Exploring the Non-thermal Universe with Gamma Rays

1. Non-thermal radio emission from binary systems Josep M. Paredes Exploring the Non-thermal Universe with Gamma Rays On the occasion of the 60th birthday of Felix Aharonian Barcelona, November 6 – 9, 2012

2. OUTLINE • Stellar radio sources • Microquasars • Non-accreting pulsars • New cases • Cataclysmic binaries • Colliding wind binaries

3. The radio flux density Sνat a frequency ν of a star measured at the Earth can be expressed in terms of the Rayleigh-Jeans approximation to the BB formula k: Boltzmann’s constant TB: brightness temperature ΩS: solid angle subtended by the star Sν =2 kν2 TB ΩS c2 The maximum distance at which a star like the Sun would be observable at 5 GHz with a radio telescope with a typical minimum flux density of 1 mJy: the quiet Sun: 0.2 pc the slowly varying component: 0.3 pc the very strong solar radio bursts: 7.6 pc (Proxima Centauri is 1.3 pc away) Solar-like emission would be seen from only a few nearby stars Detectable radio starshave either unusually large emitting surfacesgenerally associated with stars with massive stellar winds or enhanced brightness temperaturesstars exhibiting powerful non-thermal radio emission

4. Hertzprung-Russell diagram based on stellar radio detections (Güdel 2002, Annu. Rev. Astron. Astrophys. 40, 217) Radio emission has been detected from all stages of stellar evolution across the HR diagram Most of the main sequence and sub-giant objects are non-thermal emitters Most of the giants and many O-B stars are thermal emitters (they are big) Novae and X-ray binaries not shown since the active source is related to accretion onto a WD and NS or BH respectively

5. Types of radio stars Stars undergoing mass loss OB and Wolf-Rayet stars:FF emission from an optically thick wind. Non-thermal emission: relativistic e- embedded in the wind (Fermi acceleration in shock fronts) or collision between the winds of two components of a binary Be stars:FF emission as a consequence of the mass loss trough an equatorial disk by centrifugal effects caused by the rapid rotation Single red giants and supergiants:the winds are cool and only partially ionised. Detectable if near VV Cephei stars:cool supergiant + B V companion. FF emission from a subregion of the supergiant wind ionized by UV photons from the hotter companion Pre-main sequence stars:T Tauri stars. Classical (CTT): FF from ionised gas. Weak-lined (WTT): non-thermal radio emission from magnetically active regions

6. Stars exhibiting enhanced solar activity The sun exhibits non-thermal emission and flare activity associated with high-energy particles in the chromosphere and corona. The same phenomenon on larger scales is characteristics of many cool (primarily K-M) stars on and above MS. The energy supply is thought to be the release of stellar magnetic field energy by reconnection of field lines. The fields may be generated by the dynamo mechanism. Flare stars:Single stars that exhibit intense flares from X-rays to radio waves. The coronal gas is bound by magnetic loops (several 1000 G) covering most of the stellar surface. Stellar flares due to coherent emission (at lower frequencies) and incoherent gyrosynchrotron emission (higher frequencies). Prototype: UV Ceti, YZ CMi, AD Leo Close binaries:RS CanumVenaticorum stars: Solar type + cool subgiant. Gyrosynchrotron emission. Magnetic activity is enhanced by the high rotation rate of the active subgiant which is synchronised to the period of orbital revolution by tidal coupling. Particle acceleration may also occur in the intrabinary plasma if the interaction between the fields of the two stars produce reconnection. Pre-main sequence stars: Some of the properties of WTT stars are similar to the RS CVn stars. Gyrosynchrotron emission from starspot regions

