Extragalactic Background Light Density Research Articles

Constraints on extragalactic background light using TeV observations of BL Lacertae objects

ABSTRACTThe extragalactic background light (EBL) in the infrared to ultraviolet bands partly absorbs very high energy (VHE; E ≥ 100 GeV) γ-ray photons travelling over cosmological distances via pair production. In this paper, to get stronger constraints on EBL, we use the deliberate selection of the EBL model and data of five BL Lacs with better statistics and the harder spectra to limit the EBL density and the radiation mechanism of BL Lacs. We constrain the upper limit of the EBL density by fitting the spectral energy distributions of TeV BL Lacs and find that our results are compatible with the published measurement, reaching 50 $\rm {nW \,m^{-2} \,sr^{-1}}$. We also obtain that the EBL is not necessarily transparent to VHE photons. We fix the intrinsic spectral index Γi of TeV BL Lacs as 1.0 and 1.5 under observation evidence and model assumption. Comparing the EBL density given by galaxy count and Spitzer observations, we then obtain that 1ES 1101−232 has Γi ≤ 1.0 and 1ES 0229+200 should have Γi not harder than 1.0. We demonstrate that the common radiation assumption of BL Lacs, in which the Γi is softer than 1.5, should be revisited. Furthermore, we propose that the upper EBL density could be given by fitting the hardest energy spectra of TeV BL Lacs.

Optical spectroscopy of blazars for the Cherenkov Telescope Array – II

ABSTRACT Active galactic nuclei (AGNs) make up about 35 per cent of the more than 250 sources detected in very high-energy (VHE) gamma rays to date with the imaging atmospheric Cherenkov telescopes. Apart from four nearby radio galaxies and two AGNs of unknown type, all known VHE AGNs are blazars. Knowledge of the cosmological redshift of gamma-ray blazars is key to enabling the study of their intrinsic emission properties, as the interaction between gamma rays and the extragalactic background light (EBL) results in a spectral softening. Therefore, the redshift determination exercise is crucial to indirectly placing tight constraints on the EBL density, and to studying blazar population evolution across cosmic time. Due to the powerful relativistic jets in blazars, most of their host galaxies’ spectral features are outshined, and dedicated high signal-to-noise (S/N) spectroscopic observations are required. Deep medium- to high-resolution spectroscopy of 33 gamma-ray blazar optical counterparts was performed with the European Southern Observatory, New Technology Telescope, Keck II telescope, Shane 3-metre telescope, and the Southern African Large Telescope. From the sample, spectra from 25 objects display spectral features or are featureless and have high S/N. The other eight objects have low-quality featureless spectra. We systematically searched for absorption and emission features and estimated, when possible, the fractional host galaxy flux in the measured total flux. Our measurements yielded 14 firm spectroscopic redshifts, ranging from 0.0838 to 0.8125, one tentative redshift, and two lower limits: one at $z > 0.382$ and the other at z > 0.629.

Probing the Extragalactic Mid-infrared Background with HAWC

The extragalactic background light (EBL) contains all the radiation emitted by nuclear and accretion processes in stars and compact objects since the epoch of recombination. Measuring the EBL density directly is challenging, especially in the near-to-far-infrared wave band, mainly due to the zodiacal light foreground. Instead, gamma-ray astronomy offers the possibility to indirectly set limits on the EBL by studying the effects of gamma-ray absorption in the very high energy (VHE: >100 GeV) spectra of distant blazars. The High Altitude Water Cherenkov Gamma Ray Observatory (HAWC) is one of the few instruments sensitive to gamma rays with energies above 10 TeV. This offers the opportunity to probe the EBL in the near/mid-IR region: λ = 1–100 μm. In this study, we fit physically motivated emission models to Fermi-LAT gigaelectronvolt data to extrapolate the intrinsic teraelectronvolt spectra of blazars. We then simulate a large number of absorbed spectra for different randomly generated EBL model shapes and calculate Bayesian credible bands in the EBL intensity space by comparing and testing the agreement between the absorbed spectra and HAWC extragalactic observations of two blazars. The resulting bands are in agreement with current EBL lower and upper limits, showing a downward trend toward higher wavelength values λ > 10 μm also observed in previous measurements.

