Variability of blazars: Optical and GeV

C. S. Stalin

Indian Institute of Astrophysics

Abdo et al. 2011, ApJ, 736, 131 Jorstad et al. 2013, ApJ, 773, 147

Compton dominance < 1; gamma-ray emission due to synchrotron self Compton mechanism

Compton dominance > 1; gamma-ray emission due to EC processes

Blazars

Leptonic / hadronic / hybrid models

If leptonic models are correct, how do they reflect in variability ?

In leptonic scenario, low (optical) and high (gamma) energy emission are from the same population of electrons and thus both these variations are correlated

➢Correlation between optical and GeV bands are studied for about a dozen sources

➢Close correlations noticed

Bonning et al. 2012 (12 sources) Chatterjee et al. 2012 (6 sources)

E W. Bonning et al. 2012

3C 454.3

3C 279 (z = 0.536): A major flare in April 2014

➢First FSRQ detected in VHE (E > 100 GeV; MAGIC in 2008)

➢Underwent a major flare during 25 March – 13 April 2014

➢Brightest gamma-ray flare ever observed in this source

➢Two other flares observed; June 2015 and December 2013

Flaring state

Paliya et al. 2015, ApJ, 803, 15

April 2014 flare

Second flare: June 2015: SEDs are explained by Leptonic Scenario

Flares of 2013 December and 2014 April

Paliya et al. 2016, ApJ, in press

2014 Flare Leptonic, 2013 Flare NOT leptonic; a source can behave differently at different times

How Variability can test these models?

➢Leptonic Scenario: optical and GeV variations are correlated

➢Hadronic scenario: Optical and GeV variations are not correlated

We have less than half – a – dozen studies that focus on this

This could be due to lack of good simultaneous data

Case 1: FSRQ PKS 0208-512

Correlation during Flares 1 and 3

No correlation during Flare 2

R. Chatterjee et al. 2013

Case 2: FSRQ 2142-75

Correlation during Flare A, but no correlation during Flare B

M. S. Dutka et al. 2013

Case 3: NLSy1 1H 0323+342

Paliya et al. 2014, ApJ, 789, 143

Case 4: FSRQ 3C 279

D. Bhattacharya et al. (2016, to be submitted)

In different band different physical processes may be operating?

A systematic search for correlated/uncorrelated opical – GeV variations in blazars

Sample: All blazars from 3FGL (Ackermann et al. 2015) : Cross-correlated with CRTS (optical V – band) Only those sources that have good V-band data considered ( 1218 sources)

FSRQ (LSP) --> 343FSRQs (ISP) --> 39FSRQs (HSP) --> 3BL Lacs (LSP) --> 216BL Lacs (ISP) --> 269BL Lacs (HSP) -> 348

Analyzed 7 years of Fermi – LAT data (Analysis in progress)Optical -> CRTSLooked for correlated optical/GeV variations

Case 1

Case 2

Case 3

Conclusions

➢Optial – GeV variability patterns of blazars clearly tell that the variability behavior shown by them is complex

➢ correlated optical – GeV flux variations

➢ optical flares with no GeV counterparts

➢ GeV flares with no optical counterparts

Blazars do not show similar variability characteristics at all times

Leptonic/Hadronic?

One zone/multi-zone ?

Hybrid models?

Optical – GeV connection: Towards the answer

➢GeV: Fermi is there

➢Optical: need more planned monitoring (flux + polarization)

➢X-ray : this too is needed (would be nice to try with ASTRSAT when a blazar flares)

➢More understanding needed driven by good quality observations

Observations

Theory

How ASTROSAT can help us?

ASTROSAT + ground based optical/IR observations (flux and polarization) can give a better data set for broad band modeling of blazars. This in conjunction with improved models will help us in constraining the emission processes in blazars.

For BL Lacs, ASTROSAT X-ray bands lie in the falling part of the first peak as well as in the valley

For FSRQs, ASTROSAT X-ray bands lie in the rising part of the second peak

Careful sample selection + observation of few targets (BL Lacs + FSRQs)

Future: X-ray polarization will help (leptonic v/s hadronic)

Now: