Synchrotron Proton Blazar Research Articles

FERMILARGE AREA TELESCOPE OBSERVATIONS OF MARKARIAN 421: THE MISSING PIECE OF ITS SPECTRAL ENERGY DISTRIBUTION

We report on the gamma-ray activity of the high-synchrotron-peaked BL Lacertae object Mrk 421 during the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT) gamma-ray spectrum above 0.3 GeV can be well-described by a power-law function with photon index Gamma=1.78 +/- 0.02 and average photon flux F(>0.3 GeV)=(7.23 +/- 0.16) x 10^{-8} ph cm^{-2} s^{-1}. Over this time period, the Fermi-LAT spectrum above 0.3 GeV was evaluated on 7-day-long time intervals, showing significant variations in the photon flux (up to a factor ~3 from the minimum to the maximum flux), but mild spectral variations. The variability amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in gamma-rays measured by Fermi-LAT, and these two energy ranges are not significantly correlated. We also present the first results from the 4.5-month-long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign (2009 January 19 to 2009 June 1). During this campaign, Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broad band SED was reproduced with a leptonic (one-zone Synchrotron Self-Compton) and a hadronic model (Synchrotron Proton Blazar). Both frameworks are able to describe the average SED reasonably well, implying comparable jet powers but very different characteristics for the blazar emission site.

M 87 as a misaligned synchrotron-proton blazar

The giant radio galaxy M 87 is usually classified as a Fanaroff-Riley class I source, suggesting that M 87 is a mis- aligned BL Lac object. Its unresolved nuclear region emits strong non-thermal emission from radio to X-rays which has been interpreted as synchrotron radiation. In an earlier paper we predicted M 87 as a source of detectable gamma ray emission in the context of the hadronic Synchrotron-Proton Blazar (SPB) model. The subsequent tentative detection of TeV energy photons by the HEGRA-telescope array would, if confirmed, make it the first radio galaxy to be detected at TeV-energies. We discuss the emission from the unresolved nuclear region of M 87 in the context of the SPB model, and give examples of possible model rep- resentations of its non-simultaneous spectral energy distribution. The low-energy component can be explained as synchrotron radiation by a primary relativistic electron population that is injected together with energetic protons into a highly magnetized emission region. We find that the γ-ray power output is dominated either by µ ± /π ± synchrotron or proton synchrotron radiation depending on whether the primary electron synchrotron component peaks at low or high energies, respectively. The predicted γ-ray luminosity peaks at ∼100 GeV at a level comparable to that of the low-energy hump, and this makes M 87 a promis- ing candidate source for the newly-commissioned high-sensitivity low-threshold Cherenkov telescopes H.E.S.S., VERITAS, MAGIC and CANGAROO III. Because of its proximity, the high-energy spectrum of M 87 is unaffected by absorption in the cosmic infrared (IR) background radiation field, and could therefore serve as a template spectrum for the corresponding class of blazar if corrected for mis-alignment effects. This could significantly push efforts to constrain the cosmic IR radiation field through observation of more distant TeV-blazars, and could have a strong impact on blazar emission models. If M 87 is a mis- aligned BL-Lac object and produces TeV-photons as recently detected by the HEGRA-array, in the context of the SPB model it must also be an efficient proton accelerator.

Modeling the Multiwavelength Spectra and Variability of BL Lacertae in 2000

BL Lacertae was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. In this paper, we are using leptonic and hadronic jet models to fit the observed broadband spectra and spectral variability patterns. We start out with global spectral models. Subsequently, we investigate various flaring scenarios for comparison with the observed short-term variability. For our leptonic jet model, we find that the short-term variability, in particular the optical and X-ray spectral variability, can be best represented with a flaring scenario dominated by a spectral-index change of the spectrum of ultrarelativistic electrons injected into the jet. Based on this result, a detailed model simulation, reproducing the observed optical and X-ray spectral variability and broadband SED of BL Lacertae simultaneously, is presented. Our leptonic modeling results are compared to fits using the hadronic synchrotron-proton blazar (SPB) model. That model can reproduce the observed SEDs of BL Lacertae in a scenario with muon-synchrotron dominated high-energy emission. It requires a significantly higher magnetic field than the leptonic model (~ 40 G vs. ~ 2 G in the leptonic model) and a lower Doppler factor associated with the bulk motion of the emission region (D ~ 8 vs. D ~ 18 in the leptonic model). The hadronic model predicts a significantly larger > 100 GeV flux than the leptonic models, well within the anticipated capabilities of VERITAS and MAGIC.

M87 – a misaligned synchrotron-proton blazar?

The Fanaroff–Riley (FR) class 1 radio galaxy M87 is widely believed to be a misaligned blazar of BL Lac type. Its unresolved nuclear region is a strong non-thermal emitter of radio to X-ray photons that have been interpreted as synchrotron radiation. The recent detection of TeV-photons by the HEGRA-telescope array, if confirmed, would make it the first radio galaxy detected at TeV-energies. We discuss M87’s core emission in the context of the hadronic Synchrotron-Proton Blazar model. By modeling M87’s non-simultaneous spectral energy distribution we predict the peak power of the γ-ray component at ∼100 GeV at a flux level detectable for the new Cherenkov telescopes H.E.S.S., VERITAS and MAGIC. Thanks to M87’s proximity TeV photon absorption in the cosmic background radiation field is negligible. This offers for the first time the opportunity to directly trace the intrinsic high energy cutoff of a TeV-emitting blazar-like object.

