A new method to constrain the distance of blazars with unknown redshift using
combined observations in the GeV and TeV regimes will be presented. The
underlying assumption is that the Very High Energy (VHE) spectrum corrected for
the absorption of TeV photons by the Extragalactic Background Light (EBL) via
photon-photon interaction should still be softer than the extrapolation of the
gamma-ray spectrum observed by Fermi/LAT. Starting from the observed spectral
data at VHE, the EBL-corrected spectra are derived as a function of the
redshift z and fitted with power laws. Comparing the redshift dependent VHE
slopes with the power law fits to the LAT data an upper limit to the source
redshift can be derived. The method is applied to all TeV blazars detected by
LAT with known distance and an empirical law describing the relation between
the upper limits and the true redshifts is derived. This law can be used to
estimate the distance of unknown redshift blazars: as an example, the distance
of PKS 1424+240 is inferred.Comment: Contribution to SciNeGHE 2010, Trieste, Italy, September 2010; 4
  pages, 2 figur

Bonnoli, Giacomo

Maraschi, Laura

Mariotti, Mose'

Prandini, Elisa

Tavecchio, Fabrizio

English

arXiv

A new method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes will be presented. The underlying assumption is that the Very High Energy (VHE) spectrum corrected for the absorption of TeV photons by the Extragalactic Background Light (EBL) via photon-photon interaction should still be softer than the extrapolation of the gamma-ray spectrum observed by Fermi-LAT. Starting from the observed spectral data at VHE, the EBL-corrected spectra are derived as a function of the redshift z and fitted with power laws. Comparing the redshift-dependent VHE slopes with the power-law fits to the LAT data an upper limit to the source redshift can be derived. The method is applied to all TeV blazars detected by LAT with known distance and an empirical law describing the relation between the upper limits and the true redshifts is derived. This law can be used to estimate the distance of unknown redshift blazars: as an example, the distance of PKS 1424+240 is inferred. © Società Italiana di Fisica

Prandini E.

Bonnoli G.

Maraschi L.

Mariotti M.

Tavecchio F.

Archivio istituzionale della ricerca - Università di Padova

Blazars distance indications from Fermi and TeV data

Prandini, E.

Maraschi, L.

Mariotti, M.

Tavecchio, F.

Archivio della Ricerca - Università degli Studi di Siena

A new method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes will be presented. The underlying assumption is that the Very High Energy (VHE) spectrum corrected for the absorption of TeV photons by the Extragalactic Background Light (EBL) via photon-photon interaction should still be softer than the extrapolation of the gamma-ray spectrum observed by Fermi-LAT. Starting from the observed spectral

data at VHE, the EBL-corrected spectra are derived as a function of the redshift z and fitted with power laws. Comparing the redshift-dependent VHE slopes with the power-law fits to the LAT data an upper limit to the source redshift can be derived. The method is applied to all TeV blazars detected by LAT with known distance and an empirical law describing the relation between the upper limits and the true redshifts is derived. This law can be used to estimate the distance of unknown redshift blazars: as an example, the distance of PKS 1424+240 is inferred

