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Discovery of very high energy gamma-ray emission from Centaurus A with H.E.S.S
HESS Collaboration, F. Aharonian
TL;DR
Cen A had been proposed as a possible source of ultra-high-energy cosmic rays, while associations between cosmic rays and nearby active galactic nuclei remained under discussion. H.E.S.S. observations detected a 5.0σ VHE excess at Cen A, with a power-law spectrum and faint flux; further localization and variability studies are needed to distinguish emission models.
Problem
Cen A had been proposed as a UHECR source, while possible correlations between UHECR arrival directions and nearby active galactic nuclei remained under discussion.
Source: §1 · p. 2Method
H.E.S.S. used its four-telescope Cherenkov array to observe Cen A between April 2004 and July 2008, recording 115.0 hours of good-quality data.
Source: §2 · p. 2Results
5.0σ excess at Cen A; the power-law photon index is 2.73 ± 0.45_stat ± 0.2_sys, and the integral flux above 250 GeV is 0.8% of the Crab Nebula flux.
Source: §2 · p. 3Takeaways & Limitations
If the emission comes from a jet, Cen A's proximity makes it a laboratory for investigating blazar emission processes and jet physics.
Source: §3 · p. 4Further information on the excess position, spectral shape, and variability is required to distinguish between emission models.
Source: §3 · p. 4
Abstract
from arXiv · showhide
We report the discovery of faint very high energy (VHE, E>100 GeV) gamma-ray emission from the radio galaxy Centaurus A in observations performed with the H.E.S.S. experiment, an imaging atmospheric Cherenkov telescope array consisting of four telescopes located in Namibia. Centaurus A has been observed for more than 120 h. A signal with a statistical significance of 5.0 sigma is detected from the region including the radio core and the inner kpc jets. The integral flux above an energy threshold of ~250 GeV is measured to be ~0.8 % of the flux of the Crab Nebula (apparent luminosity: L(>250 GeV)~2.6x10^39 erg s^-1, adopting a distance of 3.8 Mpc. The spectrum can be described by a power law with a photon index of 2.7 +/- 0.5_stat +/- 0.2_sys. No significant flux variability is detected in the data set. However, the low flux only allows detection of variability on the timescale of days to flux increments above a factor of ~15-20 (3 sigma and 4 sigma, respectively). The discovery of VHE gamma-ray emission from Centaurus A reveals particle acceleration in the source to >TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters.
1. INTRODUCTION
Centaurus A’s radio, X-ray, and gamma-ray properties make it a compelling target for very-high-energy observations. This Letter reports H.E.S.S. discovery of VHE emission from Cen A and discusses its properties and implications.
1. INTRODUCTION
- Cen A’s radio structures extend from the core and inner jets to giant outer lobes, while its inner kpc jet also shows complex X-ray emission.The host’s kpc-scale jet and active nucleus are established sources of strong non-thermal emission.Source: §1 · p. 2
- Earlier VHE observations of Cen A followed a tentative 4.5σ detection during a 1970s X-ray outburst, but later experiments reported upper limits.Source: §1 · p. 2
- Cen A has been proposed as a possible source of ultra-high-energy cosmic rays, and some observed events may be associated with it.Source: §1 · p. 2
- Before this report, M 87 was the only firmly established extragalactic VHE source with weakly beamed emission.M 87 showed VHE outbursts on timescales of days [Aharonian et al. 2006a; Albert et al. 2008].Source: §1 · p. 2
- The Letter reports the discovery of VHE emission from Cen A with H.E.S.S. and discusses the detected emission’s properties and implications.Source: §1 · p. 2
2. H.E.S.S. OBSERVATIONS AND RESULTS
H.E.S.S. detected very-high-energy emission from Centaurus A at 5.0σ, with a spectrum consistent with a power law and no significant variability in the observed data. The data constrain the source’s position and extension, while the low flux limits sensitivity to short-term flares.
