Compton X-ray and Gamma-ray Emission from Extended Radio Galaxies
Compton X-ray and γ-ray Emission from Extended Radio
Galaxies
C. C. Cheung1
Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA
Abstract. The extended lobes of radio galaxies are examined as sources of X-ray and γ-ray emission via inverse Compton
scattering of 3K background photons. The Compton spectra of two exemplary examples, Fornax A and Centaurus A, are
estimated using available radio measurements in the ∼10’s MHz – 10’s GHz range. For average lobe magnetic fields of
∼ 0.3–1 µG, the lobe spectra are predicted to extend into the soft γ-rays making them likely detectable with the GLAST
LAT. If detected, their large angular extents (∼1◦ and 8◦) will make it possible to “image” the radio lobes in γ-rays. Similarly,
this process operates in more distant radio galaxies and the possibility that such systems will be detected as unresolved γ-ray
sources with GLAST is briefly considered.
Keywords: gamma-ray sources (astronomical); radiofrequency spectra; imaging; radiogalaxies
PACS: 98.54.Gr,98.58.Fd
INVERSE COMPTON "IMAGES" OF LARGE RADIO GALAXIES
Inverse Compton (IC) scattering of the CMB is a mandatory process in synchrotron emitting sources. This emission
becomes most prominent in regions of weaker B-field like the extended lobes of radio galaxies. Many such IC/CMB
lobe X-ray sources are now known (e.g., Croston et al. 2005; Kataoka & Stawarz 2005) and we explore the possibility
of the IC spectra extending into the γ-ray band. This is independent of possible γ-ray emission from the unresolved
nuclei of radio galaxies, i.e., from the misaligned blazar (Sreekumar et al. 1999; Bai & Lee 2001; Foshini et al. 2005).
The case of the nearby (D=18.6 Mpc) double-lobed radio galaxy, Fornax A was discussed in Cheung (2007). Radio
flux density measurements down to ∼30 MHz (Isobe et al. 2006) were used to estimate the IC/CMB spectra of the
lobes. Normalizing the IC spectra to the X-ray detections of the lobes (which indicate B∼1.5µG on average; Feigelson
et al. 1995, Isobe et al. 2006), the presence of high frequency radio emission observed in the >∼ 10–90 GHz range with
WMAP (with Fν ∝ ν−1.5) imply a detectable soft γ-ray signal. As this emission is not expected to be time variable,
the LAT can simply integrate on this position during its normal scanning mode to test this prediction.
Here, we similarly consider the case of Centaurus A which is only 3.5 Mpc away. It is long known to have structure
extended over ∼8◦ in declination (Cooper et al. 1965, and references therein). We use the extensive compilation by
Alvarez et al. (2000) of the various components of the radio source; Figure 1 shows a low resolution 408 MHz image
from Haslam et al. (1982). The outer (degree-scale) giant lobes (GLs) visible in Figure 1 account for >∼ 2/3 of the
total 408 MHz emission at ∼1000 Jy each; the arcmin-scale inner lobes (ILs) are only 3–4 times fainter than each GL.
The northern GL was searched for such IC emission with ASCA data but the extended X-rays could not be uniquely
attributed to such a process (Isobe et al. 2001).
Repeating the analysis as for Fornax A, it appears that the extended components of Cen A will also emit γ-rays at
a level detectable by GLAST. The various data from ∼10 MHz to 43 GHz are consistent with a single spectral index
α=0.7. Since the luminosities of both the ILs and GLs are similar (within ∼20%), only the SEDs of the southern ones
are plotted in Figure 1. Utilizing these radio measurements, the expected IC/CMB spectra for example B-field strengths
are drawn. The integrated Compton Gamma-Ray Observatory (CGRO) COMPTEL detections of Cen A (Steinle et al.
1998) at ∼1021 Hz already limit B >∼ 1µG for both the northern and southern GLs (since they have similar radio
spectra); a similar extrapolation for the ILs give B >∼ 0.3µG.
Thermal emission will be a complicating factor at energies below ∼10 keV, so hard X-ray and soft γ-ray measure-
ments are better suited for detecting the suspected IC/CMB emission. Additionally, since Fornax A and Cen A are
1 Jansky Postdoctoral Fellow. The National Radio Astronomy Observatory is operated by Associated Universities, Inc. under a cooperative
agreement with the U.S. National Science Foundation.
http://arxiv.org/abs/0704.0835v1
208.000 204.000 200.000 196.000
-38.000
-40.000
-42.000
-44.000
-46.000
-48.000
Right Ascension (J2000)
GL (south)
GL (north)
Centaurus A                   408 MHz
0.85 deg
FIGURE 1. [Left] Radio image of Cen A at 0.85◦ resolution which is comparable to the angular resolution of GLAST/LAT.
