Radio-loud AGN contribution to the Extragalactic Gamma-Ray Background
Origin of extragalactic -ray background (EGRB) is still a matter of debate. EGRB can either have truly diffuse or unresolved discrete point sources origin. Majority of the Fermi and EGRET detected identified sources are blazar. So, they are expected to be a significant contributors to the EGRB. In order to estimate their contribution to the EGRB one needs to construct their luminosity functions. Here, we examine and construct the evolution and luminosity function of blazars using first LAT AGN catalog. We consider both pure luminosity and pure density evolution models for flat spectrum radio quasars (FSRQs). We also describe the methodology to estimate the contribution from misaligned blazars to the EGRB.
Observational -ray astronomy got a massive boost after the launch of the Fermi Gamma-ray Space Telescope (Fermi) on 11th June, 2008. The Large Area Telescope (LAT) onboard Fermi provides an increase in sensitivity by more than an order of magnitude over EGRET and AGILE.
The origin of extragalactic -ray background (EGRB) is one of the fundamental unsolved problem in astrophysics. EGRB can arise from some diffuse processes like black hole evaporation, large scale structure formation, matter-antimatter annihilation, etc (e.g., Loeb & Waxman (2000); Gabici & Blasi (2003); Cohen, Rjula & Glashow (1998)). Alternatively, due to the limited instrument sensitivity, unresolved -ray sources could contribute significantly to the observed EGRB.
Recently, the EGRB has been rederived from Fermi Large Area Telescope (LAT) data (Abdo et al., 2010b). EGRB is consistent with a featureless power law of index . The integrated flux above MeV is photon cm s sr.
Fermi already detected sources in 11 months of observation (First Fermi catalog; Abdo et al. (2010c)) and is expected to detect many more. In contrast to the less than active galactic nuclei (AGNs) in the EGRET catalog, the first Fermi-LAT AGN catalog (Abdo et al., 2010a) consist of blazars. Fermi also detected 11 non-blazar AGN (Abdo et al., 2010d), three normal (M , LMC and Milky Way) and two starburst galaxies (M and NGC ). Thus, Fermi provides an excellent opportunity to understand the origin of EGRB.
Since the dominant class of identified EGRET and Fermi sources are blazars,
they are expected to contribute to the EGRB. The contribution from blazars to the EGRB is
estimated using two different approaches:
(a) Considering a relationship between the blazar -ray luminosity with their luminosity at some other wavelength, their -ray luminosity function is assumed to be the scaled luminosity function at that wavelength (e.g., Stecker & Salamon (1996); Stecker, Salamon & Malkan (1993); Narumoto & Totani (2006, 2007); Abazajian, Blanchet & Harding (2010); Inoue (2011); Li & Cao (2011); Cavadini, Salvaterra & Haardt (2011); Stecker & Venters (2011)).
(b) Constructing the -ray luminosity function directly from observed -ray blazars (e.g., Chiang et al. (1995); Chiang & Mukherjee (1998); Bhattacharya, Sreekumar & Mukherjee (2009a, b)).
In an earlier work (Bhattacharya, Sreekumar & Mukherjee, 2009a, b) following a similar approach of Chiang & Mukherjee (1998) but with almost twice the number of sources, we constructed the -ray luminosity functions of flat spectrum radio quasars (FSRQs) and BL Lacs from EGRET-detected sources only. We found strong positive evolution for FSRQs, while BL Lacs did not show any significant evolution. The maximum contribution from blazars to EGRB was found to be %. However, due to the limited number of sources in the EGRET catalog, the luminosity function and hence, the contribution from blazars was not well constrained. Mcke & Pohl (2000) estimated the contribution from FSRQs and BL Lacs to the EGRB in the context of AGN unification paradigm, and found that unresolved blazars (both “aligned” and “misaligned”) contribute to the EGRB. In a similar approach, Dermer (2007) estimated that FSRQs and BL Lacs contribute % and at GeV respectively. From the first three months of Fermi observation, Abdo et al. abdo2009a () found that FSRQs exhibit strong evolution while BL Lacs show no evolution. Using the 1st Fermi catalog, Abdo et al. (2010e) found from the source count distribution that the point source contribution is of the EGRB.
Here, we construct the luminosity function of FSRQs and BL Lacs following the methodology described in Bhattacharya, Sreekumar & Mukherjee (2009a). Utilizing the Fermi observed sources we construct a complete source list, apply the method to search for the presence of evolution, find parameters by modified method, and finally construct the luminosity function using method and maximum likelihood estimator.
In the next section we discuss the evolution of blazar source class. We discuss in section 3 the luminosity function construction of FSRQs. The methodology to calculate the misaligned blazars contribution to the EGRB is given in section 4. We conclude with our discussion in section 5.
Ii Evolution of blazars
We study the luminosity evolution of blazars by considering a sample that consists of sources from first LAT AGN catalog having or above detection significance with -ray flux (E MeV) ph cms. Abdo et al. (2010e) showed that Fermi-LAT detects spectrally hard sources at flux level generally fainter than those of soft sources. They showed that the LAT sample (including all sources) is complete above ph cms (Figure 1 of Abdo et al. (2010e)). However, the maximum photon index for blazars is 2.98, much lower than the maximum photon index of all sources of 1st Fermi catalog, and from figure 1 of Abdo et al. (2010e), one can see that the blazar sample is complete to a flux below ph cms. As a secondary check to the completeness of our sample above ph cms, we performed the test of this sample and obtained that matches within of a sample of limiting flux ph cms. Our sample consists of FSRQs. and BL Lacs ( with known ). The comes out to be , which indicates strong evolution. For BL Lacs (with known sources only), comes out to be , which indicates no significant evolution. If we include the sources with unknown considering they are at the average of the sample, comes out to be . Since, a good fraction of BL Lacs in our sample do not have redshift information, it is not possible to construct a meaningful luminosity function of BL Lacs at this moment.