7. Chemically peculiar stars CP stars or Bp-Ap stars: MS stars characterised by over and under abundances of certain chemical elements and by strong (1-10 kG) dipolar magnetic fields. No convection: B may be a fossil remnant of dynamo fields generated in the PMS phase. Radio characteristics (flat radio spectrum, circular polarisation, non-thermal TB) consistent with gyrosynchrotron emission Radio emission related to mass transfer in binaries The transfer process may involve Roche lobe overflow or accretion from a stellar wind from a normal star to a white WD, NS or BH. Cataclysmic variables: Classical novae: semi-detached binaries (MS+WD). Nova outbursts (intervals of ~ 104 yr) occurs when accreted H rich material accumulates on the outer surface of the WD leading to a thermal runaway and explosive ejection of the outer envelope. FF emission from the expanding ionised ejecta. Synchrotron radiation from particles accelerated in a shock (within the ejecta or interaction between the nova ejecta and a dense gas cloud). Magnetic cataclysmic variables (AM Her stars): late-type MS+magnetic WD. The mass transfer process is modified by the presence of strong magnetic fields (105-107 G). Highly variable flare-like non-thermal radio emission. Symbiotic stars: interacting binaries (cool giant (read giant) + hot companion). FF emission from a circumstellar ionised envelope. X-ray binaries: x-ray emission produced by accretion. HMXB (130) and LMXB (150). 20% detected in radio. Synchrotron radiation from relativistic particles interacting with magnetic fields.

8. Binaries with synchrotron radio emission Be/X-ray transients HMXRB SG/obscure binaries X-ray binaries BH binaries LMXRB 1. Accreting binaries Microquasars Relativistic jet Cataclysmic binaries Colliding-wind massive binaries (OB+WR, OB+OB,WR+WR) 2. Non-accreting binaries Pulsar wind binaries (e.g. PSR B1259-63) Sources of relativistic particles  HE and VHE gamma-rays could be produced

9. Radio emission Synchrotron Radiation e- e- e- UV - Opt Donor star e- OB Star X-ray Disk black body or Corona power-law e- e- Gamma-ray Inverse Compton Scattering Possible scenarios • An accretion disk is formed by mass transfer. • Display bipolar jets of relativistic plasma. • The jet electrons produce radiation by synchrotron emission when interacting with magnetic fields. • VHE emission is produced by inverse Compton scattering when the jet particles collide with stellar UV photons, or by hadronic processes when accelerated protons collide with stellar wind ions. Microquasar Γe~105 [Mirabel & Rodríguez 1998, Nature 392, 673; Aharonian & Atoyan 1998, NewAR 42, 579; Romero et al. 2003, A&A, 410, L1; Bosch-Ramon et al. 2006, A&A, 447, 263] • The relativistic wind of a young (ms) pulsar is contained by the stellar wind. • Particle acceleration at the termination shock leads to synchrotron and inverse Compton emission. • After the termination shock, a nebula of accelerated particles forms behind the pulsar. • The cometary nebula is similar to the case of isolated pulsars moving through the ISM. Non-accreting pulsar [Maraschi & Treves 1981, MNRAS, 194, P1; Dubus 2006, A&A, 456, 801; Sierpowska-Bartosik & Torres 2007, ApJ, 671, L145] From Moldón 2012

11. Microquasars Cygnus X-1 Stellar Mass Black Hole 5 pc (8’) diameterring-structure of bremsstrahlungemitting ionized gas at theshockbetween (dark) jet and ISM. ● HMXB, O9.71+BH VLA VLBA+VLA WSRT Gallo et al. 2005, Nature 15 mas 30 AU Stirling et al. 2001, MNRAS 327, 1273 Martí et al. 1996, A&A 306, 449

12. Cygnus X-3 Strong radio outbursts Microquasars ● HMXB, WR+BH? • Exhibits flaring to levels of 20 Jy or more • Modelling Cyg X-3 radio outbursts: particle injection into • twin jets Martí et al. 1992, A&A 258, 309 VLBA VLA, 5 GHz Miller-Jones et al. 2004, ApJ 600, 368 Martí et al. 2001, A&A 375, 476

