![]() | Annu. Rev. Astron. Astrophys. 2001. 39:
249-307 Copyright © 2001 by Annual Reviews. All rights reserved |
3.7. CIB Limits from TeV
-Ray
Observations
Indirect evidence for the brightness of the CIB comes from observations
of TeV
-rays from
blazars. A beam of high-energy photons can be strongly attenuated by
e+e- pair production through collisions with
low-energy photons (
Nikishov 1962,
Jelley 1966,
Gould & Schréder
1966).
The
+
b
e+
+ e- interaction cross section of a
-ray photon
of energy
E
with isotropically distributed background photons
b
of energy
b is
sharply peaked at ~ 1.5 × 10-25 cm2
(Nikishov 1962).
This peak occurs at energies for which the product
E
b
4
(me c2)2
1 MeV2, or
b(µm)
1.24
E
(TeV) (Guy et al. 2000).
Hence, infrared photons are particularly effective at attenuating
-rays of
energies ~ 1-100 TeV.
If the production of second-generation
-ray
photons appearing from the direction of the source can be neglected,
then the observed
-ray flux
at energy E
from the source,
Jobs(E
), is
simply related to the intrinsic source flux,
J0(E
), by
![]() |
(1) |
where is the
optical depth for
-
interaction. For a given
E
, the optical depth is proportional to the number
density of background photons. Therefore, very high energy
-ray
sources can, in principle, be used to probe the intensity of the CIB if
they happen to be relatively nearby
(Stecker et al. 1992).
Determination of the CIB from
-ray
observations requires: (a) knowledge of the intrinsic spectrum of
the
-ray
source, (b) knowledge of the spectral shape of the CIB, and
(c) the assumption that the attenuation takes place in the
intergalactic medium by the CIB instead of in the source itself or its
immediate vicinity
(Böttcher & Dermer
1995,
Protheroe & Biermann
1997,
Bednarek & Protheroe
1999).
The probing of the CIB with very high energy
-ray
photons has recently become practical with the development of imaging
atmospheric Cerenkov telescopes, such as the Whipple Observatory, the
High Energy Gamma-ray Array (HEGRA), and the Cerenkov Array at Themis
(CAT). The first extragalactic
-ray source
detected at TeV energies was the blazar Markarian (Mrk) 421, located at
redshift z
0.031
(Punch et al. 1992).
Two more sources, Mrk 501 and 1ES2344 + 514, at respective redshifts
z
0.034 and
z
0.044, have been
detected since, but the latter source was only detected once, with
marginal significance.
Catanese & Weekes
(1999)
have reviewed the
-ray
observatories and observations (see also
McConnell & Ryan
2000).
Since all of the detected extragalactic TeV
-ray
sources are located at very low redshift, one can safely neglect
evolutionary effects in the CIB, which may be important in calculating
the opacity to more distant sources
(Section 4.4).
In the first applications of this technique, Stecker & De Jager (1993), De Jager et al. (1994), Dwek & Slavin (1994) combined GeV observations of Mrk 421 obtained by the Energetic Gamma-ray Experiment Telescope on board the Compton Gamma-ray Observatory (Lin et al. 1992) with TeV observations obtained by the Whipple Observatory (Punch et al. 1992), to infer the intrinsic energy distribution of the source and to search for evidence of a high-energy cutoff in its spectrum due to attenuation. Background limits derived from their analyses are shown in Figure 4. The problems of ascertaining reliable evidence for intergalactic attenuation were illustrated by Biller et al. (1995), who showed two equally acceptable fits to the spectrum of Mrk 421, the first requiring no attenuation, the second requiring significant CIB attenuation. Taking into account the uncertainties in the extrapolation of the source spectrum from GeV to TeV energies and in the spectral shape of the CIB, Biller et al. (1995) derived a conservative upper limit of ~ 250 h nW m-2 sr-1 for the CIB at 10 µm.
![]() |
Figure 4. Cosmic infrared background limits
implied by TeV
|
Stanev & Franceschini
(1998)
were the first to apply the attenuation analysis to Mrk 501, using
high-quality TeV
-ray data
obtained by the HEGRA experiment between March and October 1997, when
the source was unusually bright. The observed spectrum was fit by
varying the spectral index of the
-ray source
spectrum and the normalization of the CIB spectrum, which was taken to
be the model spectrum of
Franceschini et
al. (1991).
The best fit to the observations was consistent with no absorption. A
less model-dependent result was obtained by adopting the flattest
allowable
-ray source
spectrum and a flat
I
spectrum
within the CIB wavelength bin that contributes most to the attenuation
at a given
-ray
energy. This approach yielded CIB upper limits, the most restrictive
being 7 h nW m-2 sr-1 in the 4 to 25
µm wavelength range.
A similar approach was adopted by
Funk et al. (1998),
who fitted the Mrk 501 spectrum by an unattenuated power law up to 10
TeV, which suggests that
(10 TeV)
< 1. By assuming a spectral shape for the CIB based on the models of
MacMinn & Primack
(1996),
they derived an upper limit of 4 h nW m-2
sr-1 at ~ 40 µm, consistent with previous
estimates.
