X-ray absorption by IGM
Moshe-Ishay Cohen
May 2014
Agenda
• Motivation – The missing atoms
• Topics:−GRBs−X-ray absorption− IGM−Optical depth
• Conclusions
• Discussion
• Bibliography
Motivation – the missing atoms
Dark Energy72.00%
Dark Matter23.00%
Stars0.25%Gas in galaxies, groups and clus-
ters0.25%
Intergalactic UV absorption sys-tems2.00%
???2.50%
energy distribution
GRB – Gamma Ray Bursts
• Gamma Ray Bursts are flashes of gamma rays
caused apparently by energetic explosions.
• They are the brightest known source in the
universe.
• They are followed by a long “afterglow” of the
entire spectra.
• They emits a lot of radiation. But we don’t get all
of it…
X-Ray absorption from host
• The missing radiation from the GRBs is
assumed to go either by an obstacle
near the source, or by junks of matter
(IGM – Inter Galactic Medium) in the
way.
• Absorption from near host obstacle is
depended on distance (because energy
dependency)
IGM – Inter Galactic Medium
• The material found scattered in the space
between galaxies is called the IGM.
• The Inter galactic space (IGS) is consisted
mostly of vacuum, and have estimated
average density of 1 atom per m3.
• The IGM is filaments of matter in IGS, and
have estimated average density of 100 atoms
per m3.
• Most of the IGM is ionized.
• https://
www.youtube.com/watch?v=Qys80HF9-PE
• https://
www.youtube.com/watch?v=74IsySs3RGU
Optical depth
• During the journey to the satellite
some of the radiation from the GRBs
is absorbed in the IGM.
• We define τ as the optical depth.
• We have good measurements of τ.
0
viewd source
dI dIk I j Ids d
I e Id k ds n ds
Optical depth measurement
Optical depth (2)
• In each segment of the way dl what
governs the absorption rate is:−The density in this segment n(l)−The cross section for absorption for the
materials (Z) in the segment for the specific energy (E(l)).
, ,
, ,
l ctd n l l E l Z l dl
dtdt dl c dzn z z E z Z z c dzdzdz
Optical depth factors - density• The density is given by the
hydrogen density
30
27 30
0
1
30.67 1.7 10
8
H
bH
n z n z
Hn cm
Gm
Radiative processes
• We are looking for radiation
absorption.
• Photo excitation – Absorption lines:−The absorption lines have very strong
cross section, but have very narrow bandwidth of wavelengths to happen, and there for are neglected
• Photo ionization:−Happens for energetic enough photons,
and has a very wide bandwidth of wavelengths to happen.
Optical depth factors – cross section
• The cross section in
the area of interest
is governed by metals.
• The cross section is approximated
as:
2.50
2.5
2.5
0
; 1 ;
, 1
, ,0 1
E E E z Z Z z
E z z
E z Z z E z
0.5 keV
Optical depth factors – metallicity evolution
• Metals evolve as the universe ages,
so we expect to see less metals as
we move to higher redshift.
• A general metallicity evolution is
given by: 1
k
k z z
Optical depth (3) - results• Combining all the factors we get:
0
3
0 0
2.5 300
0 020
0
3 2
0
0.51
.5
0 0
30
, , , ,
1,0
1 1 1
,01
1 ; , , ,
2
0 1
1 11 ;
1 1
1
z
IGM
z k
M
z k
k
k
E k
M
eVz
c dtE z Z n z E z Z dz
dz
z z dzn cZE
H z z z
z dzn cZE
H z
n z n z E z Z E z Z z
dtz z
dz H z z
Z
k
Data Vs. model (1) – z>4.4
Data Vs. model (2) – z<4.3
Conclusions
• Low redshift optical depth is dominated by
absorption at host galaxy.
• The IGM can explain the optical depth
saturation for high z GRBs (z>2).
• The redshift of saturation depends on the
metallicity of the IGM.
• If the model is correct then we found the
missing baryons
Discussion
• Does the high metallicity
assumption holds?
• Non-Homogeneous IGM
• Behavior of Quasars
• Future missions – high spectral
resolution for defining absorber
(spectral lines)
Bibliography
• Behar E., Dado S., Dar A., Laor A.,
2011, Can the Soft X-Ray Opacity
Toward High-redshift Sources Probe
the Missing Baryons? ApJ, 734, 26.