Redshift determination of blazars for the Cherenkov Telescope Array

AbstractBlazars are the most numerous type of observed high-energy gamma-ray emitters. However, their emission mechanisms and population properties are still not well-understood. Crucial to this understanding are their cosmological redshifts, which are often not easy to obtain. This presents a great challenge to the next-generation ground-based observatory for very-high-energy gamma rays, the Cherenkov Telescope Array (CTA), which aims to detect a large number of distant blazars to study their intrinsic emission properties and to place tight constraints on the extragalactic background light density, amongst others. The successful investigation of these subjects needs a precise redshift determination. Motivated by these challenges, the CTA redshift task force initiated more than 3 years ago a spectroscopic observing program using some of the largest optical and infrared telescopes to measure the redshifts of a large fraction of blazars that are likely to be detected with CTA. In this proceedings, we give an overview of the CTA redshift task force, discuss some of the difficulties associated with measuring the redshifts of blazars and present our sample selection and observing strategies. We end the proceedings with reporting selected results from the program, the on-going collaborative efforts and our plans for the future.

Studies of extragalactic background light with TeV BL Lacertae objects

ABSTRACT Very high energy (VHE; E ≥ 100 GeV) gamma-rays from cosmological distances are attenuated by the extragalactic background light (EBL) in the infrared to ultraviolet bands. By contrasting measured versus intrinsic emission,we can derive the EBL photon density. However, we do not know the intrinsic spectra and the EBL separately, only their combined effect. Here we first present a flexible model-dependent optical depth method to study the EBL by fitting the emission spectra of TeV BL Lacertae objects (BL Lacs) via a one-zone leptonic synchrotron self-Compton model (SSC). We have little information about electron energy distributions (EEDs) in the jet, which is critically important to build spectral energy distributions (SEDs) in the SSC scenario. Based on current particle acceleration models, we use two types of EEDs to fit the observed spectra: a power-law log-parabola (PLLP) EED and a broken power-law (BPL) EED. We find that the upper limit of the EBL density is about 30 n W m−2 sr−1, which is similar to the published measurement. Furthermore, we propose an unprecedented method to test the radiation mechanisms involved in TeV objects, by simply comparing the reduced EBL density with the limit obtained by galaxy counts. We demonstrate that for some BL Lacs, at least, the one-zone SSC model should be reconsidered.

Extragalactic background light: Constraints from the TeV gamma-ray observations

The cosmological processes including the history of stars and galaxies formation will be observed in the energy spectrum of Extragalactic Background Light (EBL) radiation. Observations of the very high energy γ-ray sources at redshifts from z = 0.018 up to z = 2.979 with SHALON telescopes gives an opportunity to constrain the EBL density, as the TeV γ-rays can be absorbed due to interaction of low-energy photons of EBL. The detection of TeV γ-ray sources at high redshifts is the evidence of less average spectral density of EBL and thus the less star formation rate at early evolution stage as well as the less expansion rate of the universe, than it is previously believed.

Measurement of the extragalactic background light using MAGIC and Fermi-LAT gamma-ray observations of blazars up to z = 1

We present a measurement of the extragalactic background light (EBL) based on a joint likelihood analysis of 32 gamma-ray spectra for 12 blazars in the redshift range z = 0.03 to 0.944, obtained by the MAGIC telescopes and Fermi-LAT. The EBL is the part of the diffuse extragalactic radiation spanning the ultraviolet, visible and infrared bands. Major contributors to the EBL are the light emitted by stars through the history of the universe, and the fraction of it which was absorbed by dust in galaxies and re-emitted at longer wavelengths. The EBL can be studied indirectly through its effect on very-high energy photons that are emitted by cosmic sources and absorbed via photon-photon interactions during their propagation across cosmological distances. We obtain estimates of the EBL density in good agreement with state-of-the-art models of the EBL production and evolution. The 1-sigma upper bounds, including systematic uncertainties, are between 13% and 23% above the nominal EBL density in the models. No anomaly in the expected transparency of the universe to gamma rays is observed in any range of optical depth.We also perform a wavelength-resolved EBL determination, which results in a hint of an excess of EBL in the 0.18 - 0.62 $\mu$m range relative to the studied models, yet compatible with them within systematics.