Biller et al. (1998)
derived new CIB upper limits using minimal assumptions on the source or
the CIB spectra. Approximating the CIB by a series of flat
I
spectra in
discrete wavelength intervals, they calculated the
-ray
opacity at various
-ray
energies. Constraining the overall index of the differential photon
spectrum of the source to be between 2.3 and 2.8, they derived an upper
limit of 16 h nW m-2 sr-1 in the 4 to 15
µm interval.
Mannheim (1998)
derived an upper limit on the maximum allowed attenuation assuming a
power law TeV
-ray
spectrum for Mrk 501, which implied a CIB limit of (3-5) h nW
m-2 sr-1 at 25 µm.
Recent simultaneous multiwavelength observations of Mrk 501 and Mrk 421
have provided new constraints on the intrinsic source spectrum. The
observed double-humped nature of the spectra of Mrk 421 and 501, with peaks at X-ray (~ 10 keV) and
-ray (~ 1
TeV) energies, and the rapid, correlated variations of their X-ray and
TeV spectra
(Catanese & Weekes
1999),
are most readily explained in the framework of the homogeneous
synchrotron self-Compton (SSC) model. The SSC model and other competing
models are described by
Dermer et al. (1997),
Sikora (1997),
McConnell & Ryan
(2000).
In the SSC model, the same electrons that produce the synchrotron
radiation also upscatter the synchrotron photons in the jet to TeV
energies by the inverse Compton process. The model produces a
double-peaked spectral energy distribution with a synchrotron (S) peak
in the X-ray regime and an inverse Compton (IC) peak in the ~ 0.1-1 TeV
energy regime.
Guy et al. (2000)
pointed out that the position and intensity of the IC peak in Mrk 501 is affected by the
-ray
interactions with the optical and near-infrared photons of the
EBL. Adopting the spectral shape of the EBL predicted by
Primack et al. (1999)
(model LCDM; see Section 5.2.3), they
calculated the TeV opacity and
reconstructed the source spectrum for various scaling factors for the
EBL intensity. Making the general assumption that the flux of the
source,
F
, should
not rise between 4 and 17 TeV, and that its spectral slope at 600 GeV
should be similar to that of the X-ray spectrum at 1 keV, they found
that the scaling factor must be less than about 2.5, setting CIB upper
limits
(1
) of 58 h, ~ 6
h, and ~ 35 h nW m-2 sr-1 at 1, 20,
and 80 µm, respectively.
Other studies have used the spectral observations, such as the locations
of the S and IC peaks, their variability, and the hardening of the
synchrotron spectrum, to constrain the parameter space of the SSC
model. For example,
Konopelko et
al. (1999)
argue that the intrinsic source spectrum of Mrk 501 must be flat in
F
in order
to comply with the observational constraints and therefore see the
observed hardening of the spectrum with energy as evidence for
intergalactic absorption. Adopting the
Malkan & Stecker
(1998)
model for the EBL with the addition of a stellar emission component (the
details of which have not been published), they derived an essentially
flat spectrum from 300 GeV to 20 TeV. However, these results are at
variance with those derived by
Guy et al. (2000),
presumably because of differences in the stellar emission models that
produce the ~ 1 TeV IC peak in the source spectrum.
Sambruna et al. (2000)
succeeded in modeling the observed spectrum of Mrk 501 with the SSC
model. Such a fit would imply no evidence for any intergalactic
attenuation. However, the model overpredicts the 10 TeV flux by a factor
of 2.5, a discrepancy regarded by them as only marginal evidence for
intergalactic attenuation.
New upper limits on the CIB were derived by Dwek
(2001:
Appendix) and
Renault et al. (2001),
who explored the range of "allowable" CIB spectra with only the minimal
assumption that the intrinsic
-ray
spectrum of Mrk 501 be nonincreasing at energies above ~ 4 TeV. Dwek
reported preliminary upper limits of 7 h and 14 h nW
m-2 sr-1 in the 6 to 30 µm interval
and at 60 µm, respectively. Renault et al. found an upper
limit of 7 h nW m-2 sr-1 in the 5 to 15
µm interval.
Figure 4 summarizes representative upper limits
on the CIB determined from TeV
-ray
observations. Dashed and solid lines are limits determined from
observations of Mrk 421 and 501, respectively. Limits based on Mrk 501
are seen to be generally more restrictive. The lowest limits are in the
5 to 30 µm range since the observed ~ 1 to 15 TeV
-rays
interact most strongly with infrared photons in this wavelength
range. The shaded region in Figure 4 is provided
to allow comparison with other EBL measurements (see
Section 3.10).
The determinations of CIB intensities from
-ray
attenuation studies are somewhat model dependent and are further
uncertain because of the possibility that some attenuation may occur
within the
-ray
source. However, even with the current limited data and uncertainties,
the
-ray
observations are providing valuable constraints on the CIB in the
difficult 5 to 60 µm range.