TeV γ-rays and cosmic rays from the nucleus of M87, a mis-aligned BL Lac object

The unresolved nuclear region of M87 emits strong non-thermal emission from radio to X-rays. Assuming this emission to originate in the pc scale jet aligned at θ∼30° to the line of sight, we interpret this emission in the context of the synchrotron proton blazar model. We find the observed nuclear jet emission to be consistent with M87 being a mis-aligned BL Lac object and predict γ-ray emission extending up to at least 100 GeV at a level easily detectable by GLAST and MAGIC, and possibly by VERITAS depending on whether it is high-frequency or low-frequency peaked. Predicted neutrino emission is below the sensitivity of existing and planned neutrino telescopes. Ultra-high-energy neutrons produced in pion photoproduction interactions decay into protons after escaping from the host galaxy. Because energetic protons are deflected by the intergalactic magnetic field, the protons from the decay of neutrons emitted in all directions, including along the jet axis where the Doppler factor and hence emitted neutron energies are higher, can contribute to the observed ultra-high-energy cosmic rays. We consider the propagation of these cosmic ray protons to Earth and conclude that M87 could account for the observed flux if the extragalactic magnetic field topology were favourable.

Gamma Ray and Infrared Emission from the M87 Jet and Torus

The existence of intrinsic obscuration of Fanaroff-Riley I objects is a controversial topic. M87, the nearest such object, is puzzling in that although it has very massive central black hole it has a relatively low luminosity, suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) it is not known with certainty whether or not M87 has a dusty torus. Infrared observations indicate that if a torus exists in M87 it must have a rather low infrared luminosity. Using arguments from unification theory of active galactic nuclei, we have earlier suggested that the inner parsec-scale region of M87 could still harbour a small torus sufficiently cold such that its infrared emission is dwarfed by the jet emission. The infrared emission from even a small cold torus could affect through photon-photon pair production interactions the escape of 100 GeV to TeV energy gamma rays from the central parsec of M87. The TeV gamma-ray flux from the inner jet of M87 has recently been predicted in the context of the Synchrotron Proton Blazar (SPB) model to extend up to at least 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection of gamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported. We discuss the interactions of gamma-rays produced in the inner jet of M87 with the weak infrared radiation expected from a possible dusty small-scale torus, and show that the HEGRA detection shows that the temperature of any torus surrounding the gamma-ray emission region must be cooler than about 250 K. We suggest that if no gamma-rays are in future detected during extreme flaring activity in M87 at other wavelength, this may be expected because of torus heating.

BL Lac objects in the synchrotron proton blazar model

We calculate the spectral energy distribution (SED) of electromagnetic radiation and the spectrum of high-energy neutrinos from BL Lac objects in the context of the synchrotron proton blazar model. In this model, the high-energy hump of the SED is due to accelerated protons, while most of the low-energy hump is due to synchrotron radiation by co-accelerated electrons. To accelerate protons to sufficiently high energies to produce the high-energy hump, rather high magnetic fields are required. Assuming reasonable emission region volumes and Doppler factors, we then find that in low-frequency peaked BL Lacs (LBLs), which have higher luminosities than high-frequency peaked BL Lacs (HBLs), there is a significant contribution to the high-frequency hump of the SED from pion photoproduction and subsequent cascading, including synchrotron radiation by muons. In contrast, in HBLs we find that the high-frequency hump of the SED is dominated by proton synchrotron radiation. We are able to model the SED of typical LBLs and HBLs, and to model the famous 1997 flare of Markarian 501. We also calculate the expected neutrino output of typical BL Lac objects, and estimate the diffuse neutrino intensity due to all BL Lacs. Because pion photoproduction is inefficient in HBLs, as protons lose energy predominantly by synchrotron radiation, the contribution of LBLs dominates the diffuse neutrino intensity. We suggest that nearby LBLs may well be observable with future high-sensitivity TeV γ-ray telescopes.

A proton synchrotron blazar model for flaring in Markarian 501

The spectral energy distribution (SED) of gamma-ray (γ-ray) loud BL Lac objects typically has a double-humped appearance usually interpreted in terms of synchrotron self-Compton models. In proton blazar models, the SED is instead explained in terms of acceleration of protons and subsequent cascading. We discuss a variation of the synchrotron proton blazar model, first proposed by Mücke and Protheroe (Proc. Workshop GeV–TeV Astrophysics: Toward a Major Atmospheric Cherenkov Telescope VI, Snowbird, Utah, submitted for publication), in which the low energy part of the SED is mainly proton synchrotron radiation by electrons co-accelerated with protons, which produce the high energy part of the SED mainly as synchrotron radiation. As an approximation, we assume non-relativistic shock acceleration which could apply if the bulk of the plasma in the jet frame were non-relativistic. Our results may therefore change if a relativistic equation of state was used. We consider the case where the maximum energy of the accelerated protons is above the threshold for pion photoproduction interactions on the synchrotron photons of the low energy part of the SED. Using a Monte Carlo/numerical technique to simulate the interactions and subsequent cascading of the accelerated protons, we are able to fit the high-energy γ-ray portion of the observed SED of Markarian 501 during the April 1997 flare. We find that the emerging cascade spectra initiated by γ-rays from π 0 decay and by e ± from μ ± decay turn out to be relatively featureless. Synchrotron radiation produced by μ ± from π ± decay, and even more importantly by protons, and subsequent synchrotron-pair cascading, is able to reproduce well the high energy part of the SED. For this fit, we find that synchrotron radiation by protons dominates the TeV emission, pion photoproduction being less important with the consequence that we predict a lower neutrino flux than in other proton blazar models.