VERITAS Observations of Relativistic Jets

Transcription

1 VERITAS Observations of Relativistic Jets 1 for the VERITAS Collaboration 1Barnard College, Columbia University Sensitivity improvement: 1% Crab in ~25 hr Sensitive energy range: 100 GeV to 30 TeV Spectral reconstruction: begins at ~150GeV. Energy resolution: ~15% - 20% Angular resolution: < 0.1o at 1 TeV, 0.14o at 200 GeV (68% values)

2 Outline Highlights from the Extragalactic Program TeV Blazar Sample Modeling blazar SEDs M87 Galactic sources of HE relativistic outflows Gamma-ray binaries Pulsar wind nebulae (PWN)

3 Blazars Detected by VERITAS Key Science Project: Discovery, MWL & ToO observations ~400 hr/yr including moonlight data AGN Type z Mkn 421 HBL Mkn 501 HBL ES HBL ES HBL detections 10 discoveries 4 VHE intermediate BL Lacs Predominately nearby - EBL horizon? Target selection Mostly X-ray candidates Now Fermi-LAT motivated All LAT detected except 1ES W Comae IBL RGB J * HBL H HBL ES IBL ES HBL ES HBL ES IBL RX J Blazar ES HBL RBS 0413 HBL ES HBL PG HBL 0.43 < z < ES HBL 0.341? 3C 66A IBL 0.444? PKS IBL/HBL? VER J Blazar?

4 Blazar sequence expanding on TeV source classes Sambruna 1996; Fossati et al Non-thermal, continuum spectra. Dramatic peak at γ-ray energies. Emission extends to GeV-TeV. Blazars detected by Fermi HBLs: ν pk ~ Hz IBLs: ν pk ~ 10 Sambruna Hz 1996; LBLs: ν pk ~ 10 Fossati Hz et al Extreme HBLs > Hz Absence of intrinsic γγ pair absorption beaming in blazars. High isotropic γ-ray luminosity ~ erg/s Optical depth >> 1 γ-ray emission originates in strongly beamed sources. Non-HBLs VERITAS: W Comae 3C 66A 1ES ES

5 IBLs: VERITAS Discovery of W Com Intermediate-peaked (IBL). First IBL to be detected > 200 GeV Strong flare 2008 March (ATel 1422) 2008 Jan Apr: ~5 σ in ~40 hr observations FOV shows recent detection of B ES W Comae B Acciari et al ApJ 690, L73 Benbow et al. ICRC % of excess from 4-night flare in 2008 March 275γ, 8.6σ; τ 1.3 ± 0.3 days, 9% CU

6 Discoveries: Finding Blazars behind the Galactic plane A number of unidentified Fermi sources are expected to be blazars behind the Galactic plane. VHE telescopes are a good tool for identifying blazars at low latitudes (better localization, higher sensitivity to flux variability).

7 Blazars behind the Galactic plane Identified as VHE candidates because of cluster of E>50 GeV (Fermi) photons at source position. 1 period = 1 month = 1 dark run VER J VER J γ-ray excess, 15.6 σ, 5% Crab Position compatible with radio & X-ray source RGB J z unknown. Follow-up optical spectroscopy revealed continuum dominated spectrum, no absorption lines, typical of BL Lacs Unidentified radio & X-ray source RGB J located 6 o off the plane VERITAS: 19 h in Mar-Apr10 γ-ray excess, 5.3 σ, 2% Crab Follow-up optical spectroscopy at Lick: z= E. Aliu et al. 2011, in prep.

8 Low-latitude blazar: RX J E. Aliu et al. 2011, in prep. 1 period = 1 month = 1 dark run Follow-up optical spectroscopy at the Shane 3m telescope at Lick Observatory. The obtained results reveal a continuum dominated spectrum typical of BL Lactype blazars. Observed absorption lines are compatible with z=

9 Blazar Models What have we learnt with VERITAS? Particle acceleration & radiation in blazar jets Collimated ejection of plasma with bulk Lorentz factor >>1 Radiation peaks in SED. Non-thermal synchrotron & IC (Ieptonic), or hadronic processes Sources of soft photons: synchrotron self- Compton radiation, accretion disk photons, radiation scattered by BLR,.. Synchrotron & SSC flux: F ~ δ 3+α External Compton : F ~ δ 4+2α (δ = 1/[Γ(1-βcos θ)] Doppler factor) VERITAS data modeled using leptonic one-jet model (Boettcher & Chiang 2002) Observed radiation originating from ultra-rel. e - in a spherical emission region (R B ), moving at β Γ c. Relativistic jet outflow with Γ 10 Urry & Padovani 1995 Size of emission region constrained by shortest timescale of variab. R B <cδt var D/(1+z)