MAGIC observations of the February 2014 flare of 1ES 1011+496 and ensuing constraint of the EBL density

In February-March 2014, the MAGIC telescopes observed the high-frequency peaked BL Lac 1ES 1011+496 (z=0.212) in flaring state at very-high energy (VHE, E>100GeV). The flux reached a level more than 10 times higher than any previously recorded flaring state of the source. We present the description of the characteristics of the flare presenting the light curve and the spectral parameters of the night-wise spectra and the average spectrum of the whole period. From these data we aim at detecting the imprint of the Extragalactic Background Light (EBL) in the VHE spectrum of the source, in order to constrain its intensity in the optical band. For this we implement the method developed by the H.E.S.S. collaboration in which the intrinsic energy spectrum of the source is modeled with a simple function, and the EBL-induced optical depth is calculated using a template EBL model. The likelihood of the observed spectrum is then maximized, including a normalization factor for the EBL opacity among the free parameters. From the data collected differential energy spectra was produced for all nights of the observed period. Evaluating the changes in the fit parameters we conclude that the spectral shape for most of the nights were compatible, regardless of the flux level, which enabled us to produce an average spectrum from which the EBL imprint could be constrained. The likelihood ratio test shows that the model with an EBL density 1.07(-0.20,+0.24)_{stat+sys}, relative to the one in the tested EBL template (Dominguez et al.2011), is preferred at the 4.6 sigma level to the no-EBL hypothesis, with the assumption that the intrinsic source spectrum can be modeled as a log-parabola. This would translate into a constraint of the EBL density in the wavelength range [0.24 um,4.25 um], with a peak value at 1.4 um of F=12.27_{-2.29}^{+2.75} nW m^{-2} sr^{-1}, including systematics.

Understanding the TeV emission from a distant blazar PKS 1424+240 in a lepto-hadronic jet model

We investigate the formation of TeV spectrum of a distant blazar PKS 1424+240 residing at a redshift $z\geq0.6$ in two scenarios in the frame of a lepto-hadronic jet model taking both the uncertainties of the extragalactic background light (EBL) and its redshift into account. In the first scenario, TeV emission is attributed to the synchrotron emission of pair cascades resulting from $p\gamma$ interaction; in the second scenario, TeV emission is attributed to the proton-synchrotron emission, and an internal absorption due to interaction with the photons around the jet is included. The results show that in the first scenario the 68\% upper limit of its redshift within which this scenario can explain the VERITAS TeV spectrum in 2009 well is $\sim0.75$; in the second scenario, this upper limit of the redshift becomes $\sim1.03$. However, the second scenario can be excluded because it requires an unreasonable photon field around the jet with a luminosity of $\sim10^{43}\rm \ erg\ s^{-1}$. In conclusion, the jet model can explain its TeV spectrum with a low EBL density if $0.6<z<0.75$.

