Multiwavelength observations of the blazar BL Lacertae: a new fast TeV gamma-ray flare

Transcription

1 Multiwavelength observations of the blazar BL Lacertae: a new fast TeV gamma-ray flare Qi Feng1 for the VERITAS Collaboration, S.G. JORSTAD, A.P. MARSCHER, M.L. Lister,Y.Y. Kovalev, A.B. Pushkarev, T. Savolainen, I. AGUDO, S.N. MOLINA, J.L. GOMEZ, V.M. Larionov, G.A. Borman, A.A. Mokrushina, and P.S. Smith 1McGill University 1 ICRC July 13, 2017, Busan

2 Radio-loud AGN + beaming = blazar AGN: active (accreting) galactic nuclei; Radio loud: ~10% of AGN have a relativistic jet; Blazar: ~10% of radio-loud AGN point their jet toward us. High luminosity & compact central region: extreme physics! Urry & Padovani

3 Radio-loud AGN + beaming = blazar AGN: active (accreting) galactic nuclei; Radio loud: ~10% of AGN have a relativistic jet; Blazar: ~10% of radio-loud AGN point their jet toward us. High luminosity & compact central region: extreme physics! blazar Urry & Padovani

4 Radio-loud AGN + beaming = blazar AGN: active (accreting) galactic nuclei; Radio loud: ~10% of AGN have a relativistic jet; Blazar: ~10% of radio-loud AGN point their jet toward us. High luminosity & compact central region: extreme physics! blazar Urry & Padovani

5 What s to learn from fast TeV flare of blazars Causality: fast flare (minutes) => small size Flux R Δtint=0 Flux Time Δtobs R (1+z)/(δc) Δtobs R (1+z)/(δc) when Δtint is unknown => upper limit on R Time Gamma rays escaped pair-production: - the emitting location is far away from BLR - large doppler factor to escape synchrotron photons Ɣ e + Ɣ e - 3

6 What s to learn from fast TeV flare of blazars For BL Lac: A radio core, stationary radio knots, and many superluminal knots A coincident superluminal radio knot after the first fast TeV flare in 2011; implication on the location of the emitting region Flux (10-6 m -2 s -1 ) m -2 s >200 GeV Minutes The first TeV flare from BL Lac on 2011 Jun 28 decay time ~13 minutes Cohen et al GHz GHz Arlen et al

7 A new TeV flare from BL Lac on 2016 Oct 5 Peak flux ~180% Crab Avg flux ~90% Crab I(t) = ( I peak e (t t peak)/t rise, t 6 t peak ; I peak e (t t peak)/t decay, t > t peak ; 5 rise time = 140 ( ) minutes decay time = 36 (+ 8-7) minutes

8 Markov-Chain Monte Carlo: To get posterior probability distribution of the parameters. Ensemble sampling: 100 random walkers 1000 steps after 1000 burning steps 100,000 sims Peak flux 6 Peak time Rise time Decay time

9 Adding a constant baseline makes the best-fit variability time much faster Motivation: 1. BL Lac is known for its multiple radio cores 2. A concurrent 1-week GeV flare was observed red χ 2 = 1.18 I(t) = ( I base + I 0 e (t t peak)/t rise, t 6 t peak ; I base + I 0 e (t t peak)/t decay, t > t peak ; Considering the statistical and systematic uncertainties, we can t significantly reject either model. 7 rise time = 65 (+19-8) minutes decay time = 2.6 ( ) minutes I_base = (1.2 ± 0.1) x 10-6 ph m -2 s -1

10 Baseline constraints from observations Integral flux upper limit TeV at 99% confidence level assuming a photon index of -3.3 Peak ~180% Crab On Oct 6 (live time 37.6 min) the UL is 2x10-7 m -2 s -1 Avg ~90% Crab On Oct 22 - Nov 19 (live time min) the UL is 2.8x10-8 m -2 s -1 ~8% Crab +1 day ~1% Crab +3 weeks 8

