Variability of Extragalactic VHE γ-ray Emitters

Transcription

1 Variability of Extragalactic VHE γ-ray Emitters Wystan Benbow Harvard-Smithsonian Center for Astrophysics APS April 2011 Meeting, Anaheim, CA

2 Very High Energy (VHE) γ-ray Astronomy VHE: ~100 GeV to ~100 TeV Necessarily ground-based! 1.8 m ~100 meters

3 3rd Generation VHE γ-ray Observatories MAGIC II: 1% Crab in 50 h VERITAS: 0.7% Crab in 50 h HESS: 0.7% Crab in 50 h (in 2004) VERITAS = 4 x 100 m; HESS = 4 x 120 m; MAGIC = 2 x 85 m ~500 scientists worldwide; VERITAS is the most-sensitive

4 Very High Energy γ-ray Sky VHE γ-ray Sky Catalog Overlaid onto Fermi-LAT 1FGL Sky Map Source Counts vs Time for the 3 Highest Energy Wavebands 122 VHE γ-ray sources as of 4/28/11; ~10 different source classes ~115 discovered after 2002; Start of operation of 3rd generation instruments Catalog comparable to revised 3rd EGRET MeV-GeV catalog Largely pulsar wind nebulae, unidentified sources & active galactic nuclei (AGN) 47 extragalactic VHE γ-ray sources: All but 2 are AGN

5 Practicalities of VHE γ-ray Astronomy Data: ~1000 h / yr per observatory 40-70% on extragalactic sources Sensitivity limit: ~1% Crab (5σ in ~25 h) Crab = Brightest steady source (~1 trigger) Brightest (flare) ~20 Crab; Weakest ~0.003 Crab Systematic errors: Γ ~ 0.1, Flux ~ 20% Systematics are largely due to issues with the absolute energy scale; NB: Atmosphere is calorimeter For relative studies within an observatoryʼs data, systematics can be negligible (<1 month) Preliminary Angular res.: r 68 < 0.1º; Pointing: ~20ʼʼ All known extragalactic VHE sources are point-like & source sizes are such that no morphological changes will be observed Energy resolution: ~15%; Technique limited

6 47 Extragalactic VHE Sources 45 AGN = 41 blazars + 4 radio galaxies 2 starburst galaxies; Constant flux No VHE-detected GRB/GRB afterglow Fermi: Observed GeV γʼs after powerful GRB Delayed X-ray flares detected by Swift Milagro: ~3σ evidence for >650 GeV emission Long-lived afterglows: Models => blazar-like SED VERITAS: >35 GRB afterglows observed Fermi / Swift / AGILE / INTEGRAL triggers Observe if <3 h old & θ<70º; ~30 h / y No detections, but all are high-z or z =? MAGIC & HESS: Similar GRB programs & results Also not detected in VHE: Galaxy clusters, dwarf galaxies, local group galaxies, globular clusters GRB Best T ~ 90 s Acciari et al., ApJ, submitted

7 Active Galactic Nuclei: Basics AGN: ~5% of galaxies; Small, bright nucleus in otherwise typical galaxy Highly variable & much brighter than host Non-thermal emission in all wavebands Powered by accretion onto Msun BH Jets = collimated, relativistic outflow of plasma ~perpendicular to disk; radio emitting; 10% of AGN have jets VHE γ-rays produced in jets! Urry & Padovani, 1995 FR I FR II Radio galaxy = AGN viewed off jet axis Blazar = AGN viewed down jet axis Blazars = BL Lac & FSRQ BL Lac = FR I (core-dominated, low luminosity) FSRQ = FR II (lobe-dominated; high luminosity) VLA, 5 GHz VLA, 5 GHz

8 M 87: The best radio galaxy at VHE discovered in VHE band in 1999; M87 Aharonian et al., A&A, 403, L1, : HESS detects variable, point-like, hard-spectrum excess from the core Only other 3 too faint: Cen A (5σ in >100 h), NGC 1275 & IC 310: ~5σ in 1-day flares HESS Localization d ~16 kpc MBH 6 x 109 Msun Jet angle ~ 20º VERITAS, MAGIC, HESS, others, in prep. M87: HESS day-scale variability in 2005 Science, 314, 1424, 2006

