Comment on the first-order Fermi acceleration at ultra-relativistic shocks

Ostrowski, M.; Bednarz, J.

doi:10.1051/0004-6361:20021173

Cited by 72 publications

(69 citation statements)

References 16 publications

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“…If, indeed, the optical emission is synchrotron emission from a fast-cooling electron distribution, then electrons have been primarily accelerated to a power-law distribution with an injection index of q ¼ p À 1 ¼ 3:5. This is much steeper than the canonical spectral index of q $ 2:2Y2:3 found for acceleration on relativistic, parallel shocks (e.g., Gallant et al 1999;Achterberg et al 2001), and could indicate an oblique magnetic field orientation (e.g., Ostrowski & Bednarz 2002;Niemiec & Ostrowski 2004), which would yield a picture consistent with the predominantly perpendicular magnetic field orientation observed on parsec scales (Jorstad et al 2004;Ojha et al 2004;Lister & Homan 2005). The observed hard lag (B vs. R) may then be the consequence of a gradual spectral hardening of the electron acceleration (injection) spectrum throughout the propagation of a relativistic shock front along the jet.…”

Section: Discussionmentioning

confidence: 71%

The WEBT Campaign on the Blazar 3C 279 in 2006

Böttcher¹,

Basu²,

Joshi³

et al. 2007

ApJ

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The quasar 3C 279 was the target of an extensive multiwavelength monitoring campaign from 2006 January through April. An optical-IR-radio monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration was organized around target-of-opportunity X-ray and soft -ray observations with Chandra and INTEGRAL in 2006 midJanuary, with additional X-ray coverage by RXTE and Swift XRT. In this paper we focus on the results of the WEBT campaign. The source exhibited substantial variability of optical flux and spectral shape, with a characteristic timescale of a few days. The variability patterns throughout the optical BVRI bands were very closely correlated with each other, while there was no obvious correlation between the optical and radio variability. After the ToO trigger, the optical flux underwent a remarkably clean quasi-exponential decay by about 1 mag, with a decay timescale of d $ 12:8 days. In intriguing contrast to other (in particular, BL Lac type) blazars, we find a lag of shorter wavelength behind longer wavelength variability throughout the RVB wavelength ranges, with a time delay increasing with increasing frequency. Spectral hardening during flares appears delayed with respect to a rising optical flux. This, in combination with the very steep IR-optical continuum spectral index of 0 $ 1:5Y2:0, may indicate a highly oblique magnetic field configuration near the base of the jet, leading to inefficient particle acceleration and a very steep electron injection spectrum. An alternative explanation through a slow (timescale of several days) acceleration mechanism would require an unusually low magnetic field of B P 0:2 G, about an order of magnitude lower than inferred from previous analyses of simultaneous SEDs of 3C 279 and other flat-spectrum radio quasars with similar properties.

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Section: Discussionmentioning

confidence: 71%

The WEBT Campaign on the Blazar 3C 279 in 2006

Böttcher¹,

Basu²,

Joshi³

et al. 2007

ApJ

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“…Several electron injection/acceleration scenarios have been proposed, e.g. impulsive injection near the base of the jet (e.g., [22,23]; such a scenario might also apply to originally Poyntingflux dominated jets, see [80]), isolated shocks propagating along the jet (e.g., [52,39,78,81]), internal shocks from the collisions of multiple shells of material in the jet [83], stochastic particle acceleration in shear boundary layers of relativistic jets (e.g., [64,73]), magnetic reconnection in Poynting-flux dominated jets (e.g., [80]), or hadronically initiated pair avalanches [37].…”

Section: Blazar Modelsmentioning

confidence: 99%

“…Significant progress has recently been made in our understanding of particle acceleration at relativistic shocks (e.g., [1,64,62,88]) and the conversion of relativistic bulk kinetic energy into relativistic particles and ultimately into radiation [69,76,86,53]. While particle acceleration at relativistic parallel shocks might produce electron injection spectra ofṄ e (γ) ∝ γ −q with 2.2 < ∼ q < ∼ 2.3 (e.g., [1]), oblique shocks tend to produce much softer injection spectral indices (e.g., [64]).…”