Bonnoli, G.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10863-4Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011Blazars distance indications from Fermi and TeV dataE. Prandini(1)(∗), G. Bonnoli(2), L. Maraschi(3), M. Mariotti(1)and F. Tavecchio(2)(1) Dipartimento di Fisica and INFN, Sezione di Padova - via Marzolo 835134, Padova, Italy(2) Osservatorio Astronomico di Merate - via E. Bianchi 46, 23807, Merate (Lc), Italy(3) Osservatorio Astronomico di Brera - via Brera 28, 20121, Milano, Italy(ricevuto il 25 Febbraio 2011; pubblicato online il 4 Maggio 2011)Summary. — A new method to constrain the distance of blazars with unknownredshift using combined observations in the GeV and TeV regimes will be presented.The underlying assumption is that the Very High Energy (VHE) spectrum correctedfor the absorption of TeV photons by the Extragalactic Background Light (EBL)via photon-photon interaction should still be softer than the extrapolation of thegamma-ray spectrum observed by Fermi-LAT. Starting from the observed spectraldata at VHE, the EBL-corrected spectra are derived as a function of the redshift zand fitted with power laws. Comparing the redshift-dependent VHE slopes with thepower-law fits to the LAT data an upper limit to the source redshift can be derived.The method is applied to all TeV blazars detected by LAT with known distanceand an empirical law describing the relation between the upper limits and the trueredshifts is derived. This law can be used to estimate the distance of unknownredshift blazars: as an example, the distance of PKS 1424+240 is inferred.PACS 95.55.Ka – X- and γ-ray telescopes and instrumentation.PACS 98.54.Cm – Active and peculiar galaxies and related systems (including BLLacertae objects, blazars, Seyfert galaxies, Markarian galaxies, and active galacticnuclei).PACS 98.70.Vc – Background radiations.1. – IntroductionThe extragalactic TeV sky catalogue (E > 100GeV) counts nowadays 45 objects(1).Many of these sources have been recently detected also at GeV energies by the Fermisatellite [1], allowing for the first time a quasi-continuous coverage of the spectral shape(∗) E-mail: prandini@pd.infn.it(1) For an updated list see: http://www.mppmu.mpg.de/~rwagner/sources/c© Societa` Italiana di Fisica 241242 E. PRANDINI, G. BONNOLI, L. MARASCHI, M. MARIOTTI and F. TAVECCHIOof extragalactic VHE emitters over more than 4 decades of energy. The large majorityof extragalactic TeV emitting objects are blazars, radio-loud active galactic nuclei witha relativistic jet closely oriented toward the Earth, as described in [2]. Here, we discussa method, recently published in [3], to derive an upper limit on the redshift of a blazar,based on the comparison between the spectral index at GeV energies as measured by LAT(unaffected by the cosmological absorption up to redshifts far beyond those of interesthere) and the TeV spectrum corrected for the absorption. Starting from the derivedlimits, we find a simple law relating these values to real redshift, which can be used toguess the distance of unknown redshift blazars. We assume a cosmological scenario withh = 0.72, ΩM = 0.3 and ΩΛ = 0.7.2. – ResultsThe photon flux emitted by a blazar in both GeV and TeV regimes can be well approxi-mated by power laws, of the form dN/dE = f0(E/E0)−Γ, where Γ is the power-law index.At VHE, the photons of the spectrum interact with the extragalactic background light(EBL), via electron-positron pair creation. Quantitatively, the effect is an exponentialattenuation of the flux by a factor τ(E, z), where τ is the optical depth, a function of bothphoton energy and source redshift. Thus, the observed differential energy spectrum froma blazar, Fobs, is related to the emitted one, Fem, according to Fobs(E) = e−τ(E)Fem(E).In order to estimate a safe upper limit to the source distance, we can reasonablyassume that the intrinsic spectrum at TeV energies cannot be harder than that in theadjacent GeV band. Indeed, from the brightest objects studied at both GeV and TeVenergies it appears that the SED is continuous, with a broad peak not requiring ad-ditional spectral components [4]. Hence, a natural assumption is to require that theslope measured in the GeV energy range is a limit value for the power-law index of thede-absorbed TeV spectrum.For the study, we consider the blazar sample listed in table I and containing all theextragalactic TeV emitters located at redshift larger than z = 0.01, detected by LATafter taking 5.5 months of data [1]. In order to estimate the redshift z∗ for which theTeV spectral slope equals to the GeV one, the measured spectral points of each sourcehave been corrected for the corresponding absorption factor [7], starting from redshiftz = 0.01, and the resulting spectrum fitted with a power law. The procedure, applied infine steps of redshift, is iterated until the slope of the de-absorbed spectrum equals theone measured by LAT. The corresponding redshift, z∗, reported in table I, is the limitvalue on the source distance.Among the 16 sources considered in this study, 14 blazars have well-known red-shift and are used to test the method, while the remaining two blazars (3C 66A andS5 0716+714) have uncertain redshift, and are considered separately. The errors on z∗are estimated taking into account both errors on the TeV and