9 M87: The 2008 Flare VHE flaring along with simultaneous birth of radio-knot & nuclear X-ray flare Strongly suggests core as a source of VHE emission! Acciari et al., Science, 325, 444, 2009

10 21 observations in 15 days Sub-day-scale variability (0.6 days) M 87: The 2010 flare R < δ x 1015 cm; (67 AU = cm) Coincides w/ X-ray (Chandra) core brightening again, but no radio core brightening

11 Blazar Properties SED = Power vs Energy BL Lacs = continuum Blazar SEDs have 2 bumps Average Spectral Shape of Blazars Sambruna et al., ApJ, 463, 1996 Low-E: Synchrotron radiation of relativistic electrons in jet B-field High-E: controversial Leptonic models: Relativistic e- upscatter photons (via inverse- Compton scattering) to high-e Seed photons: SSC vs EC Fermi VHE Hadronic models: Interaction of relativistic protons w/ matter, ambient photons, B-field, or photons & B-field Variability & simultaneous MWL studies are critical 75% of VHE blazars are HBL: 30 HBL, 5 IBL, 3 LBL, 3 FSRQ Most Fermi-LAT AGN are FSRQ/LBL

12 VHE Blazars: Variability common, but... 63% of VHE blazars have shown VHE variability 11 Non-HBL Blazars: Detection only during flares is the norm 3 FSRQ: 3C 279, PKS , PKS Only PKS is steady but weak (1% Crab); Others only seen during during 1-night flares 3 LBL: AP Lib, BL Lac & S Only AP Lib has steady emission; Others only weakly detected during brief (multi-day) flares 5 IBL: W Com, 3C 66A, PKS , 1ES , 1ES All but 1 seen during flares; evidence coming in that low-states are just detectable (~1% Crab) 30 HBL: 60% show variability, but itʼs often weak; Only ~10 are well-studied 5 weakly detected & only seen at few % Crab during brief flares; Otherwise not detected 5 have weak (factor of 2-3x), month-scale variations; All are 2-3% Crab sources; incl. 1ES (IGMF impl.) 3 have ~1-day factor of few flares in otherwise steady, moderate-flux (few % Crab) data Only 5 HBL with extended, day-scale VHE flaring episodes: These are the 5 brightest!

13 Modeling VHE Blazar Emission VHE blazar papers since ~2008 all have simultaneous MWL SEDs for modeling Swift, LAT, Chandra, RXTE, XMM + optical & radio HBL: modeling 1-zone SSC works well SSC model parameters: Size of emission region: R ~ 1015 cm (~100 AU) Homogeneous magnetic field: ~0.1 G Doppler factor: δ ~ Electron distribution: RGB J ApJ, 715, L49, 2010 Broken (γ break) power-law, between γmin & γmax Normalization ~104 cm -3, n1 ~ 2, n2 ~ 3.5 γ min ~10 3, γmax ~10 6, γbreak ~ 10 5 SSC model works during HBL flares (usually) 1ES ApJ, submitted, 2011

14 Modeling during Mrk 421 Flaring in 2008 Acciari et al., ApJ, submitted, days SSC works in all flare states for Mkn 421 even with factor of ~10 variations over long periods of time

15 Mkn 501: Flare vs Quiet ApJ, 729, 2, Flare 2009 quiescent state Synch. peak moves by 2 orders of mag., VHE peak moves little: Onset of Klein-Nishina effects SSC scenario successfully models both states: Transition largely due to change in electron distribution

16 IBL vs HBL Modeling 5 VHE IBL - First 4 discovered by VERITAS 3C 66A (ApJ, 693, L104, 2009 & ApJ, submitted, 2010) & W Com (ApJ, 684, L73, 2008 & 707, 612, 2009) in flares SEDs: Needs SSC + EC component PKS (ApJ, 708, L100, 2010) -- steady in ʼ10, 1ES (in prep.) - steady flux over 3-years SED: SSC works: No EC needed Both are borderline HBL 1ES (too new) & PKS lower in ʻ11 PKS C 66A