We consider both pure luminosity evolution (PLE) and pure density evolution (PDE) for FSRQs.
Both power law () and exponential (exp()) evolution function have been considered. Here denotes look-back time. The modified method is used to find the evolution parameter. For the optimum parameter value, is expected to be uniformly distributed between 0 and 1, thus . We find . For each value of evolution parameter (), the distribution of is compared with a uniform distribution using KS-test which shows distribution of is uniform for . The optimum value of is found to be . Similarly, for pure density evolution, the parameter values of exponential and power law evolution models are, and respectively. The upper panel of Fig 1 shows the variation of with different values of power law PDE parameter . Horizontal dotted lines show the error in .
Iii Luminosity function construction of FSRQs
For pure luminosity evolution, after de-evolution of the sources at , we constructed their luminosity function by method which suggests a luminosity function described by a broken power law. We then used maximum Likelihood function for the redshift distribution of sources, considering a broken power law luminosity function,
Here is the de-evolved luminosity function and is the normalization of the luminosity function. The break luminosity () and the lower and upper end index ( and respectively) are constrained by maximizing the likelihood function.
For pure density evolution, we consider a similar form of luminosity function and constrained the parameters by maximizing the Likelihood function for the redshift distribution of sources.
The normalization constant of luminosity function has been determined by equating the total number of FSRQs in our sample to that expected from the model luminosity function. The source distribution () has been over plotted with the model distribution for PLE and PDE models in Fig 3. Exponential and power law evolution models are considered for both PLE and PDE models.
|(in erg s)|
Iv Misaligned blazar contribution
According to the AGN Unification scenario, FR II galaxies are the parent population of FSRQs whereas, FR I galaxies are the parent population of BL Lacs. The AGN jet emission is expected to fall very rapidly with increasing jet to line-of-sight angle. Nevertheless, considering the large population of these misaligned blazars compared to nearly-aligned ones, these sources could contribute significantly to EGRB. Fermi has already detected 11 misaligned blazars.
The observed jet luminosity at a jet to line-of-sight angle () can be written as
From the knowledge of one can in principle, estimate the contribution from off-axis AGNs to the EGRB.
iv.1 Construction of
Our aim is to model the SED of all Fermi-detected off-axis sources using standard leptonic models with structured jets. From the knowledge of these SED modeling and VLBI observations one can fix the emission processes in the jet and other jet parameters and can construct . Next step is to constrain by comparing -ray detected misaligned blazars with equivalent on-axis objects (by matching sources with similar jet power, ) and then validate our predictions by modeling few misaligned blazars that are not detected by Fermi.
From the observed -ray luminosities of off-axis sources
and their jet inclination angle one can also construct .
The difference in the observed -ray luminosities between two radio galaxies
can arise from two different reasons:
a due to their different jet to line-of-sight angle,
b due to their different intrinsic luminosities.
If one considers that the total radio luminosity is a tracer of unbeamed luminosity, then after an appropriate scaling, one can construct from scaled -ray luminosities and jet inclination angles of off-axis sources.
We have initiated this work. Our aim is to analyze Fermi data to date on radio galaxies and model it to derive jet parameters. We have completed the analysis and the SED modeling of 3C111 and 3C120 (Figure 4 shows the SED fitting of 3C 120.)
Both the sources are well fitted with simple SSC emission model. is also 5 for both the sources.
Using first Fermi catalog, we investigated the luminosity function and nature of evolution of FSRQs and BL Lacs separately in -rays. The nature of evolution of FSRQs and BL Lacs as found in this work, are in agreement with earlier works (e.g., Bhattacharya, Sreekumar & Mukherjee (2009a); Chiang & Mukherjee (1998)). We considered both PLE and PDE models for FSRQs. Both power law and exponential evolution functions have been considered. The luminosity function is considered to be of a broken power law form and the parameters are found using likelihood analysis. From this analysis it is not possible to strongly conclude on the true evolution model. Fig 3 suggests that PDE model explains observations marginally better than that of PLE model. In contrast to earlier works of determination of luminosity function from -ray observations (Bhattacharya, Sreekumar & Mukherjee, 2009a; Chiang et al., 1995; Chiang & Mukherjee, 1998), our derived luminosity function is well constrained. In a recent work, Ajello et al. Ajello et al. (2011) showed that the redshift of the peak in the number density of FSRQs is luminosity dependent. Hence, they considered luminosity dependent density evolution for FSRQs (similar to that of radio-quiet AGN). They considered a much lower limiting flux ( ph cm s) and hence, a much larger sample ( FSRQs) than us.
For misaligned blazars, we plan for SED fitting of all detected off-axis sources and then construct from model. This work is under progress and will be reported elsewhere.
Acknowledgements.We are thankful to Dipankar Bhattacharya, IUCAA, for his valuable comments and suggestions. This work is partially supported by the NASA grant NNX09AT71G.
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