A correlation between hard gamma-ray sources and cosmic voids along the line of sight

We estimate the galaxy density along lines of sight to hard extragalactic gamma-ray sources by correlating source positions on the sky with a void catalog based on the Sloan Digital Sky Survey (SDSS). Extragalactic gamma-ray sources that are detected at very high energy (VHE; E>100 GeV) or have been highlighted as VHE-emitting candidates in the Fermi Large Area Telescope hard source catalog (together referred to as "VHE-like" sources) are distributed along underdense lines of sight at the 2.4 sigma level. There is also a less suggestive correlation for the Fermi hard source population (1.7 sigma). A correlation between 10-500 GeV flux and underdense fraction along the line of sight for VHE-like and Fermi hard sources is found at 2.4 sigma and 2.6 sigma, respectively. The preference for underdense sight lines is not displayed by gamma-ray emitting galaxies within the second Fermi catalog, containing sources detected above 100 MeV, or the SDSS DR7 quasar catalog. We investigate whether this marginal correlation might be a result of lower extragalactic background light (EBL) photon density within the underdense regions and find that, even in the most extreme case of a entirely underdense sight line, the EBL photon density is only 2% less than the nominal EBL density. Translating this into gamma-ray attenuation along the line of sight for a highly attenuated source with opacity tau(E,z) ~5, we estimate that the attentuation of gamma-rays decreases no more than 10%. This decrease, although non-neglible, is unable to account for the apparent hard source correlation with underdense lines of sight.

Probing the peak of the star formation rate density with the extragalactic background light

The extragalactic background light (EBL), i.e., the diffuse meta-galactic photon field in the ultraviolet to infrared, is dominated by the emission from stars in galaxies. It is, therefore, intimately connected with the integrated star formation rate density (SFRD). In this paper, the SFRD is constrained using recent limits on the EBL density derived from observations of distant sources of high and very-high energy gamma-rays. The stellar EBL contribution is modeled utilizing simple stellar population spectra including dust attenuation and emission. For modeling the SFRD up to z=4 a broken power law function in z+1 is assumed. A wide range of values for the different model parameters (SFRD(z), metallicity, dust absorption) is investigated and their impact on the resulting EBL is studied. The calculated EBL densities are compared with the specific EBL density limits and constraints on the SFRD are derived. For the fiducial model, adopting a Chabrier initial mass function (IMF) and a second power law index for the SFRD of beta=0.3, the SFRD is constrained to <~ 0.1 M_solar yr^-1 Mpc^-3 and < 0.2 M_solar yr^-1 Mpc^-3 for a redshift of z~1 and z~2, respectively. The limits for a redshift of z~1 are in tension with SFRD measurements derived from instantaneous star formation tracers. While the tension for the conservative fiducial model in this study is not yet overly strong, the tension increases when applying plausible changes to the model parameters, e.g., using a Salpeter instead of a Chabrier IMF or a adopting a sub-solar metallicity.

Potential of EBL and cosmology studies with the Cherenkov Telescope Array

Very high energy (VHE, E >100GeV) γ-rays are absorbed via interaction with low-energy photons from the extragalactic background light (EBL) if the involved photon energies are above the threshold for electron–positron pair creation. The VHE gamma-ray absorption, which is energy dependent and increases strongly with redshift, distorts the VHE spectra observed from distant objects. The observed energy spectra of the AGNs carry, therefore, an imprint of the EBL. The detection of VHE gamma-ray spectra of distant sources (z=0.11−0.54) by current generation Imaging Atmospheric Cherenkov Telescopes (IACTs) enabled to set strong upper limits on the EBL density, using certain basic assumptions about blazar physics. In this paper it is studied how the improved sensitivity of the Cherenkov Telescope Array (CTA) and its extended energy coverage will enlarge our knowledge about the EBL and its sources. CTA will deliver a large sample of AGN at different redshifts with detailed measured spectra. In addition, CTA will provide the exciting opportunity to use gamma ray bursts (GRBs) as probes for the EBL density at high redshifts.

