TeV blazars and their distanceE. Prandini, Padova University & INFN

G. Bonnoli, L. Maraschi, M. Mariotti and F. Tavecchio

Cosmic Radiation Fields - Sources in the early UniverseHamburg 9-12 November 2010

E. Prandini TeV blazars and their distance CRF2010, Hamburg2

Outline TeV blazars and EBL Combined GeV+TeV spectra:

TEST the max slope hypothesis Use it to set u.l. on the distance of unknown z

sources Propose an empirical method to estimate blazars z

Update: new data

E. Prandini TeV blazars and their distance CRF2010, Hamburg3

TeV Blazars

Blazars: “radio loud” AGNs with the jet almost aligned to the line of sight of the observer

log(E)

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eV keV MeV GeV TeV

Simplified SED model of emitted spectrum of TeV blazars

Fermi TeV

The blazar emission is non thermal and covers the entire e.m. spectrum

It is composed by two bumps:

Syncrotron emission High energy emission

E. Prandini TeV blazars and their distance CRF2010, Hamburg4

The effect of EBL on blazars spectra

The blazar EMITTED spectrum is partially absorbed and deformed (at TeV instruments energy range)

The absorption is related to the DISTANCE of the source

BUT: some blazars have unknown distance!

log(E)

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eV keV MeV GeV TeV

SED

EBL: the EMITTED spectrum is deformed

Hauser & Dwek (2001)

E. Prandini TeV blazars and their distance CRF2010, Hamburg5

Constraints from the absorption

log(E)

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eV keV MeV GeV TeV

SED

EBL model

Hypotheses on the intrinsic spectrum

Blazar distance

E. Prandini TeV blazars and their distance CRF2010, Hamburg6

Constraint on the distance With Fermi + TeV spectral points the second bump is resolved!

Use the Fermi slope as limiting slope for the TeV de-absorbed spectrum in order to set a limit on the source distance: z*

Observed spectrum De-absorbed spectrum

Abdo et al. (2010)

E. Prandini TeV blazars and their distance CRF2010, Hamburg7

The sample Fermi TeV sources (from Abdo et al.

2009) + TeV spectra from last generation of Cherenkov Telescopes (Magic, Veritas, H.E.S.S.)

14 sources with well known redshift

2 sources of uncertain redshift (S5 0716+714 and 3C 66A)

Spectral break

E. Prandini TeV blazars and their distance CRF2010, Hamburg8

Results: z* VS z[true]

All the limits (z*) are above the bisector

Open points: uncertain redshift: the only values below the bisector!

THIS TEST CONFIRMS THAT THE SLOPE MEASURED BY FERMI CAN BE USED AS A LIMIT ON THE VHE SLOPE FOR

CONSTRAINING THE REDSHIFT

bisectorUncertain

redshift

E. Prandini TeV blazars and their distance CRF2010, Hamburg9

A step further: from limit to estimate

Is there any relation among z* and z[true]?

Following Stecker & Scully, 2010 (1): linear expression for

the steepening of the observed TeV slope

z* is also related to the steepening: LINEAR RELATION

Linear fit:z* = A + B z[true]

Ref (1): Stecker and Scully, 2010 ApJ 709 L124

E. Prandini TeV blazars and their distance CRF2010, Hamburg10

Reconstructed redshift

We can use the fit to ESTIMATE THE REDSHIFT of a source (and not only a limit)

Method: De-absorb TeV data according to the Fermi

spectral measure Estimate z* (and obtain a LIMIT on the distance) Invert the linear relation and estimate z[rec]

z[rec] = (z* - A)/B

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Test on known distance blazars

E. Prandini TeV blazars and their distance CRF2010, Hamburg

Residuals distribution z[true]-z[rec]

Sigma of the Gaussian fit: 0.05 Uncertain redshift sources are

well outside the distribution...

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Comparison between different EBL Models

Similar results with extreme EBL models: Low EBL: Kneinske & Dole 2010 Mean EBL: Franceschini et al. 2008 High EBL: Stecker et al. 2006

E. Prandini TeV blazars and their distance CRF2010, Hamburg

E. Prandini TeV blazars and their distance CRF2010, Hamburg13\

Conclusions of the Preliminary study Combining Fermi and TeV spectra: the max hardness

hypothesis is successfully tested on known redshift sources LIMIT on z

Moreover: we found a linear relation between the z limit (z*) and the real redshift (z[true]), that can be inverted and used for the estimate of the unknown distance of blazars, z[rec]. The result is EBL model independent!

According to our results, the “uncertain” z attributed to 3C66A and S5 0716+714 is largely overestimated or these sources are peculiar

More details in: E. Prandini et al. “Constraining blazar distances with combined Fermi and TeV data: an empirical approach”, MNRAS 405, L76-L80 (2010)

E. Prandini TeV blazars and their distance CRF2010, Hamburg14\

Update

Since last year: New Fermi catalogue (1 y)

New TeV sources

New EBL model (Dominguez et al. 2010)

E. Prandini TeV blazars and their distance CRF2010, Hamburg15\

The updated sample

E. Prandini TeV blazars and their distance CRF2010, Hamburg16\

Results: Franceschini & Dominguez models

Confirm previous results

The two EBL models give very similar results

Prandini et al. , in prep.

Prandini et al. , in prep.

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Application: the distance of PKS 1424+240

E. Prandini TeV blazars and their distance CRF2010, Hamburg

estimate on the distance of unknown redshift sources observed at both TeV and GeV ranges: PKS 1424+240

Deabsorbed spectrum of PKS 1424+240 (Fermi 1y catal., Franceschini et al. EBL model)

z* = 0.45 ± 0.15

z[rec] = 0.26 ± 0.06

Prandini et al. , in prep.

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Conclusions… The updated work basically

confirms the results Fermi slope as limiting slope for

TeV blazars

Empirical law allows an estimate on blazars distance

Increase statistics (new TeV sources)

Use simultaneous Fermi - TeV data

Extend the z range (CTA?)

… & Outlook

Newly detected TeV extragalactic objects:

E. Prandini TeV blazars and their distance CRF2010, Hamburg

Danke!

E. Prandini TeV blazars and their distance CRF2010, Hamburg19\

Backup slides

E. Prandini TeV blazars and their distance CRF2010, Hamburg20

The TeV extragalactic sky45 sources (November 2010)

E. Prandini TeV blazars and their distance CRF2010, Hamburg21

Assumptions

Not simultaneous GeV-TeV observations

But slopes seem somehow less variable than the corresponding fluxes (es. 1ES 1218+304 Veritas Coll.)

Different TeV instruments, sensitivities and energy thresholds (systematics?)

IC peak position: could be different

E. Prandini TeV blazars and their distance CRF2010, Hamburg22

Results: z* VS z[true] in LINEAR SCALE

E. Prandini TeV blazars and their distance CRF2010, Hamburg23

Application: the distance of PKS 1424+240

E. Prandini TeV blazars and their distance CRF2010, Hamburg24

Steepening of blazars spectra

Stecker & Scully, 2010

E. Prandini TeV blazars and their distance CRF2010, Hamburg25\

Results: Linear Relation

Linear releation still fits well!

E. Prandini TeV blazars and their distance CRF2010, Hamburg26\

z*: updated values

E. Prandini TeV blazars and their distance CRF2010, Hamburg27\

z[rec]