LAT slopes. Figure 1shows the comparison between the known redshift, x-axis, and z∗. All the z∗ lie abovethe bisector (dashed line) meaning that their values are larger than those of the real red-shift z[true]. This is expected since we are not considering the presence of the intrinsicbreak in the blazar spectra, and confirms that the method can be used to set safe upperlimits on blazars distance. The only exceptions are the two sources with uncertain dis-tance, S 0716+714 and 3C 66A (open circles). This could be either due to some intrinsicproperties of the sources or to a wrong estimate of their distances. In the latter case, ourmethod would constrain, at the two-sigma level, the redshift of S5 0716+714 below 0.39and that of 3C 66A below 0.44.BLAZARS DISTANCE INDICATIONS FROM FERMI AND TeV DATA 243Table I. – TeV blazars used in this study. The sources used in this study are listed in the firstcolumn, their redshift (second column), their Fermi-LAT slope (third column), the VHE slopeof the observed differential energy spectrum fit (fourth column) and the value z∗ (last column).Detailed references can be found in [3].Source name z[real] Fermi-LAT slope TeV slope z∗Mkn 421 0.030 1.78± 0.03 2.3± 0.1 0.08± 0.02Mkn 501 0.034 1.73± 0.06 2.3± 0.1 0.10± 0.021ES 2344+514 0.044 1.76± 0.27 2.9± 0.1 0.20± 0.06Mkn 180 0.045 1.91± 0.18 3.3± 0.7 0.20± 0.121ES 1959+650 0.047 1.99± 0.09 2.6± 0.2 0.09± 0.04BL Lacertae 0.069 2.43± 0.10 3.6± 0.5 0.23± 0.12PKS 2005−489 0.071 1.91± 0.09 3.2± 0.2 0.19± 0.04W Comae 0.102 2.02± 0.06 3.7± 0.2 0.23± 0.05PKS 2155−304 0.116 1.87± 0.03 3.4± 0.1 0.22± 0.011ES 0806+524 0.138 2.04± 0.14 3.6± 1.0 0.23± 0.151ES 1218+304 0.182 1.63± 0.12 3.1± 0.3 0.21± 0.081ES 1011+496 0.212 1.82± 0.05 4.0± 0.5 0.49± 0.12S5 0716+714 0.310(a) (b) 2.16± 0.04 3.4± 0.5 0.21± 0.09PG 1553+113 0.400(c) 1.69± 0.04 4.1± 0.2 0.57± 0.053C66A 0.444(a) 1.93± 0.04 4.1± 0.4 0.34± 0.053C279 0.536 2.34± 0.03 4.1± 0.7 0.75± 0.72(a) Uncertain.(b) From [5].(c) From [6].z[true]-110z*-1101 / ndf 2χ  9.919 / 12Prob   0.6231p0        0.01223± 0.054 p1        0.1147± 1.365 Fig. 1. – z∗ vs. true redshift derived with the procedure described in the text. The open pointsare the two uncertain redshift sources, namely 3C 66A and S5 0716+714, not used in the fitcalculation (continuous line). The dashed line is the bisector.244 E. PRANDINI, G. BONNOLI, L. MARASCHI, M. MARIOTTI and F. TAVECCHIO Energy [GeV]210]-1 TeV-1 s-2dN/dE [cm-1210-1110-1010PKS 1424+240 observed spectrumPKS 1424+240 deabsorbed spectrum (z = 0.382)Power law fitFig. 2. – Measured (open points) and de-absorbed (filled points) spectrum of PKS 1424+240 atredshift z = 0.382.In [8], a linear expression for the steepening of the observed TeV slope due to EBLabsorption is derived. Since in our procedure z∗ is related to this steepening, it isnatural to assume that also z∗ and z[true] are related by a linear function, of the formz∗ = A + Bz[true]. The meaning of the coefficients is rather transparent: basically Ais a measure of the intrinsic spectral break of the sources, while, following [8], B is ameasure (increasing values for decreasing EBL level) of the optical depth of the EBLmodel used. We interpolate with this linear function the data with well-known distanceof fig. 1. The linear fit (continuous line) has a probability of 62%. Once derived thisempirical relation, one can use it to determine the redshift of sources with uncertaindistance. For S5 0716+714 the reconstructed redshift is z[rec] = 0.11± 0.05, while thatof 3C 66A is z[rec] = 0.21± 0.05. The error quoted is estimated in [3].3. – The redshift of PKS 1424+240As a final example of application, we use our procedure on PKS 1424+240, a blazar ofunknown redshift recently observed in the VHE regime by Veritas [9]. The slope spectrummeasured by Fermi -LAT between 0.2 and 300GeV is 1.85± 0.05. The corresponding z∗redshift at which the de-absorbed TeV spectrum slope becomes equal to it, is 0.382 ±0.105, fig. 2, using the EBL model [7]. This result is in agreement with the value of0.5± 0.1, reported in [9], calculated applying the same procedure but only simultaneousFermi data. Our estimate on the most probable distance for PKS 1424+240, obtainedby inverting the z∗ formula, is z[rec] = 0.24± 0.05, where, as before, the error quoted isestimated in [3].∗ ∗ ∗GB, LM and FT acknowledge a 2007 Prin-MIUR grant for financial support.REFERENCES[1] Abdo A. A. et al., Astrophys. J., 707 (2009) 1310.[2] Urry C. M. and Padovani P., Publ. Astron. Soc. Pac., 107 (1995) 803.BLAZARS DISTANCE INDICATIONS FROM FERMI AND TeV DATA 245[3] Prandini E., Bonnoli G., Maraschi L., Mariotti M. and Tavecchio F., Mon. Not.R. Astron. Soc., 405 (2010) L76.[4] Aharonian F. et al., Astrophys. J., 696 (2009) L150.[5] Nilsson K., Pursimo T., Sillanpa¨a¨ A., Takalo L. O. and Lindfors E., Astron.Astrophys., 487 (2008) L29.[6] Danforth C. W., Keeney B. A., Stocke J. T., Shull J. M. and Yao Y., Astrophys.J., 720 (2010) 976.[7] Franceschini A., Rodighiero G. and Vaccari M., Astron. Astrophys., 487 (2008) 837.[8] Stecker F. W. and Scully S. T., Astrophys. J., 709 (2010) L124.[9] Acciari V. A. et al., Astrophys. J., 708 (2010) L100.

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Blazars distance indications from Fermi and TeV data

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