Limits on the extragalactic background light in theFermiera

Very high energy (VHE, energy $E \gtrsim 100$\,GeV) \gamma-rays from cosmological sources are attenuated due to the interaction with photons of the extragalactic background light (EBL) in the ultraviolet to infrared wavelength band. The EBL, thus, leaves an imprint on the observed energy spectra of these objects. In the last four years, the number of extragalactic VHE sources discovered with imaging atmospheric Cherenkov telescopes (IACTs), such as MAGIC, H.E.S.S., and VERITAS, has doubled. Furthermore, the measurements of the \emph{Fermi} satellite brought new insights into the intrinsic spectra of the sources at GeV energies. In this paper, upper limits on the EBL intensity are derived by considering the most extensive VHE source sample ever used in this context. This is accomplished by constructing a large number of generic EBL shapes and combining spectral informations from \emph{Fermi} and IACTs together with minimal assumptions about the source physics at high and very high \gamma-ray energies. The evolution of the EBL with redshift is accounted for and the possibility of the formation of an electromagnetic cascade and the implications on the upper limits are explored. The EBL density at $z=0$ is constrained over a broad wavelength range between 0.4 and 100\,\mu m. At optical wavelengths, the EBL density is constrained below 24\,nW\,m$^{-2}$\,sr$^{-1}$ and below 5\,nW\,m$^{-2}$\,sr$^{-1}$ between 8\,\mu m and 31\,\mu m.

FERMI /LAT OBSERVATIONS OF 1ES 0229+200: IMPLICATIONS FOR EXTRAGALACTIC MAGNETIC FIELDS AND BACKGROUND LIGHT

We report the observation in the GeV band of the blazar 1ES 0229+200, which over recent years has become one the primary sources used to put constraints on the Extragalactic Background Light (EBL) and Extragalactic Magnetic Field (EGMF). We derive constraints on both the EBL and EGMF from the combined Fermi-HESS data set taking into account the direct and cascade components of the source spectrum. We show that the limit on the EBL depends on the EGMF strength and vice versa. In particular, an EBL density twice as high as that derived by Franceschini et al. (2008) is allowed if the EGMF is strong enough. On the other hand, an EGMF strength as low as 6x10^-18 G is allowed if the EBL density is at the level of the lower bound from the direct source counts. We present the combined EBL and EGMF limits on as an exclusion plot in two dimensional parameter space: EGMF strength vs. EBL density.

Constraining extragalactic background light from TeV blazars

Our goal is to research the upper limits on the extragalactic background light (EBL). The upper limits on the extragalactic background light (EBL), using the Fermi and very high energy (VHE) spectra recently observed in TeV blazars, are presented. We use an assumption that the VHE intrinsic photon index cannot be harder than the Fermi index measured by the Fermi-LAT. Totally, these upper limits on the EBL are compatible with ones given by most of EBL models. However, the models of high EBL density are denied by TeV blazars.

Potential of the next generation VHE instruments to probe the EBL (I): The low- and mid-VHE

The diffuse meta-galactic radiation field at ultraviolet to infrared wavelengths – commonly labeled extragalactic background light (EBL) – contains the integrated emission history of the universe. Difficult to access via direct observations, indirect constraints on its density can be derived through observations of very-high energy (VHE; E > 100 GeV) γ-rays from distant sources: the VHE photons are attenuated via pair-production with the low energy photons from the EBL, leaving a distinct imprint in the VHE spectra measured on earth. Discoveries made with current generation VHE observatories like H.E.S.S. and MAGIC enabled strong constraints on the density of the EBL, especially in the near-infrared. In this article the prospect of future VHE observatories to derive new constraints on the EBL density are discussed. To this end, results from current generation instruments will be extrapolated to the future experiment’s sensitivity and investigated for their power to enable new methods and improved constraints on the EBL density.