2. H.E.S.S. OBSERVATIONS AND RESULTS
- Observations from 2004–2008 provided 115.0 hours of live time; standard Hillas-type analysis used an energy threshold of about 250 GeV, and independent analysis and calibration chains agreed.The observations used reflected-background estimation with pointing offsets of 0.5°–0.7° from the radio core.Source: §2 · pp. 2–3
- 5.0σ significance was measured for an excess of 330 events at the Centaurus A radio core position.The point-source analysis reported N_ON = 4199, N_OFF = 42513, and α = 0.091.Source: §2 · p. 3
- Figure 4 places the measured VHE spectrum alongside prior VHE limits and tentative detections, EGRET GeV measurements, and NASA Extragalactic Database data.Source: Figure 4 · p. 3
- The best-fit position is compatible with the radio core and inner kiloparsec jet, while the extension is constrained to less than 0.2° at 95% confidence.Figure 2 overlays the VHE position and extension limit on an optical image with radio contours.Source: §2 · p. 3, Figure 2 · p. 3
- The spectrum is well described by a power law with photon index Γ = 2.73 ± 0.45_stat ± 0.2_sys, and the flux above 250 GeV is 0.8% of the Crab Nebula flux.The measured integral flux is (1.56 ± 0.67_stat) × 10^-12 cm^-2 s^-1; the apparent luminosity is 2.6 × 10^39 erg s^-1 at 3.8 Mpc.Source: §2 · p. 3, Figure 3 · p. 3
- No significant variability was detected, but a one-night flare would need to rise by about 15-fold for 3σ or 20-fold for 4σ detection.The analysis found no significant variability on 28-minute, nightly, or monthly timescales.Source: §2 · p. 3
3. DISCUSSION
The discussion compares candidate origins for Cen A’s VHE emission using its spectrum and position, but the available data do not yet distinguish among models. It identifies improved observations as necessary to resolve the emitting region and model.
Spectral constraints
- The H.E.S.S. spectrum lies below previous VHE upper limits, while the EGRET GeV spectrum roughly matches it when extrapolated, favoring the softer end of EGRET’s index range.The paper says further positional, spectral, and variability information is needed to distinguish models.Source: §3 · pp. 3–4
Core and jet models
- Magnetospheric acceleration could explain the emission, while inner-jet models include a spine-sheath scenario and a multiblob SSC prediction compatible with the H.E.S.S. data.The magnetospheric model is considered potentially applicable because Cen A’s black-hole mass and VHE luminosity are about ten times lower than M 87’s.Source: §3 · p. 4
Extended-emission models
- A roughly 4-arcmin isotropic pair halo would appear point-like to H.E.S.S.; its predicted steady flux and TeV photon index agree with the measurements.The proposed halo could be resolved by CTA.Source: §3 · p. 4
Hadronic models
- In a hadronic scenario, ultrarelativistic protons released in the kpc-scale jet could be confined in the host galaxy and produce VHE gamma rays through interactions with dense material.The proposed targets include Cen A’s dust lane and the merger remnant’s thermal gas.Source: §3 · p. 4
Other proposed origins
- Some UHECR-injection model solutions and a model of inverse-Compton emission from the southwest inner-lobe shock predict VHE spectra compatible with H.E.S.S.The shock model considers scattering of starlight and the cosmic microwave background by high-energy particles.Source: §3 · p. 4
Emission location
- The VHE excess matches the core, pc/kpc inner jets, and inner radio lobes, not the proposed outer giant lobes.Only upper limits were derived for the southern outer lobe.Source: §3 · p. 4
Alternative sources
- An ensemble of supernova remnants or pulsar wind nebulae would require an unrealistically large number of sources at typical assumed luminosities.The assumed typical luminosity is about 10^34–10^35 erg s^-1 above 250 GeV per source.Source: §3 · p. 4
Outlook
- More sensitive observations and improved astrometry and angular resolution could localize the emission, while current sensitivity is likely insufficient to establish whether VHE emission is general among AGN.The paper identifies CTA as a possible means to resolve emitting sites and detect a larger source sample.Source: §3 · pp. 4–5