13. Binary pulsar systems Young pulsar wind interacting with the companion star PSR B1259-63 The first variable galactic source of VHE PSR B1259-63 / LS 2883:O8.5-9 Ve (Negueruela et al. 2011, ApJL, 732, L11) • Dense equatorial circumstellar disk + 47.7 ms radio pulsar, P = 3.4 yr, e = 0.87 • No radio pulses are observed when the NS is behind the circumstellar disk (free-free absorption) • Orbital plane of the pulsar inclined with respect to the disk (Melatos et al. 1995, MNRAS 275, 381; Chernyakova et al. 2006, MNRAS 367, 1201) Aharonian et al. 2009, A&A 507, 389 Abdo et al. 2011, ApJ 736, L11 Chernyakova et al. 2009, MNRAS 397, 2123 HESS June 2007 Johnston et al. 1999 PSR B1259-63. Nearly all the spin-down power is released in HE gamma rays (Abdo et al. 2011). Doppler boosting suggested (Tam et al. 2011), but very fine tuning is needed(!). PSR B1259 / LS 2883

14. Binary pulsar systems Extended radio structure Australian Long Baseline Array (LBA) 2.3 GHz Moldón et al. 2011, ApJ 732, L10 Total extension of the nebula: ~ 50 mas, or 120 ± 24 AU The red crosses marks the region where the pulsar should be contained in each run Kinematical model Moldón et al. 2011, ApJ 732, L10 Shock between the relativistic pulsar wind and a spherical stellar wind (Dubus 2006, A&A 456, 801) The evolution of the nebular flow after the shock is described in Kennel & Coroniti (1984) This is the first observational evidence that non-accreting pulsars orbiting massive stars can produce variable extended radio emission at AU scales PSR B1259 / LS 2883

15. Microquasaror pulsar scenario ? HMXB, B0Ve+NS? COS-B γ-ray source CG/2CG 135+01 Hermsen et al. 1977, Nature 269, 494 LS I +61 303 0.8-0.5 Radio (P=26.496 d) Taylor & Gregory 1982, ApJ 255, 210 Optical and IR Mendelson & Mazeh 1989, MNRAS 239, 733; Paredes et al. 1994 A&A 288, 519 X-raysParedes et al. 1997 A&A 320, L25; Torres et al. 2010, ApJ 719, L104 HE gamma-rays Abdo et al. 2009, ApJ 701, L123 VHE gamma-rays Albert et al. 2009, ApJ 693, 303 periodicity 4.4 yr periodicityRadio (P= 1667 d) Paredes 1987, PhD Thesis;Gregory 2002, ApJ 575, 427 Strickman et al. 1998, ApJ 497, 419 0.5-0.8 Paredes et al. 1990, A&A 232, 377 Gregory 2002, ApJ 575, 427 0.5-0.8

16. Microquasaror pulsar scenario ? Jet-like features have been reported several times, but show a puzzling behavior (Massi et al. 2001, 2004). VLBI observations show a rotating jet-like structure(Dhawan et al. 2006, VI Microquasars Workshop, Como, Setember 2006) VLBA G F H E A I D B C J NOT TO SCALE Observer 3.6cm images, ~3d apart, beam 1.5x1.1mas or 3x2.2 AU. Semi-major axis: 0.5 AU LS I + 61 303

17. Microquasaror pulsar scenario ? Repeatability of the LS I +61 303 morphology Highly stable morphology probably indicates interaction of steady outflows Striking similarity 8 GHz , 2007 September (Moldón 2012, PhD thesis) 8 GHz , 2006 July Dhawan et al. (2006) orbital phase 0.62 5 GHz , 2006 October 25 (contours) 8 GHz , 2006 February 2 (gray scale) (Albert et al. 2008) 20062007 LS I + 61 303

18. Microquasaror pulsar scenario ? Orbital morphological variability LS 5039 HMXB, O6.5V+NS? • Images at the same phase • have similar morphology • Periodic orbital modulation • of the radio morphology of • LS 5039, stable over several • years • Changes in the radio • structure are compatible with • the presence of a young non • accreting NS Moldón et al. 2012, A&A arXiv 1209.6073 LS 5039

19. VLBA image Model Model of the VLBI structure of LS 5039 considering an outflow starting at 10a and using a similar bending as in Bosch-Ramon et al. 2012. Includes synchrotron and adiabatic losses. Moldón et al. 2012, A&A arXiv 1209.6073 LS 5039