17 VHE Blazar Flaring HESS: H Monthly light curve A&A, 516, 56, 2010 HESS: PKS daily light curve A&A, 430, 865, % Crab on Average 15% Crab on Average HESS: PKS year (monthly) light curve HESS: PKS min light curve A&A, 520, 83, 2010 ApJ, 664, L71, 2007 ~90,000 γ 70% in 4 nights 700% Crab on Average Only 4 VHE blazars (1ES , PKS 2155, +) have ever been observed >1 Crab; Only 2 (Mrk 501 +) w/ more than one episode & only Mrk 421 >2 Crab more than once

18 What about flaring HBL? Illustrative case using Mkn 421 in 2008 Same trends seen in long-term studies Similar results seen for a few other VHE HBL VHE spectral hardening w/ increased flux Mkn 421 ApJ, submitted X-ray & VHE fluxes are linearly correlated VHE/X-ray spectral hardening also correlated Low flicker states: No correlation seen; or VHE correlation w/ optical & not X-ray Mkn 421 ApJ, submitted Mkn 421 ApJ, submitted

19 Orphan Flare of 1ES Krawczynski et al., ApJ, 601, 151, 2004 Typical VHE flux is ~0.1 Crab Circles: HEGRA Stars: Whipple Nearby VHE blazar (HBL) well-studied since 1998 Except for 2002 outburst, no flux variations > 2x seen June 4, 2002: Orphan flare : VHE bright & X-ray low Challenges 1-zone SSC model; Hadronic? Implies hadronic process, but trials impossible to calculate Tagliaferi et al. 2008: X-ray varies by ~2x; but VHE flux same; SSC ok Interesting, but not significant, AMANDA neutrino result Resconi, 2005 Aspen Conf.

20 4 Most Extreme VHE Flares w/ 3rd Generation PKS (90 min) & Mkn 501 on Same Scales HESS: PKS Flare on 7/30/ h of data - ~6 h Simultaneous w/ Chandra HESS: PKS (7/28/06) MAGIC: Mkn 501 (7/9/05) 1 min bins 1-3 minute variability No MWL 2 min bins Hour-scale variability 20 min. episodes Taken from Hinton 2007 VERITAS: ~5 h Exposure on Mkn 421 A&A, 502, 749, min bins 5-10 minute variability No MWL

21 The Big Flare of PKS (z = 0.116) 9-yrs HESS monitoring: 4 nights >2 Crab 90 min. episode: Factor of 23 flux range ~12000 γ,168σ, γ-rate: 2.5 Hz (after cuts) F var = 0.58 ± 0.03; ~2x higher than X-ray Almost no spectral change from low state, despite factor of ~50 flux change 5 bursts; Fit GRB generalized Gaussian Best τ r = 173 ± 28 s; Fastest τr = 67 ± 44 s Best: R < δ x 4.7 x 1012 cm < δ x 0.31 AU BH is ~109 Msun: δ > ~100 R / Rsch Energetics + VHE photon escape: Γ > 50 GRB like Doppler factors or variability not related to black hole Γ1 = 2.7, Γ2 = 3.5 Ebreak = 430 GeV

22 PKS : Use X-ray timing techniques PDS: Significant power down to 600 s P ~ ν-2 (red-noise like) ν-1 rejected; Too much power at high-ν Remarkably similar to X-ray power spectra Log-normal process suggests multiplicative (not additive) process Typically related to undamped fluctuations in accretion rate Scenarios of binary BH system proposed Tidal disruption of massive object? e.g SwJ1644? Alternative fast moving jet w/ several compact (faster moving) sources Simple SSC explanation Larger component dominates steady state Fourier PDS for PKS Flare Shaded = 90% CL region Dashed-line = Noise level No sign of high-ν cutoff suggests even faster variability may be detectable with a more sensitive instrument

23 PKS : Highlights from a Chandra Flare ~7.5 h HESS exposure, 6 h w/ Chandra Brighter, but slower (~1 h) than fast flare Changes: 20x VHE, 2x X-ray, 15% optical VHE/X-ray harder when brighter VHE spectra more curved with higher flux Synchrotron & inverse-compton peaks donʼt move X-ray/VHE flux correlation strong & cubic No lags between bands or within bands VHE/X-ray spectra correlated; similar time evolution No optical corr. aside from rise at flare onset A&A, 502, 749, 2009 Extremely Compton-dominated (10x) flare Never seen before Again, multiple SSC components needed

