, has re-established communication contact with Earth, showing that the spacecraft is safe and fully under control. The news was confirmed this morning by the mission control team at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany.

Radio contact was re-established on Wednesday 22 October around 18:00 Central European Summer Time (CEST). This followed an unexpected radio silence from the spacecraft which started on Saturday 18 October 2008 when XMM-Newton, coming out of a nominal period of non-radio visibility along its orbit around Earth, did not succeed in sending the expected signal to Earth.

X-ray telescopes usually also have gratings that can be inserted into the light path, so that photons of different energies get dispersed by different angles, and whose actual energies can then be inferred accurately by measuring how far away on the detector they ended up. The accuracy of the inference is usually limited by the width of the PSF. Thus, a major contributor to the LRF (Line Response Function) is the aforementioned scattering.

A correct accounting of the spread of photons of course requires a full-fledged response matrix (RMF), but as it turns out, the line profiles can be fairly well approximated with Beta profiles, which are simply Lorentzians modified by taking them to the power β –

where B(1/2,β-1/2) is the Beta function, and N is a normalization constant defined such that integrating the Beta profile over the real line gives the area under the curve as N. The parameter β controls the sharpness of the function — the higher the β, the peakier it gets, and the more of it that gets pushed into the wings. Chandra LRFs are usually well-modeled with β~2.5, and XMM/RGS appears to require Lorentzians, β~1.

The form of the Lorentzian may also be familiar to people as the Cauchy Distribution, which you get for example when the ratio is taken of two quantities distributed as zero-centered Gaussians. Note that the mean and variance are undefined for that distribution.

With the 5th year WMAP data release, there were many WMAP related papers and among them, most statistical papers are listed. WMAP specific/related:

• [astro-ph:0803.0586] J. Dunkley et. al.
    Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Likelihoods and Parameters from the WMAP data (likelihoods)

• [astro-ph:0803.0715] B. Gold et. al.
    Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission (MCMC)

• [astro-ph:0803.0889] Ichikawa, Sekiguchi, & Takahashi
    Probing the Effective Number of Neutrino Species with Cosmic Microwave Background

And others:

• [astro-ph:0802.4464] M. Sahlén et.al.
    The XMM Cluster Survey: Forecasting cosmological and cluster scaling-relation parameter constraints

• [astro-ph:0803.0918] J.M. Colberg et.al.
    The Aspen–Amsterdam Void Finder Comparison Project (TFE, tessellation field estimator)

• [astro-ph:0803.0885] J.Ballot et.al.
    On deriving p-mode parameters for inclined solar-like stars (MLE, maximum likelihood estimator)

By the way, I noticed [astro-ph:0802.4464] used Monte Carlo Markov Chain, whereas [astro-ph:0803.0715] used Markov chain Monte Carlo. Do they mean different? Or the former is a typo?

The probability distribution of photon counts from the Optical Monitor on XMM Newton satellite (XMM-OM) and the UVOT on the Swift satellite follows a binomial distribution due to detector characteristics. Incident count rate was derived as a function of the measured count rate, which was shown to follow a binomial distribution.

Discrepancy between the mapping the 1σ Poisson incident count error onto measured count error and the mapping the 1σ binomial measurement count error onto incident counts was illustrated with an example of large counts per frame. Although this discrepancy for small counts per frame was commented to be negligible, the authors urge to use their binomial distribution based formalism to derive the errors in measurements.