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Register a new userHigh-frequency-peaked BL Lacertae objects as spectral candles to measure the Extragalactic Background Light in the Fermi and air Cherenkov telescopes era
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Nijil Mankuzhiyil
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The Extragalactic Background Light (EBL) is the integrated light from all the stars that have ever formed, and spans the IR-UV range. The interaction of very-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located at cosmological distances, with the intervening EBL results in electron-positron pair production that leads to energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity in the interpretation of measured VHE gamma-ray spectra: neither the intrinsic spectrum, nor the EBL, are separately known -- only their combination is. In this paper we propose a method to measure the EBL photon number density. It relies on using simultaneous observations of BL Lac objects in the optical, X-ray, high-energy (HE: E>100 MeV) gamma-ray (from the Fermi telescope), and VHE gamma-ray (from Cherenkov telescopes) bands. For each source, the method involves best-fitting the spectral energy distribution (SED) from optical through HE gamma-rays (the latter being largely unaffected by EBL attenuation as long as z<1) with a Synchrotron Self-Compton (SSC) model. We extrapolate such best-fitting models into the VHE regime, and assume they represent the BL Lacs intrinsic emission. Contrasting measured versus intrinsic emission leads to a determination of the photon-photon opacity to VHE photons. Using, for each given source, different states of emission will only improve the accuracy of the proposed method. We demonstrate this method using recent simultaneous multi-frequency observations of the high-frequency-peaked BL Lac object PKS 2155-304 and discuss how similar observations can more accurately probe the EBL.
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The Extragalactic Background Light (EBL) is the integrated light from all the stars that have ever formed, and spans the IR-UV range. The interaction of very-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located at cosmological distances, with the intervening EBL results in electron-positron pair production that leads to energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity in the interpretation of the measured VHE blazar spectra: neither the intrinsic spectra, nor the EBL, are separately known - only their combination is. In this paper we propose a method to measure the EBL photon number density. It relies on using simultaneous observations of blazars in the optical, X-ray, high-energy (HE: E>100 MeV) gamma-ray (from the Fermi telescope), and VHE gamma-ray (from Cherenkov telescopes) bands. For each source, the method involves best-fitting the spectral energy distribution (SED) from optical through HE gamma-rays (the latter being largely unaffected by EBL attenuation as long as z<1) with a Synchrotron Self-Compton (SSC) model. We extrapolate such best-fitting models into the VHE regime, and assume they represent the blazars intrinsic emission. Contrasting measured versus intrinsic emission leads to a determination of the gamma-gamma opacity to VHE photons - hence, upon assuming a specific cosmology, we derive the EBL photon number density. Using, for each given source, different states of emission will only improve the accuracy of the proposed method. We demonstrate this method using recent simultaneous multi-frequency observations of the blazar PKS2155-304 and discuss how similar observations can more accurately probe the EBL.
Here we present highlights from VERITAS observations of high-frequency-peaked BL Lac objects (HBLs). We discuss the key science motivations for observing these sources -- including performing multiwavelength campaigns critical to understanding the emission mechanisms at work in HBLs, constraining the intensity and spectra shape of the extragalactic background light, and placing limits on the strength of the intergalactic magnetic field.
The existence of radio weak BL Lac objects (RWBLs) has been an open question, still unsolved, since the discovery that quasars could be radio-quiet or radio-loud. Recently several groups identified RWBL candidates, mostly found while searching for low energy counterparts of the unidentified/unassociated gamma-ray sources listed in the Fermi catalogs. Confirming RWBLs is a challenging task since they could be confused with white dwarfs (WDs) or weak emission line quasars (WELQs) when there are not sufficient data to precisely draw their broad band spectral energy distribution and their classification is mainly based on a featureless optical spectra. Motivated by the recent discovery that Fermi BL Lacs appear to have very peculiar mid-IR emission, we show that it is possible to distinguish between WDs, WELQs and BL Lacs using the [3.4]-[4.6]-[12]$mu$m color-color plot built using the WISE magnitudes when the optical spectrum is available. On the basis of this analysis, we identify WISE J064459.38+603131 and WISE J141046.00+740511.2 as the first two genuine RWBLs, both potentially associated with Fermi sources. Finally, to strengthen our identification of these objects as true RWBLs, we present multifrequency observations for these two candidates to show that their spectral behavior is indeed consistent with those of the BL Lac population.
In the third catalog of active galactic nuclei detected by the $Fermi$ Large Area Telescope Clean (3LAC) sample, there are 402 blazars candidates of uncertain type (BCU). The proposed analysis will help to evaluate the potential optical classification flat spectrum radio quasars (FSRQs) versus BL Lacertae (BL Lacs) objects of BCUs, which can help to understand which is the most elusive class of blazar hidden in the Fermi sample. By studying the 3LAC sample, we found some critical values of $gamma$-ray photon spectral index ($Gamma_{rm ph}$), variability index (VI) and radio flux (${rm F_R}$) of the sources separate known FSRQs and BL Lac objects. We further utilize those values to defined an empirical high-confidence candidate zone that can be used to classify the BCUs. Within such a zone ($Gamma_{rm ph}<2.187$, log${rm F_R}<2.258$ and ${ rm logVI <1.702}$), we found that 120 BCUs can be classified BL Lac candidates with a higher degree of confidence (with a misjudged rate $<1%$). Our results suggest that an empirical high confidence diagnosis is possible to distinguish the BL Lacs from the Fermi observations based on only on the direct observational data of $Gamma_{rm ph}$, VI and ${rm F_R}$.
We introduce a new method to determine the redshift of unknown-redshift BL Lac Objects. The method relies on simultaneous multi-wavelength (MWL) observations of BL Lac objects in optical, X-ray, HE (E>100 MeV) gamma-rays and VHE (E>100 GeV)gamma-rays. It involves best-fitting spectral energy distribution (SED) from optical through HE gamma-rays with a Synchrotron-Self-Compton (SSC) model. We extrapolate such best fitting model into VHE regime, and assume that it represents the intrinsic emission of the object. We then compare the observed VHE flux which has been affected by the interaction with Extragalactic Background Light (EBL). Constraining the measured vs intrinsic emission leads to the determination of gamma-gamma opacity. Comparing the obtained opacity with the predicted opacity based on EBL model, we obtain the redshift of the photon source.
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