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.