[seminar] Astro Journal Club on 16 March [note the different place]

[Oskari Miettinen]

Dear all,

Our Astro Journal Club will be held on Wednesday at 3:00 pm in the coffee
room of the Physics Department (i.e. the Blue Room). This time we will
discuss about the analysis of X-ray observations of galaxy clusters.

Presenter: Krešimir Tisanić
Paper title: Decoding X-ray observations from centres of galaxy clusters
using MCMC
Authors: Lakhchaura, K., Deep Saini, T., and Sharma, P.
Paper status: submitted to MNRAS

Summary:
Traditionally the thermodynamic profiles (gas density, temperature, etc.)
of galaxy clusters are obtained by assuming spherical symmetry and
modeling projected X-ray spectra in each annulus. The outer annuli
contribute to the inner ones and their contribution needs to be subtracted
to obtain the temperature and density of spherical shells. The usual
deprojection methods lead to propagation of errors from outside to in and
do not model the covariance of parameters in different radial shells. In
this paper we describe a method based on a free-form model of clusters
with cluster parameters (density, temperature) given in spherical shells,
which we jointly forward fit to the X-ray data by constructing a Bayesian
posterior probability distribution that we sample using the MCMC
technique. By systematically marginalising over the nuisance outer shells,
we estimate the inner entropy profiles of clusters and fit them to various
models for a sample of Chandra X-ray observations of 17 clusters. We show
that the entropy profiles in almost all of our clusters are best described
as cored power laws. A small subsample is found to be either consistent
with a power law, or alternatively their cores are not fully resolved
(smaller than, or about few kpc). We find marginal evidence for bimodality
in the central values of entropy (and cooling time) corresponding to
cool-core and non cool-core clusters. The minimum value of the ratio of
the cooling time and the free-fall time (min[t_cool/t_ff]; correlation is
much weaker with core entropy) is anti-correlated with Halpha and radio
luminosity. Halpha emitting cold gas is absent in our clusters with
min(t_cool/t_ff)>~10.

Link to the paper:
http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1601.02347

Hope to see you all on Wednesday!

Cheers,
Oskari