24 3 Other Important Notes from PKS All but July 2006 Quiescent State is seen (15% Crab) A&A, 520, 83, 2010 Spectral evolution is perhaps a bit more complicated than harder vs brighter More variable at higher E

25 VHE Flares & Lorentz Invariance Violation Quantum gravity & effective field theory: possible energy dependence in c Small effect; Linear or quadratic in E Near Planck energy: E P = 1.22 x GeV Detectable for cosmological sources Result assumes photons emitted at same time Flaring-AGN / GRBs: High-E photons lag lower-e Best VHE result: PKS flare HESS: PRL, 101, , 2008 Others: Mkn 421 (Whipple) & Mkn 501 (MAGIC) 200 < E < 800 GeV E > 800 GeV 10 4 sims No delay => Linear effect at E > 6% E P NB: Fermi-GRB: 31 GeV γ < 1 s late E > few M P; Nature, 462, 331, 2009 τpeak = 20 s, RMS of τpeak = 28 s; 21% of τpeak < 0 s

26 Cosmology: Flares & Extragalactic Background Light Diffuse EBL: ~Analogous to CMB Combined flux of all extragalactic sources over entire history of Universe A&A, 479, L9, 2007 EBL absorbs γʼs: F int = Fo e -τ(e, z) Softens VHE spectra; eventual horizon Constrain w/ distant, hard Γ VHE blazars 1ES (z = 0.186): Γ obs ~ 2.9 1ES (z = 0.139): Γ obs ~ 2.5 up to 10 TeV Assume Γ int > 1.5 => How high can EBL be? 1ES (z=0.182); Γ obs ~ 3.1 EBL limits within 20% of best, rules out models Increase limits: Produce Γ int < 1.5 => Outer jet models VHE flare in 1ES 1218 rules out steadystate models that refute EBL limits 1ES ApJ, 709, L163, 2010

27 Major Blazar Efforts in the Future Major annual MWL (incl. all VHE) efforts organized for: Mkn 421 & Mkn 501 Simultaneous X-ray & VHE every 2-3 days; LAT is always on; + optical/radio Mkn 421 flared in papers in the works Mkn 501 in 2009: ApJ, 727, 129, 2011 VERITAS deep 6-year campaigns on 14 blazars ( h shots on HBL, IBL, LBL) + LAT + X-ray/optical/radio

28 CTA: 10x Sensitivity, 3x Angular resolution Worldwide effort 730 Scientists $400M Construction to begin in 2014 Comparable to VERITAS by ʻ17 Think of PKS flare w/ 10x smaller errors!

29 Conclusions VHE source catalog rapidly growing: 47 extragalactic objects; ~40% of total ~60% show some form of variability, but only 10% have major flaring episodes; AGN only Sub-hour for only 3 objects: PKS (1-3 min), Mkn 501 (2 min), Mkn 421 (5 min) 4 episodes in past decade despite intense monitoring; Found during ~month-length elevated states Blazars: VHE variations are fastest & highest-amplitude of any wave-band Rest is short-lived and/or only factor of ~2; Typically sensitivity limited Repeated M87 flares show that VHE flare emission is from region very near black hole For VHE blazars: SSC modeling works for HBL (75% of pop) even during flaring episodes IBL: Perhaps an additional EC component is needed during flares Rapid variability: Either need GRB like doppler factors or some other mechanism Indications that perhaps rapid variability comes from additional compact SSC components, but other ideas too... Future is bright: Extensive VHE + LAT MWL campaigns; CTA is < decade away -- no longer sensitivity limited

30 Two VHE Starburst Galaxies γʼs expected from central regions: High rate of massive star formation => CRs Nature, 462, 770, 2009 High gas density; CRs + gas => o => γʼs VERITAS discovery of M82 (ʼ07-09) 137 h; 91γ, ~5σ; 0.9% Crab; Γ = 2.5 ± 0.6 HESS discovery of NGC 253 (ʻ03-09): 119 h, 250γ, ~5σ; 0.3% Crab: Science, 326, 1080, 2009 M82 CR density: 250 ev cm-3 ; ~500x Milky Way Links CR acceleration to star formation SN+Stellar winds = likely CR accel. sites M82/NGC 253 flux constant as expected Variability would destroy CR interpretation; NB: Low stats Abdo et al., 709, L152, 2010