John Peterson
Purdue University
X-ray Gratings 2007
Boston, MA

Intracluster medium
Optical X-ray
Heated due to large gravitational potentials
Temperatures ~ 1-10 keV (10 7 to 10 8 K)
Densities ~ 10 -5 to 10 -1 particles per cubic cm
Sizes ~ 1 to 10 Mpc (10 24 to 10 25 ) cm

<=X-ray Spectra (prior to 2000)
X-ray Spectrum dominated by line emission and
Bremmstrahlung from collisionally ionized plasma
Plasma out of LTE optically thin
At densities and temperatures (in core), t recombination t cool
= 10 6 years (for Fe XVII at 1 keV)
= (5/2 n k T)/(n 2  ) = 10 8 to 10 9 years t formation
= 5 10 9 years
Collisional ionizations balanced by recombinations
Line emission dominated by collisional excitations+cascades,
Radiative recombination, and dielectronic recombination
Same model as stellar coronae

Long-standing prediction that cores of clusters should cool by emitting X-rays in less than a Gyr (Fabian & Nulsen
1977, Cowie & Binney 1977, Mathews & Bregman 1978)
Temperature Drops (e.g. Allen et al. 2001)
From CCD spectral fits
Density rises and t cool is short
(e.g. Voigt et al. 2002) from Images

Range of emperatures approximately at constant pressure
dL x
/ dT=5/2 (Mass Deposition Rate) k/(  m p
)
unique
max
The major assumption is that the emission of
X-rays is the dominate heating or cooling term

Measuring a differential luminosity at keV temperatures
=> Need Fe L ions (temperature sensitive)
=> Need to resolve each ion separately (i.e.  /  ~ 100)
Very difficult to do in detail with CCD instrument
(ASCA, XMM-Newton EPIC, Chandra ACIS)
Works with XMM-Newton RGS (for subtle reasons)

RGS (dispersive spectrometer) :
High dispersion angles (3 degrees)
 /  ~ 3 degrees / ang. size ~ 100 for arcminute size
Soft X-ray band from Si K to C K;
FOV: 5 arcminutes by 1 degree
Analysis not simple: dispersive, background, few counts
Data
Detailed studies best done with full Monte
Carlo
Model

<= dL/dT= constant model
8 keV  3 keV  ?
Peterson et al. 2001

Decompose into temperature bins and set limits

Hot clusters
Peterson et al. 2003

Warm Clusters
Peterson et al. 2003

Cool clusters/groups
Peterson et al. 2003

Differential Luminosity vs.
Temperature
Peterson et al. 2003
Differential Luminosity vs.
Fractional Temperature

Theoretical Intepretation: Essentially Three Fine-tuning Problems
1. Energetics: Need average heating or cooling power ~ L x
2. Dynamics: Either need energy source to work at low temperatures or at t ~ t occur) cool
(before complete cooling would
Cooling time ~ T 2 / (cooling function)
If at 1/3 T max then why cool for 8/9 of the cooling time? or why at low temperatures?
3. Get Energetic and Dynamics right at all spatial positions
Soft X-rays missing throughout entire cflow volume

Current Models
1. AGN reheating: relativistic flows inflate subsonic cosmic ray bubbles & cause ripples; dissipation efficiency? & feedback mechanism?
(Rosner & Tucker 1989; Binney & Tabor 1995; Tabor &
Binney 1995; Churazov et al. 2001, Bruggen & Kaiser 2001;
Quilis et al. 2001, David et al 2001; Nulsen 2002; Kaiser &
Binney 2002; Ruszkowski & Begelman 2002; Soker & David
2003; Brighenti & Mathews 2003)
McNamara et al. 2000
Fabian et al. 2003

2. Heat transfer from the outside to the core: probably through conduction
Stability & is conduction coefficient realistic
(Tucker & Rosner 1983, Stewart et al. 1984, Zakamska & Narayan 2001; Voigt et al.2002;
Fabian, Voigt, & Morris 2002; Soker 2003; Kim & Narayan 2003)
Voigt et al. 2003