[Center] SEDs of the multiple components of the Cen A radio source with lines indicating Fν ∝ ν−0.7 spectra. The data points at
> 1018 Hz are the integrated detections with CGRO with lines indicating the expected IC/CMB spectra of the southern giant lobe
for different average B-fields. [Right] IC/CMB X-ray and γ-ray flux predictions for 17 of the highest-z radio galaxies discussed in
the text. Typical Chandra “snapshots" are 5–10 ksec exposures so these sources are all expected to be easily detectable in the X-rays
for the indicated field strengths. GLAST detections require electrons with γ >∼ 105 in a 1µG or smaller field which are optimistic.
quite extended in the sky (∼1◦ and 8◦), if they are detected with GLAST, the contributions from the two lobes will
be separable with the LAT making IC/CMB γ-ray “images” of these radio galaxies possible. These γ-ray images will
appear most similar to radio maps at frequencies, ν >∼ 10 GHz; such radio maps of Cen A’s extended components have
already been obtained by WMAP (Page et al. 2007, Fig. 2 therein) and are available for this comparison.
THE HIGHEST-REDSHIFT RADIO GALAXIES
Using the above examples as a guide, we can gauge the feasibility of detecting even more distant radio galaxies at
the higher-energies. Utilizing the recent large compilation of bright z>2.5 radio sources by Carson et al. (2007), we
consider the highest-redshift (z > 3.5) radio galaxies for illustration. The observed monochromatic Compton (X-ray,
γ-ray) to synchrotron (radio) flux ratio for IC/CMB emission has a strong redshift dependence: for α=1, it is simply
fc/ fs ≃ ucmb/uB ≃ 10(1+ z)
4δ 2/B2µG ( fν ≡ νFν , and δ is the Doppler factor which is set to 1). We use the NVSS
(Condon et al. 1998) 1.4 GHz fluxes for fs. Most of the considered sources (13/17) are detected at 74 MHz in the
VLSS database (Cohen et al. 2005) giving α74MHz−1.4GHz ∼ 0.9− 1.2, so the approximate relation is applicable.
As in the nearby sources, these distant radio galaxies are expected to be IC/CMB X-ray sources unless B ≫10µG
(Fig. 1). Chandra observations should easily detect this emission to constrain the lobe B-fields, and thus the lobe
energetics. In one of the highest-redshift (z = 3.8) radio galaxies observed so far with Chandra (Scharf et al. 2003),
it was necessary to remove the contribution from a bright nucleus (spatially) and extended IC emission from other
sources of seed photons (by spectral fitting). Such X-ray observations can guide our determination of the expected
level of (soft) γ-ray emission from the IC/CMB process; at the moment, the estimates (Fig. 1) are rather crude.
REFERENCES
1. H. Alvarez, J. Aparici, J. May, & P. Reich, Astron. Astrophys. 355, pp. 863–872 (2000).
2. J. M. Bai & M. G. Lee, Astrophys. Journal Lett. 549, pp. L173–L177 (2001).
3. J. E. Carson, T. M. Arias, & C. C. Cheung, in preparation (2007).
4. C. C. Cheung, in The Central Engine of Active Galactic Nuclei, edited by L. C. Ho & J.-M. Wang, ASP Conf. Series, in press,
arXiv:astro-ph/0612372 (2007).
5. A. S. Cohen, et al., in From Clark Lake to the Long Wavelength Array: Bill Erickson’s Radio Science, edited by N. Kassim et
al., ASP Conf. Series 345, 299–303 (2005).
6. B. F. C. Cooper, R. M. Price, & D. J. Cole, Australian Journal of Physics 18, pp. 589–625 (1965).
http://arxiv.org/abs/astro-ph/0612372
7. J. J. Condon, W. D. Cotton, E. W. Greisen, Q. F. Yin, R. A. Perley, G. B. Taylor, & J. J. Broderick, Astron. Journal 115, pp.
1693–1716 (1998).
8. J. H. Croston, et al., Astrophys. Journal 626, pp. 733–747 (2005).
9. E. D. Feigelson, S. A. Laurent-Muehleisen, R. I. Kollgaard, & E. B. Fomalont, Astrophys. Journal Lett. 449, pp. L149–L152
(1995).
10. L. Foschini et al., Astron. Astrophys. 433, pp. 515–518 (2005).
11. C. G. T. Haslam, C. J. Salter, H. Stoffel, & W. E. Wilson, Astron. Astrophys. Suppl. 47, pp. 1–142 (1982).
12. N. Isobe, K. Makishima, M. Tashiro, & H. Kaneda, in Particles and Fields in Radio Galaxies, edited by R. A. Laing &
K. M. Blundell, ASP Conf. Series 250, pp. 394–399 (2001).
13. N. Isobe, K. Makishima, M. Tashiro, K. Itoh, N. Iyomoto, I. Takahashi, & H. Kaneda, Astrophys. Journal 645, pp. 256–263
(2006).
14. J. Kataoka, & Ł. Stawarz, Astrophys. Journal 622, pp. 797–810 (2005).
15. L. Page, et al., Astrophys. Journal, in press (2007).
16. C. Scharf, et al. Astrophys. Journal 596, pp. 105–113 (2003).
17. P. Sreekumar, D. L. Bertsch, R. C. Hartman, P. L. Nolan, & D. J. Thompson, Astroparticle Physics 11, pp. 221–223 (1999).
18. H. Steinle, et al. Astron. Astrophys. 330, pp. 97–107 (1998).
	Inverse Compton "Images" of Large Radio Galaxies
	The Highest-Redshift Radio Galaxies
