The sources of the hard X-ray and soft gamma-ray …userpages.irap.omp.eu/~pvonballmoos/gamma_wave_2005/...mardi 27 septembre 2005 Gamma-WAVE - Bonifacio The sources of the hard X-ray

mardi 27 septembre 2005 Gamma-WAVE - Bonifacio

The sources of the hard X-ray

and soft gamma-ray

background(s)

Andrea Comastri & Roberto Gilli

INAF- Osservatorio Astronomico Bologna

Italy


Outline

! XRB synthesis models: where do we standand what we have learned from soft ( < 10keV) observations : obscured sources inthe Compton Thin regime.

! The content of the hard X-ray sky :Compton thick absorption and the primaryemission mechanism.

!Perspectives for future observations.


Extragalactic Background Light


Low energy ( < 10

keV ) -> Several

measurements by

imaging detectors ->

Differing up to 30-

40% and

systematically higher

than HEAO1-A2 …

The 30 keV peak has

been “observed” only

by HEAO1-A2


Some 80-90 % of the XRB below 7-8 keV is

resolved into discrete sources --> AGN (often

elusive or not recognized at optical wavelengths)


above 10 keV

the resolved fraction

is negligible and of

the order of a few

percent due to the

lack of imaging

detectors


The AGN Synthesis of the XRB! The XRB spectrum, source counts, redshift distribution can be

adequately fitted by a mixture of (X-ray) Type2/Type1 AGNspectra folded with an appropriate evolving XLF

! Obscured AGN outnumber unobscured one by a factor 3 to 5(simple Unified Model) and dominate the accretion driven energyoutput integrated over the cosmic history

! The luminosity function evolution is obscuration independent.

! The absorption distribution is either assumed from observationsof nearby AGN or is a free parameter in the XRB fit.

! The average SED is parameterized following broad band X-rayobservations of Seyfert and quasars over a range of luminositiesand redshift.

! COMPTON THICK absorption and High Energy Cut-off are themost uncertain parameters


XRB model formulae

XRB spectrum

Source counts

X ray luminosity function (XLF)

Source spectrum


Unobscured -> a Gamma=1.9 power law plus Compton reflection

and exponential cut-off (around a few hundreds of keV)

Obscured -> Unobscured plus low energy absorption


Risaliti

Nearby sample of

Sey2 galaxies

~ 50% are Compton

Thick

Tozzi

Spectral analysis of

Chandra Deep Field

South

Comastri :

1995 model

Ueda

2003 model


La Franca, Fiore , AC

et al. 2005

ApJ in press

astro-ph/0509081

Luminosity L

Dependent D

Density D

Evolution E


Redshift distribution (Opt +stat. ids)


Fit to the XRB spectrum


Self-consistent model

Counts, XLF, N(z),

N(NH) in both soft

and hard band are

reproduced

Smooth decrease of

the fraction of

obscured objects

towards high

luminosities

COMPTON THICK ?

… more later …

Gilli, AC, Hasinger

2006 in preparation


Where do we stand ?! A robust model can be (and indeed has been)

worked out which is able to reproduce themost recent observational constraints

! The predictive power (above 10 keV) islimited (being dependent on the assumptionson high energy spectra and Compton Thickabsorption distribution)

! Combining the > 10 keV background spectrum,the model and the available observations it ispossible to predict/guess the properties athigher energies


Malizia et al 2002

Recent INTEGRAL results (Soldi et al. 2005) --> diversity in the cut-off

Energies (50 keV to > > 300 keV)



! Radio loud AGN /Blazars contributesome 10% to thesoft XRB (Galbiatiet al 2004; Giommiet al 2005)

! Normalizing to 1 keVfor a range ofslopes (0.5-0.7) abreak or cut-offmust be presentaround 1 MeVotherwise the soft-gamma-raybackground would beexceeded


! The shape ofthe residualspectrum is afunction ofthe XRBmodelparameters(…almost bydefinition…)




! The shape of the unresolved background isvery similar to that of highly obscuredCompton Thick sources ….

! … however , depending from the LF evolution,the high energy cut-off e-folding energy andthe Blazar contribution it could be extremelypeaked with an over-exponential decrease …

! At the face value this would imply that thesources of the residual background must haveeither a very peaked spectrum or clusteredaround a small redshift interval or both




! The predicted

fraction of Compton

Thick object

increases steeply

below logFx = -14

(roughly consistent

with the present

estimate of Tozzi et

al. 2005 based on a

dozen of tentative

identifications


What would be needed ?

We are still working around a few keV

" need to sample high energies with enoughsensitivity

! Surprises above 10 keV ?

! Compton thick absorption ? ( Luminosity andredshift distribution ? ? ?)

! High energy cut-offs or continuum breaks ormore complex spectral shapes ?


>50% res.CXB

>80% res.CXB

F(20-40keV)<

7!10-15 cgs or

0.75 µCrab

10-15 cgs or

0.1 µCrab

F(10-30keV)<

10-14 cgs or

0.65 µCrab

2!10-15 cgs or

0.13 µCrab


Direct Imaging at E=10-80 keV

1µCrab = 250 sources deg2 =

12 sources X 15’ diam. FOV

0.5 µCrab = 550 deg2 =


0.1µCrab = 2350 deg2 =



Hard X-ray focusing mission proposed

for 2010-2015

HPD FOV Flim µCrab %CXB sources/FOV

FWHM 20-40keV

1Msec

•NuStar 40” 15’ 0.8 40% 15

•NeXT 30” 12’ 0.7 50% 12

•Simbol-X baseline 30” 7’ 1.4 35% 2

•Simbol-X ML 15’ 12’ 0.5-0.7 65% 25


Short dashed : unabsorbed

Long dashed : Compton thin

Solid : Total

Blue lines : high energy

cutoff at 400 keV

Red lines: high energy

Cutoff at 100 keV

Model parameters tuned

To account for 80% of the

XMM (2-10 keV)

background with the

“observed” NH distribution

No Compton thick and only

A fraction of the more

Obscured (logNH > 23)

Compton thin


Residual background

Spectrum after model

Subtraction

Magenta lines: analtyic

Fit from Gruber et al

Upper curve: renormalized

Upward by 30 %

(Hasinger talk , Ueda)

Green lines

Residual spectrum from

Original Gruber fit

Cyan lines

Residual spectrum form

Renormalized Gruber fit


Conclusions

! High energy (10-50 keV) imaging down tologFx about -14 " NuStar/SimbolX/Nextwould resolve a significant fraction of thehard XRB and test model predictionsespecially for heavily obscured Compton Thicksources

! A mission capable to explore even higherenergies ( say 50-500 keV) would allow toinvestigate the primary emission mechanismof AGN continuum

Documents

The sources of the hard X-ray and soft gamma-ray …userpages.irap.omp.eu/~pvonballmoos/gamma_wave_2005/...mardi 27 septembre 2005 Gamma-WAVE - Bonifacio The sources of the hard X-ray