Section: Blazar Modelsmentioning

confidence: 99%

Modeling the emission processes in blazars

Böttcher

2007

Astrophys Space Sci

335

267

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Blazars are the most violent steady/recurrent sources of high-energy gamma-ray emission in the known Universe. They are prominent emitters of electromagnetic radiation throughout the entire electromagnetic spectrum. The observable radiation most likely originates in a relativistic jet oriented at a small angle with respect to the line of sight. This review starts out with a general overview of the phenomenology of blazars, including results from a recent multiwavelength observing campaign on 3C279. Subsequently, issues of modeling broadband spectra will be discussed. Spectral information alone is not sufficient to distinguish between competing models and to constrain essential parameters, in particular related to the primary particle acceleration and radiation mechanisms in the jet. Short-term spectral variability information may help to break such model degeneracies, which will require snap-shot spectral information on intraday time scales, which may soon be achievable for many blazars even in the gamma-ray regime with the upcoming GLAST mission and current advances in Atmospheric Cherenkov Telescope technology. In addition to pure leptonic and hadronic models of gamma-ray emission from blazars, leptonic/hadronic hybrid models are reviewed, and the recently developed hadronic synchrotron mirror model for TeV γ-ray flares which are not accompanied by simultaneous X-ray flares ("orphan TeV flares") is revisited.

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“…A different critique was levied (Ostrowski & Bednarz 2002) concerning the spectral slope for particles accelerated at highly relativistic shocks. These authors pointed out that the claims of universality for the spectral index s % 4:2 (Kirk & Schneider 1987;Heavens & Drury 1988;Bednarz & Ostrowski 1998;Kirk et al 2000) possibly overstate their case, because they neglect the fact that, as the regular magnetic field in the downstream region becomes stronger than the field associated with the turbulent component, the spectral slope must increase, the spectrum softens, and the claim to universality evaporates.…”

Section: Accelerationmentioning

confidence: 99%

On the Generation of Ultra–High‐Energy Cosmic Rays in Gamma‐Ray Bursts: A Reappraisal

Vietri

Marco

Guetta

2003

ApJ

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We reexamine critically the arguments raised against the theory that ultra-high-energy cosmic rays (UHECRs) observed at Earth are produced in gamma-ray bursts (GRBs). These include the limitations to the highest energy attainable by protons around the bursts' shocks, the spectral slope at the highest energies, the total energy released in nonthermal particles, and the occurrence of doublets and a triplet in the data reported by the Akeno Giant Air Shower Array (AGASA). We show that, to within the uncertainties in our current knowledge of GRBs, none of these objections is really fatal to the scenario. In particular, we show that the total energy budget of GRBs easily accounts for the energy injection rate necessary to account for UHECRs as observed at Earth. We also compute the expected particle spectrum at Earth, showing that it fits the HiRes and AGASA data to within statistical uncertainties. We consider the existence of multiplets in AGASA data. To this end, we present a Langevin-like treatment for the motion of a charged particle in the intergalactic-medium magnetic field, which allows us to estimate both the average and the rms time delay for particles of given energy; we discuss when particles of identical energies reach the Earth in bunches or spread over the rms time delay, showing that multiplets pose no problem for an explosive model for the sources of UHECRs. We compare our model with a scenario in which the particles are accelerated at internal shocks, underlining the differences and advantages of particle acceleration at external shocks.

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Comment on the first-order Fermi acceleration at ultra-relativistic shocks

Cited by 72 publications

References 16 publications

The WEBT Campaign on the Blazar 3C 279 in 2006

The WEBT Campaign on the Blazar 3C 279 in 2006

Modeling the emission processes in blazars

On the Generation of Ultra–High‐Energy Cosmic Rays in Gamma‐Ray Bursts: A Reappraisal

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