Short-term VHE variability in blazars: PKS 2155-304

Abstract

Context. The γ-ray blazar PKS 2155-304 has attracted considerable attention because of its extreme TeV variability characteristics during an exceptional flaring period in 2006. Among the observed key findings are (i) a minimum variability timescale as short as ∼200 s and (ii) highly variable TeV emission, which in the frequency interval [10 −4 Hz, 10 −2 Hz] can be described by a log-normal distribution and suggests an underlying multiplicative (and not additive) process. Aims. Simultaneously accounting for these findings appears difficult within conventional approaches. Following earlier suggestions for the TeV blazar Mkn 501, we explore a possible scenario where PKS 2155-304 is supposed to harbor a supermassive binary black hole system and where the observed TeV variability is dominated by emission from the less massive black hole. Methods. We analyze the constraints on the very high energy (VHE) source imposed by the observed variability characteristics and the integrated VHE luminosity output, and discuss its implications for a binary black hole system. Results. We show that for a secondary mass of m BH ∼ 10 7 M , fluctuations in the disk accretion rate that feed the jet could account for the observed red-noise type variability process down to frequencies of ∼10 −2 Hz. Jet curvature induced by orbital motion, on the other hand, could further relax constraints on the intrinsic jet speeds. Conclusions. Because a binary system can lead to different (yet not independent) periodicities in different energy bands, a longterm (quasi-) periodicity analysis could offer important insights into the real nature of the central engine of PKS 2155-304.

Expected signatures from hadronic emission processes in the TeV spectra of BL Lacertae objects

“…Based on measurements of the host-galaxy luminosity by Kotilainen et al (1998) and on the relations given by Bettoni et al (2003), a mass of 1−2 × 10 9 M is estimated by Aharonian et al (2007). However, when accounting for the scatter in the relation between bulge luminosity and black-hole mass, its mass could be as low as 2 × 10 8 M (Rieger & Volpe 2010;McLure & Dunlop 2002). In addition, as pointed out by Aharonian et al (2007), the host galaxy luminosity might require further confirmation.…”

The minijets-in-a-jet statistical model and the rms-flux correlation

“…Transient emission on time scales t var ∼ T G /50 are easier to locate in a jet with a bulk Lorentz factor Γ (unless generated by a companion system, as e.g. in Rieger & Volpe 2010). Phenomena occurring on time scales T G then appear at t var ∼ T G /δ to the observer, where δ is the Doppler factor of the jet 1 .…”

Gamma-Ray Astrophysics in the Time Domain

“…using that δ = 2γ b in the head-on (i = 0) approximation. The ∼ 3-min VHE variability observed in PKS 2155-304 (z = 0.116) would then imply a black hole mass of M BH < ∼ 4 × 10 7 M only, much smaller than what is inferred from the host-galaxy luminosity relation (e.g., [18]). Faster variability could only be achieved if the jet collided with a small obstacle (size r c r s ).…”

“…Provided damping is negligible, these fluctuations can propagate inward and couple together to produce a multiplicative (power-law noise) behavior in the innermost disk part [48][49][50]. If this behavior is efficiently transmitted to the jet (particle injection rate), the gamma-ray emission could be modulated accordingly [18]; see Figure 6. This requires, amongst others, that the timescale for particle acceleration and radiative losses within the jet be correspondingly small [18].…”

“…If this behavior is efficiently transmitted to the jet (particle injection rate), the gamma-ray emission could be modulated accordingly [18]; see Figure 6. This requires, amongst others, that the timescale for particle acceleration and radiative losses within the jet be correspondingly small [18]. Log-normality at different wavelengths and over long timescales might then well be feasible (i.e., up to timescales t v (r d ) where r d is the characteristic disk radius).…”