11 Higher order exponential fit with a constant baseline can fit the ULs measured later Soft L1 loss function I_base = ± I_0 = 9.7 ± 5.6 t_peak = ± 1.1 t_rise = 7.7 ± 18 t_decay= 1.9 ± 4.4 k = 0.21 ± 0.06 red χ 2 = 73.7/42 = 1.75 I(t) = ( I base + I 0 e [ t t peak /t rise ] k, t 6 t peak ; I base + I 0 e [ t t peak /t decay ] k, t > t peak ; +1 day +3 weeks No single model can explain both the flare and the upper limits. Two components are most likely required. 9

12 while fails to fit the flare I(t) = ( I base + I 0 e [ t t peak /t rise ] k, t 6 t peak ; I base + I 0 e [ t t peak /t decay ] k, t > t peak ; Overfit No single model can explain both the flare and the upper limits. Two components are most likely required. 10

13 Spectral energy distribution (SED) 1.83 ± ± 0.07 VHE spec: Power law: index = ; red χ 2 = 34 Log-parabola: alpha = ; beta = ; red χ 2 =

14 MWL light curves Concurrent LAT flare: rise time 2.1 ± 0.2 days decay time 6.9 ± 1.8 days Drop in polarization fraction Change in polarization angle 12

15 Swift XRT observations on Oct 6, 7, 8 Oct 6 Oct 7 Oct 8 13

16 Swift XRT SED - exclusion region has no effect on the spectral results Except statistics! 14

17 VLBA 43 GHz Marscher & Jorstad 0.2 mas 1.3 pc/mas A potential new knot (K16) travelling at ~6c. 15

18 VLBA 43 GHz Marscher & Jorstad 0.2 mas 1.3 pc/mas A potential new knot (K16) travelling at ~6c. K11 at ~3.6c Arlen et al The first TeV flare from BL Lac on 2016 Jun 28 15

19 MOJAVE VLBA 15 GHz Lister, Pushkarev, Kovalev, Savolainen VLBA 15 GHz VLBA 43 GHz 1.3 pc/mas Radio knot K16/VHE gamma-ray flare 16

20 VLBA 15 GHz 17

21 Single-dish light curve at 37 and 15 GHz Merja, Kiehlmann, Hovatta+ Metsahovi OVRO 15 GHz vs 37 GHz 18 Zero lag between 15 GHz and 37 GHz indicates they are both measuring the jet outside the 15 GHz radio core.

22 Interpretation Fast flare => small size of emission region Fast decay => abrupt stop of injection R apple ct decay 1+z Gaussian injection The upper limit of the size of emission region is: ~12 Rs (Schwarzschild radius; ~1 Rs for the model with a baseline), or 1.39x10 13 m, or 4.5x10-4 pc on-off injection MBH = 3.8x10 8 M ; Doppler Factor δ= 24; Wu et al. (2009) Hervet et al. (2016) Katarzynski et al. (2003) 19

23 Interpretation Association between TeV and radio: Turbulent Extrem Multi-Zone model: Particle acceleration Radio core/knot Marscher et al. (2014) Fast gamma-ray flare Figure 1. Sketch of the geometry employed to carry out calculations withi Multiple recollimation shock + carried-away shock model: Magnetic dominated jet Class I/II Faster flow of plasma/perturbation Particle dominated jet Hervet et al. (2016) E Radio core/ stationary knot Stationary knot Superluminal knot Magnetic reconnection - gamma-ray flare 20

24 Summary VERITAS detected a fast TeV flare from BL Lac peaking at ~180% Crab. The decay time is about 36 minutes, and faster than the rise time. Size of the emitting region constrained to be 12Rs. Correlated GeV/optical flare at the time of the TeV flare, but slower. Polarization activities observed in R band and radio frequencies. VLBA 43 GHz images can be interpreted with a superluminal radio knot passing the core around the time of the TeV flare (but the interpretation s likely not unique). Such observations can potentially locate the TeV gamma-ray emitting region (a difficult task)! 21