3. Cooling through non radiative interactions with cold material:
Avoids producing soft Xrays?
(Begelman & Fabian 1990; Norman &
Meiskin 1996; Fabian et al. 2001, 2002;
Mathews & Brighenti 2003)
Fabian et al. 2002
4. Cluster Mergers (Markevitch et al. 2001)
5. Inhomogenous Metals (Fabian et al. 2001; Morris et al.200)
6. Differential Absorption (Peterson et al. 2001)
7. Cosmic Rays Interactions (Gitti et al. 2002)
8. Photoionization (Oh 2004)
9. Non-maxwellian particle ionization (Oh 2004)
Crawford et al. 2003

10. Dark Matter
(huge energy source):
Dark Matter-Baryon interactions (Qin & Wu 2001):
Requires high cross-section (  /m ~ 10 -25 cm 2 /GeV )
Dark Matter (Neutralino) Annihilations (Totani 2004):
Converts to relativistic particles
Requires a high central density for neutralino
Dark Matter-Baryon Interactions (Chuzhoy & Nusser 2004): same cross-section but make mass of dark matter ~ 1/3 of proton mass

M87

M87
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Use hundreds of gaussian blobs with own properties (e.g. temperature) instead of a parameterized model

Perseus

Perseus
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4 actual cooling flows:
Mukai et al. 2003

Long-standing problem of the origin of metals in the ICM:
Supernovae Ia (what fraction?)+ Type II (of what mass?) and of what metallicity (and therefore when)?+Stellar winds (for
CNO)+ Hypernovae?
Z i
= Yield
IA
(z)+  dM Yield
II
(z,M) dN/dM

Peterson et al. 2003
Abundances
Fe=>mostly Ia
O/Fe=0.7+/-0.2=>50% II
Ne/Fe=1.1+/0.3=>100% II
Mg/Fe=1.0+/0.3=>100% II
Si/Fe=2.3+/1=>100% II
Tamura et al. 2004 Matsushita et al. 2003
O/Fe 0.6
0.5
Mg/Fe 0.8
Si/Fe 1.4 1.2
S/Fe 1.1
1.1
Spatially resolved
Abundances much more complicated

Sersic 159-03, de Plaa et al. 2005
Spatial Distribution of Abundances
Abundances depend on temperature model sensitively
Gradient in Metals ~ 100% per 100 kpc
Gradient in O/Fe or Si/Fe < 20% per
100 kpc
NGC 5044, Buote et al. 2005

Evidence for a Low T (0.7 keV) diffuse thermal component (WHIM) still unsettled
OVII emission, Kaastra et al. 2001
Absorption (3 sigma) behind
Coma, Takei et al. 2007

Subtleties of particle background in CCD fits, de Plaa et al 2005
Large soft X-ray background from within the galaxy
(McCammon et al. 2002)

Resonant Scattering
 ~ n i
 i
(cluster size) ~ few for some transitions
( Fe XXV He  r, Fe XXIV 3d-2p, Fe XVIII 3d-2p, Fe XVII 3d-2p, possibly some Ly alpha transitions)
But doppler velocities can lower this
(thermal width ~ 100 km/s, sound speed ~ 1000 km/s)
NGC 4636, Xu et al. 2002 Perseus, Churazov et al. 2004

Cooling flow model fails to reproduce X-ray spectrum;
Several strong observational constraints (factor of 20!)
Fails despite very simple theoretical arguments
Much more theoretical work needed for fine-tuning challenges
Much more observational work is needed to constrain the spatial distribution and to connect to other wavelengths
Abundances still need more study
Soft excess inconclusive
Resonant scattering inconclusive
Note: radiative cooling is supposed to form galaxies through tiny cooling flows. Do we understand this now?