10 HBLs: RGB J Some examples of blazar modeling: 1 period = 1 month = 1 dark run Extreme HBL. Promising VHE candidate Redshift V. Acciari et al. 2010, ApJL 715, L49 Useful for EBL studies VERITAS detection triggered observations at other wavelengths, including Fermi -LAT detection First Fermi-LAT source found with VHE guidance Γ bulk = 30 γ min = γ max = q=1.5 B=0.036 Γ HE = 1.46 ± 0.17 stat ± 0.05 sys Γ VHE = 2.69 ± 0.26 stat ± 0.20 sys

11 TeV Blazars: Mrk 501 Γ bulk = 30 V. Acciari et al., (2010) arxiv: period = 1 month = 1 dark run γ min = 1 γ b = γ max = q=1.6 q b = 2.6 Every dark run in good agreement! Also see Panequeʼs poster A43 Joint MWL campaign VERITAS/MAGIC, Fermi-LAT, Suzaku. Large shift in synchrotron peak; little shift in IC peak. Both epochs well described by SSC model X-ray data used to place limits on peak frequencies: 230 kev (5.5 X Hz) high 0.6 kev (0.6 X Hz) low Shift in VHE peak not as dramatic -- Could be from onset of KN suppression -- KN effects become important above hν ~ m e c 2 in electron rest frame

12 SEDs: Constraints from MW Observations Correlated MW observations of IBL 3C 66A High optical luminosity is expected to play a key role in providing the seed population for IC scattering. Abdo et al. arxiv:

13 HBL 1ES : Variable Emission 1 period = 1 month = 1 dark run Redshift Hard intrinsic spectrum Γ 1.5 Flare Jan 25 Feb 5, 2009: 7% Crab to 20% Crab ~1 day variability time scale challenges kiloparsec jet model of hard-spectrum emission (Boettcher et al. 2008) R ct δ/(1 + z)) For typical Doppler factors δ= 20 (Marscher 2006), R<0.01pc. V. Acciari et al. 2010, ApJL 709, L163

14 TeV Blazar: Mrk 421 long-term monitoring program major flares in 2008 & 2010 initiated large MWL efforts spectral hardening with increasing flux high in VHE & X-ray since 11/09 35 h of data; ~400σ huge flare on Feb 17th 2010 variability on 5-10 min time scales >10σ per 2 minute bin ApJ submitted (2010)

15 M87 Radio Galaxy 2008 Flare Location of TeV γ ray emission?? TeV: night-by-night averaged VHE light curve in Strong variability resulted in the detection of at least 2 flares X-ray: Chandra core and HST-1 knot VLBA: 43 GHz observations of nucleus, peak and flux integrated along the jet -- radio flux of the unresolved core risng. Temporal coincidence of TeV & radio flares indicates they are related. Wagner et al Also see M. Raueʼs talk & D.E. Harris A.23

16 Outline Highlights from the Extragalactic Program TeV Blazar Sample Modeling blazar SEDs M87 Galactic sources of HE relativistic outflows Gamma-ray binaries Pulsar wind nebulae (PWN)

17 Galactic Science Program Galactic sources of HE relativistic outflows Mirabel 2006 Science Pulsar Wind Nebulae (pointed observations) o TeV emission from nebulae of energetic young pulsars. Large fraction of Galactic TeV sources.. o TeV PWNe associated with high E dot /d 2 pulsars. TeV observations of binaries: o Binaries are the only variable Galactic TeV sources. o TeV emission probes the highest energy particles accelerated. May provide the keys to an understanding of astrophysical jets. o Two Scenarios: Microquasar: gamma-rays are produced in a radio-emitting jet o Pulsar Binary: particles accelerated in the shock produced by the interaction of the pulsar wind and the wind of the companion

18 HMXB LS I+61 o 303 Compact object orbiting a Be companion star 26.5 day, inclined orbit, e=0.54, circumstellar disk Stage et al Extended radio structure; microquasar? (but radio images shows orbital morphology change) Strong VHE emission only near apastron: 15-20% of Crab Nebula Flux (MAGIC/VERITAS) GeV emission peaks near periastron; 6 GeV cut-off; orbital modulation; orbit-to-orbit variations; J.Casares et al (MNRAS 360, 1105 (2005)) Different component than in TeV?

19 HMXB LS I+61 o 303: VERITAS Follow up : 37 h data, 3.3 σ, no detection Almost entire orbit covered : 18 h data, 0.8 σ, no detection Deep exposure near apastron Stage et al Acciari et al. 2011, ApJ arxiv:

20 HMXB LS I+61 o 303: VERITAS Follow up October 2010: periastron detection 5% of the Crab Nebula flux at E>300 GeV Is this long term variability related to the environment of the Be star? Stage et al Acciari et al. 2011, ApJ arxiv:

21 HESS J : a new binary? HESS J : Only unidentified TeV source in Galactic plane that is point-like. Hinton ApJL 690 (2009) Discovered by H.E.S.S. in 2004/2006 (Γ=2.53, F(>1 TeV) ~3% Crab. MWL follow-up shows a hard spectrum X-ray source & faint radio source coincident with a massive B0pe star (MWC148) (Hinton et al 2009). Faint point-like, variable radio source (<2ʼʼ extension, mjy, Skilton et al 2009) Not detected by Fermi LAT No binary system identified (e.g. Aragona et al 2010) VERITAS non-detection , VERITAS detection in Implies variability. Variable X-ray emission measured by Swift. (Falcone et al 2010)) XMM A new TeV binary system? Coincident with Be star MWC 148? (Hinton et al 2009). An unusual isolated massive star? (confined stellar wind, Townsend et al 2007)

22 TeV Variability 30 h in Dec Jan 2009: not detected by VERITAS (ApJ 687 L94 (2009)) Excluded with ~4σ confidence that HESS J is a steady gamma-ray emitter H.E.S.S./VERITAS campaign in 2009/2010 (publication in prep) 8h in Oct 2009: no detection (UL~1.3% Crab) 20 h in Feb/March 2010: clear detection (7.5 σ, 1.5% Crab) clearly variable in VHE gamma rays is it a VHE binary? Need detection of orbital modulation (at any wavelength) HESS/VERITAS. (in prep.) VERITAS Feb-Mar 2010

23 And finally Swift X-ray monitoring establishes periodic behavior (T=320±5 days) (Bongiorno, 2011) Atel #3153 VERITAS observations Feb 7/8, 2011 triggered by X-ray activity (Atel #3152) > 8σ, F (E> 300 GeV) ~ 4% Crab Confirmed by MAGIC (Atel #3161)

24 Pulsar Wind Nebulae Most of the rotational energy in pulsars is lost in the form of a particle wind made of electrons (leptonic accelerators?) This wind form the PWN that shines at all wavelengths. Does CTA 1 emit from the shell? from the PWN? from both? CTA 1 radio shell Age~13 kyr Dist=1.4±0.3 kpc Pulsar Ė=4.5x10 35 erg/s First pulsar discovered at GeV γ- rays by Fermi-LAT (Gaensler & Slane, 2006, ARA&A, 44, 17)

25 The most recent: CTA 1 Observations: 26.5 hrs between Oct 2010 and Jan 2011 Extended emission centered around the pulsar (7.3σ/6.2σ pre/post-trials significance) Preliminary flux ~4% Crab - working on spectrum Morphology clearly suggests young PWN Properties of CTA 1 nicely fit in the middle of those of known TeV/X-ray PWN Black Contours:1420 MHz show SNR shell Red Circle: Fermi pulsar Green : 3 to 7σ TeV contours B. McArthur for the VERITAS Collab. 3 rd Fermi Symp May

26 Recent Studies of the Cygnus Region Abdo et al. 2007) MGRO J : ~80% Crab at 20 TeV (Abdo et al. 2007) Located towards the Cyg OB1 association Coincident with two EGRET UIDs (3EG J & 3EG J ) 3EG J =blazar B ? (Mukherjee et al., 2000) 3EG J = radio pulsar PSR J (Roberts et al., 2002) Both associations have been now confirmed with the LAT: 1FGL J (pulsed) and 1FGL J (very variable, V=139) (Abdo et al., 2009)

27 VERITAS Observations Cygnus Region VERITAS Observations: 75 hrs between May & December 2010 Point-- like source detected coincident with the CTB 87 PWN. VER J : 6.1σ F (E>1TeV) ~ 0.8% Crab Nebula. Can exclude at 99% level the blazar B FGLJ is variable, likely assoc. with blazar. GeV and TeV emission in this region seems to be of quiet different origin! VER J could contribute to the MGRO J E. Aliu for the VERITAS Collab. 3 rd Fermi Symp May

28 VERITAS Observations Cygnus Region Broad emission region detected in extended source analysis 0.23 integration radius, 650 GeV threshold Broad TeV Excess: 7.4 σ post Coincides with MGRO J (dashed ellipse). Substructure and/or multiple sources? Under evaluation. E. Aliu for the VERITAS Collab. 3 rd Fermi Symp May

29 The Crab pulsar Remnant of historic supernova in 1054 A.D. Dist ~ 2 kpc Period ~ 33 ms Most energetic pulsar 4.6 x erg/s One of the brightest GeV γ-ray pulsars Powers the brightest TeV γ-ray source Break between few MeV and few GeV (exponential cutoff) Acceleration regions (polar vs outer gap) and radiation mechanisms (curvature radiation) studied through the cutoff MAGIC detection above 25 GeV suggested that emission comes from a region > 6 stellar radii outer gap scenarios favored NERQUAM, Yale 2011

30 Crab: Latest Results VERITAS N. Otte for the VERITAS Collab. 3 rd Fermi Symp May

31 Crab: Latest Results VERITAS N. Otte for the VERITAS Collab. 3 rd Fermi Symp May

32 Summary of Results made with TeVCat (see: http//veritas.sao.arizona.edu)

33 Thanks - to all my VERITAS colleagues (veritas.sao.arizona.edu)