A METHOD FOR SETTING UPPER LIMITS TO THE EXTRAGALACTIC BACKGROUND LIGHT WITH FERMI -LAT AND TeV OBSERVATIONS OF BLAZARS

We propose a method for setting upper limits to the extragalactic background light (EBL). Our method uses simultaneous {\em Fermi}-LAT and ground-based TeV observations of blazars and is based on the assumption that the intrinsic spectral energy distribution (SED) of TeV blazars lies below the extrapolation of the {\em Fermi}-LAT SED from GeV to TeV energies. By extrapolating the {\em Fermi}-LAT spectrum, which for TeV blazars is practically unattenuated by photon-photon pair production with EBL photons, a firm upper limit on the intrinsic SED at TeV energies is provided. The ratio of the extrapolated spectrum to the observed TeV spectrum provides upper limits to the optical depth for the propagation of the TeV photons due to pair production on the EBL, which in turn sets firm upper limits to EBL models. We demonstrate our method using simultaneous observations from {\em Fermi}-LAT and ground-based TeV telescopes of the blazars \object{PKS 2155-304} and \object{1ES 1218+304}, and show that high EBL density models are disfavored. We also discuss how our method can be optimized and how {\em Fermi} and X-ray monitoring observations of TeV blazars can guide future TeV campaigns, leading to potentially much stronger constraints on EBL models.

Stochastic conversions of TeV photons into axion-like particles in extragalactic magnetic fields

Very-high energy photons emitted by distant cosmic sources are absorbed on the extragalactic background light (EBL) during their propagation. This effect can be characterized in terms of a photon transfer function at Earth. The presence of extragalactic magnetic fields could also induce conversions between very high-energy photons and hypothetical axion-like particles (ALPs). The turbulent structure of the extragalactic magnetic fields would produce a stochastic behaviour in these conversions, leading to a statistical distribution of the photon transfer functions for the different realizations of the random magnetic fields. To characterize this effect, we derive new equations to calculate the mean and the variance of this distribution. We find that, in presence of ALP conversions, the photon transfer functions on different lines of sight could have relevant deviations with respect to the mean value, producing both an enhancement or a suppression in the observable photon flux with respect to the expectations with only absorption. As a consequence, the most striking signature of the mixing with ALPs would be a reconstructed EBL density from TeV photon observations which appears to vary over different directions of the sky: consistent with standard expectations in some regions, but inconsistent in others.

THE EXTRAGALACTIC BACKGROUND LIGHT: LOWER VERSUS UPPER LIMITS

The discovery of distant sources of very high energy (VHE) γ-rays with hard energy spectra enabled to derive strong upper limits on the density of the extragalactic background light (EBL). These limits are close to the lower limits derived from deep source counts. A recent re-dertemination of the EBL contribution from resolved sources at 3.6 μm finds a higher EBL density, which is claimed to be in conflict with the assumptions utilized to derive the EBL upper limits from VHE spectra. Here, it is shown that is possible to recover the canonical Γ ~ 1.5 intrinsic spectra for such a higher EBL density.

First stars and the extragalactic background light: how recentγ-ray observations constrain the early universe

The formation of the first stars (Population III; PopIII) marks the end of the dark ages of the universe, a subject of lively scientific debate. Not (yet) accessible to direct observations, this early stage of the universe is mostly studied via theoretical calculations and numerical simulations. An indirect window is provided by integrated present day observables such as the metal abundance or the diffuse extragalactic photon fields. We aim to derive constraints on the properties of the PopIII and low metallicity Population II (LM PopII) stars utilizing limits on the density of the extragalactic background light (EBL), recently derived from very-high-energy (E > 100 GeV; VHE) observations. A model calculation for the evolving EBL density produced by PopIII/LM PopII stars is presented. The model utilizes stellar population spectra (SPS) for zero and low metallicity stars and accounts for the changing emission of an aging stellar population. Emission from the dense HII regions surrounding the stars (nebula) is included. The resulting EBL density for different scenarios (metallicity, star formation rate, initial mass function) is compared to the limit on the EBL density. The potential for detecting a cut-off in HE/VHE spectra is discussed. Assuming a maximum contribution from PopIII/LM PopII stars to the EBL density of 5 nW m^-2 s-1 at 2 micrometer a limit on the star formation rate (SFR) of the first stars of 0.3 to 3 M_solar Mpc^-3 yr^-1 in the redshift range 7 - 14 is derived. The limit depends on the assumed shape of the SFR and metallicity.