20. On the lack of pulsations in LS 5039 and LS I +61 303 Courtesy of Javier Moldón LS 5039 Porb = 3.9 day O6.5 V + ? LS I +61 303 Porb = 26.5 day B0Ve + ? PSR B1259-63 Porb = 3.4 yr O8.5Ve + pulsar Radio pulses disappear ? Radio pulses disappear ? Radio pulses disappear

21. New cases Skilton et al. 2009, MNRAS 399, 317 HESS J0632+057 Hinton et al. 2009, ApJ 690, L101 • Be star MWC 148 • XMM-Newton • Δ1RXS J063258.3+054857 Swift X-ray periodicity: P=321 ± 5 days  strong evidence for binary nature Bongiorno et al., 2011, ApJ 737, 11 confirmed by Casares et al., 2012, MNRAS 421, 1103 In Feb. 2011 Swift reported increased X-ray activity (Falcone et al. 2011, Atel # 315 VERITAS and MAGIC detected elevated TeV gamma-ray emission (Ong et al. 2011, Atel # 3153; Mariotti et al. 2011, Atel # 3161) VLBI counterpart (Moldón et al. 2011, Atel # 3180) • Confirmed association with Be star • Confirmed the non-thermal nature of the radio source • Discovery of extended emission Moldón et al. 2011, A&A 533, L7 HESS J0632+057

22. New cases 1FGL J1018.6-5856 Ackermann et al. 2012, Fermi Col., Science 335, 189 • 1FGL J1018.6-5856 is one of the brighter Fermi sources • LAT spectrum similar to a pulsar - but no pulsations seen • Optical counterpart ~O6V((f)), just like LS 5039 Fermi Flux modulated with a 16.6 d period Swift-XRT • X-ray flare-like behaviour near phase 0, coinciding with gamma-ray maximum • An spatially coincident variable radio source • Radio structure ? ATCA: 5.5 GHz, 9 GHz

23. Cataclysmicbinaries Abdo et al. 2010, Science 329, 817 V407 Cygni Mira-type pulsating red giant + WD (symbiotic binary) • The first nova explosion ever detected in g-rays • The gamma-ray emission has been attributed to particles accelerated by the interaction of the nova shock with the dense Mira wind JVLA Chomiuk et al. 2012, ApJ, in press arXiv 1210.6029 Radio light curve dominated by the wind of the Mira giant companion Not observed a thermal signature from the nova ejecta or synchrotron emission from the shock, due to the fact that these components were hidden behind the absorbing screen of the Mira wind.

24. Collidingwinds in massivebinarysystems Collision of supersonic winds in massive star binaries (WR have very strong winds, v∞ up to 5000 km s-1)strong shocks where both e and p can be efficiently accelerated up to relativistic energies through first-order Fermi mechanism (Eichler& Usov 1993) Non-thermal synchrotron emission from the colliding wind region in WR140 has been detected (Dougherty et al. 2005, ApJ 623, 447)  the presence of highly relativistic electrons. These systems may also be embedded in dense photon fields where IC losses would be unavoidable,  making CWBs potential high-energy emitters WR 140 AGILE (Tavani et al. 2009) and Fermi-LAT (Abdo et al. 2010) established high-energy γ-ray emission in spatial coincidence with the location of η Car. A weak but regular Fermi flux decrease over time has been detected (Reitberger et al. 2012) and interpreted in a colliding-wind binary scenario for orbital modulation of the γ-ray emission.  First unambiguous detection of GeV γ-ray emission frrom a colliding-wind massive star

25. Summary • The MQs and Binary pulsar systems with HE and/or VHE gamma-ray emission • Are (synchrotron) radio emitters • Periodic at all wavelengths (?) • Have a bright companion (O or B star)  source of seed photons for the IC emission • MQs: Jet radio structures • Binary (non-accreting) pulsar systems orbiting massive stars • – can produce variable extended radio emission at AU scales • – repeatability of their radio structures with the orbit of the binary system • (PSR B1259−63, LS 5039, LS I +61 303) • VLBI radio observations are a common link, useful to understand the behavior of gamma-ray binaries. Can put constrains on physical parameters of the system.