52nd Rencontres de Moriond 2017 Very High Energy Phenomena in the Universe Long term multi-wavelength variability studies of the blazar PKS 2155-304 Jill Chevalier D. Sanchez, P. Serpico, J-P. Lenain, G. Maurin
Centaurus A Active Galactic Nuclei
Nowadays challenges in AGN 1995: unification model from Urry & Padovani 1999: blazar sequence of Fossati Blazars Open questions: Links between the black hole, jet & accretion disk Acceleration & emission processes
Nowadays challenges in AGN Non-thermal spectrum with 2 broad bumps Low energy bump = synchrotron Still a lot of unknown in the high energy bump? Inverse Compton External Compton Photo-pion production Jill Chevalier Rencontres de Moriond - VHE session - March 2017
Leptonic or hadronic model? In many cases, leptonic and hadronic models can produce equally good fits to the SEDs
Variability in blazars Blazars are variable in all wavelengths and at all time scales: - flares minute scale - long term month/year scale BL Lacertae (Fermi) H.E.S.S. Variability is a key towards a better understanding of AGN ROTSE radio
Long term γ-ray monitoring of blazars 2008: Fermi-LAT 2004: H.E.S.S. 2005: MAGIC 2007: VERITAS More and more monitored VHE sources: - Mkn 421 - Mkn 501 - PG 1553+113 - PG 1510-089 - PKS 2155-304
γ-ray light curves of PKS 2155-304 H.E.S.S. and Fermi-LAT long term light curves from the H.E.S.S. Collaboration paper arxiv:1610.03311v1
Going further: The MWL dataset of PKS 2155-304 H.E.S.S. E > 200 GeV Fermi-LAT 0.1 < E < 300 GeV + 2 sub-bins X-ray 2 < E < 10 kev + 4 sub-bins SMARTS J, R, V & B bands
Variability profile F var (E) Fvar = normalised excess variance from Vaughan et al. (2003) F var = q S 2 2 err Strong variability with F var increasing through SED components
Variability profile F var (E) Fvar = normalised excess variance from Vaughan et al. (2003) F var = q S 2 2 err Strong variability with F var increasing through SED components Synchrotron Inverse Compton Same electron population?
Similar result in Mkn 421 ~2-3 years MWL light curves Ahnen et al. (2016) arxiv:1605.09017 MWL variability characterisation variability increases with the energy
Similar result in Mkn 421 Fermi-LAT
Cross-correlation Method: discrete correlation function (DCF) computation of the correlation for each time lag Maximum at t = 0 days DCF between Fermi-LAT and SMARTS (R) light curves Same electron population?
Cross-correlation Method: discrete correlation function (DCF) computation of the correlation for each time lag Maximum at t = 0 days DCF between Fermi-LAT and SMARTS (R) light curves Same electron population? Presence of two local maximum at t = ± 700 days Signature of periodic signal?
Periodicity in PKS 2155-304? Method: Lomb-Scargle Periodogram (LSP) tool to test the significance of a periodic signal (even in un-evenly spaced data) T Fermi = 685.5 ± 10 days T SMARTS = 731.7 ± 4.7 days
Periodicity in PKS 2155-304? Method: Lomb-Scargle Periodogram (LSP) tool to test the significance of a periodic signal (even in un-evenly spaced data) T Fermi = 685.5 ± 10 days T SMARTS = 731.7 ± 4.7 days
Other periodic signal in blazars OJ 287 (T~12 years) or Mkn 421 (T~400 days) PG 1553+113 T = 2.18 ± 0.08 years in optical and GeV light curves (arxiv:1509.02063) Why? 1. Supermassive binary BH system 2. Jet precession 3.
Log-normal behaviour Flux log Flux The log of the flux is described by a Gaussian Occurence per bin 50 40 30 Occurence per bin 30 25 20 log(φ) N(μ,σ) 20 15 10 multiplicative processes Excess variance: XS = q S 2 2 err 10 0 0 0.5 1 1.5 2 2.5 3-10 Φ (10 cm -2.s -1 ) 5 0-11 -10.8-10.6-10.4-10.2-10 -9.8-9.6-9.4 log Φ (cm -2.s -1 ) from arxiv:1610.03311v1
Log-normal behaviour Flux log Flux The log of the flux is described by a Gaussian Occurence per bin 50 40 30 Occurence per bin 30 25 20 log(φ) N(μ,σ) 20 15 10 multiplicative processes Excess variance: XS = q S 2 2 err 10 0 0 0.5 1 1.5 2 2.5 3-10 Φ (10 cm -2.s -1 ) 5 0-11 -10.8-10.6-10.4-10.2-10 -9.8-9.6-9.4 log Φ (cm -2.s -1 ) from arxiv:1610.03311v1 Public Optical and X-ray results New Probe of the accretion process?
Log-normal behaviour history First time seen in X-ray binaries and Seyfert galaxies (AGN) and related to the accretion disk (Uttley & McHardy 2001, McHardy 2008) propagation of fluctuations in the disk? (accretion rate?) Natural outcome from power law noise variability PSD ω -β Giebels & Degrange 2009 Note : this variability can not originates from the emission mechanism. No mechanism in synchrotron or IC to produce this BL Lacertae
Towards a variability modelling Can we model the variability observed with a simple model?
Towards a variability modelling with the SSC One zone time dependent SSC modelling with the electrons density described as a PowerLaw with exponential cut off Q(E,t)=N 0 E exp E cut(t) Steady state of PKS 2155-304 Electrons: - normalisation - index - cut off energy N 0 = 0.01 α = 2.3 log(γ cut ) = 5.3 Zone: - magnetic field - size - doppler factor B = 0.1 G R = 2 x 10 16 cm δ = 35
Why γ cut (t)? Low impact on the electrons producing the optical and GeV photons High impact on the electrons production the X-ray and TeV photons
Evolution of log γ cut (t) Evolution following a power law noise PSD ω -β stochastic process method of Timmer & Koenig (1995) 2 parameters: - power index β - time series variance σ the simulated time series is re-normalisazed to the mean value with variance σ Which [β,σ] configuration?
Preliminary results Best configuration: Time = 2.2 years log(γ cut ) = 5.3 β = 1 σ = 20 % of log(γ cut ) Flux optic X-ray GeV TeV Time (days)
Preliminary results Best configuration: Time = 2.2 years log(γ cut ) = 5.3 β = 1 σ = 20 % of log(γ cut ) Flux optic X-ray GeV TeV Time (days)
Outlook Long term multi-wavelength variability studies of the blazar PKS 2155-304 Similar behaviour in the optical-gev and Xray-TeV bands with F var (E) Perfect optical-gev correlation Hints towards a leptonic SSC emission model + 700 days periodicity + lognormal behaviour binary SMBH system? imprint of the accretion disk?
Outlook Long term multi-wavelength variability studies of the blazar PKS 2155-304 Similar behaviour in the optical-gev and Xray-TeV bands with F var (E) Perfect optical-gev correlation Hints towards a leptonic SSC emission model + 700 days periodicity + lognormal behaviour binary SMBH system? imprint of the accretion disk? Long term (~10 years) SSC modelling ongoing Application of the variability tests Periodicity tests: can we reproduce a similar periodic behaviour? (injection, doppler factor ) More time-dependent parameters?