Polarization Studies of Extragalactic Relativistic Jets from Supermassive Black Holes Iván Agudo
What is an active galactic nuclei (AGN)? Compact regions at the centre of galaxies with much higher than normal luminosity (up to 10 4 L typical galaxy ) AGN are the most luminous persistent sources of radiation in the Universe Though to be a result of accretion of mass to a super-massive black hole at the galaxy centres Conceptual representation of a radio loud AGN ( W. Steffen, UNAM & COSMOVISION)
Types of AGN Radio loud AGN: Radio-mm spectrum dominated by relativistic jet emission: Radio galaxies Radio loud quasars (steep) Blazars (with jets better oriented to the observer, 10º) Radio quiet AGN: Jet is very weak or not present, hence no or weak radio emission: LINERs? Seyfert galaxies Radio quiet quasars Original image from Urry & Padovani (1995)
How are relativistic jets formed? 3D RMHD simulations of relativistic jet formation From accretion phenomena into compact objects (supermassive black holes [10 6-10 9 M ] in the case of AGN jets) Essential ingredients to form the pair of jets: The gravitational potential of the rotating compact object Material from the rotating accretion disk Co-rotating magnetic fields McKinney & Blandford (2009) Conceptual representation of a radio loud AGN ( W. Steffen, UNAM & COSMOVISION)
The scales of jets in AGN Hot Spot/Lobes: ~109 rs (~100 kpc; 20 ) Matter dominated jet : ~103 109 rs (1 105 pc; 1 mas 20 ) Transition region: ~102.5±0.5 rs (< 1 pc; < 1 mas) MHD acceleration and collimation zone (ACZ): ~10 102.5±0.5 rs (1 < 100 mpc; 10 μas < 1 mas) Large scale jet dominated by particle/plasma (matter) dynamics 150 kpc Between the matter dominated region and the electromagnetic (Poynting) dominated region The jet nozzle Jet launching region of part of the accreting material; ~5 50 rs (0.5 5 mpc; 5 50 μas) Probably not resolved or just marginally Very Long Baseline Interferometry (VLBI) Only astronomical technique, so far, able to VLBA resolve jet structures in the sub parsec scales up to high z VLBI provides angular resolution better than 0.1 milliarcsecond (hundreds of times better than HST) EVN Introduction Current Challenges Current Polarization Studies Requirements for SKA1 Conceptual image of a radio loud AGN COSMOVISION New Science with SKA1 Further Considerations
(Relativistic) jets in Astrophysics First astrophysical jet was observed in 1917 by Curtis emanating from the core of the galaxy M87. Relativistic jets are usually associated with AGN, but jets are seen in multiple astrophysical scenarios: AGN (Blandford, Nature, 1977) Microquasars (Mirabel et al., Nature, 2001) GRBs (Sari et al., ApJ, 1999) Tidal disruption flares (Burrows et al., Nature, 2011) Pulsars (Velusamy, Nature, 1984) Supernovae (Paragi et al., Nature, 2010) Dwarf Nova (Körding et al., Science, 2008) AGB stars (Sahai et al., Nature, 2003) Star formation (Carrasco-Gónzalez, et al., Science, 2010) Planetary nebulae (Soker & Livio, ApJ, 1994) Sun (Cirtain et al., Science, 2007) Even in planets! (Sánchez-Lavega et al., Nature, 2008) Jets are a common phenomenon in Astrophysics, associated to compact accreting systems, from stars to a supermassive black holes They play a relevant role in modern Astrophysics, enough to dedicate 275 IAU Symposium to Jets at all scales Mirabel & Rodríguez (2002) Introduction Current Challenges Current Polarization Studies Requirements for SKA1 New Science with SKA1 Further Considerations
Current challenges on astrophysics of relativistic jets in AGN The jet formation, confinement, and acceleration mechanism The characterization of the role played by the magnetic field on the formation and collimation of jets What is the actual geometry and intensity of the magnetic field on these sources? Where (and how) are these magnetic fields dragged from? The location of the high energy emitting regions The origin of the seed photons responsible for such high energies The acceleration of electrons to high energies The jet composition in their different regions (electromagnetic, e - e +, e - p + ) and how it changes between them All of them related directly or indirectly with the properties of magnetic fields in AGN jets!
Polarimetric monitoring of AGN relativistic jets with VLBI 15 GHz VLBA monitoring program Time dependent observations of large samples allow for kinematic studies & computation of basic jet physical parameters (i.e., Lorentz factors, intrinsic geometry of jet, etc), and polarization properties along the jets MOJAVE web page: http://www.physics.purdue.edu/mojave/ e.g. Lister et al. (2009)
Multiwavelength polarimetric monitoring of AGN relativistic jets with VLBI 15 GHz 22 GHz 3C120 radio galaxy 43 GHz 12 monthly polarimetric VLBA images During 2001 Multi frequency monitoring programs also allow for rotation measure and spectral index time dependent analysis Introduction Current Challenges Current Polarization Studies Requirements for SKA1 Gómez Further et al. (2008) New Science with SKA1 Considerations
Time dependent Faraday rotation VLBI imaging of AGN relativistic jets Rota%on measure across %me Rotation measure averaged across time Time averaged polarization degree 15 GHz 22 GHz Gómez et al. (2008) 43 GHz Transversal slice Rotation Measure (rad/m2) 25 Degree of polarization (%) 4000 15 GHz 22 GHz 43 GHz RM A 20 15 3000 Stacking images helps to compensate from poor polarization sensitivity 2000 Transverse gradients across the jet (+true polarization angle) allow to confirm consistency with helical magnetic fields. 10 1000 5 South 0.0 Introduction Current Challenges 0.5 North 1.0 1.5 Distance (mas) Current Polarization Studies 2.0 2.5 Important for jet formation models 3.0 Requirements for SKA1 New Science with SKA1 Further Considerations
Deep VLBI Imaging 3C120 @ 5 GHz - VLBA B80 100 Deep VLBI imaging of jet structures In combination with detailed theoretical modeling C80 C90 Allows to estimate more complicated jet physical parameters (i.e. degree of order of magnetic field, Lorentz factor of plasma, intrinsic geometry of jet structures, etc). Core C16 A80 C99 Agudo et al. (2012)
Circular Polarization Typically 0.3%-0.5% at cm&mm λs Typically 15% of sources detected on large surveys Limited by sensitivity to CP Agudo, et al. (2010) Mechanisms for generation of circular polarization (e.g. Wardle & Homan 2003): intrinsic mechanism: Faraday conversion: CP studies along the spectrum help to identify the CP production mechanism These depend on the composition of relativistic jets (e - e + against e - p + ) Therefore, CP polarization studies are a powerful potential tool for AGN jet composition diagnostics. Homan & Lister (2006) Introduction New data: AO 0235+164 Results: AO 0235+164 Discussion : AO 0235+164 The case of OJ287 Conclusions
Multi-Spectral-Range Polarimetric Monitoring of Relativistic Jets in AGN Comprehensive AGN jet monitoring programs at all available spectral ranges allow to locate high energy emission regions Agudo et al. (2011) For this, polarimetry (in general), and VLBI polarimetric imaging (in particular) are an essential ingredients Boston University Blazar Group web page: http://www.bu.edu/blazars/vlbaproject.html Introduction Current Challenges Current Polarization Studies Requirements for SKA1 New Science with SKA1 Further Considerations
Multi-Spectral-Range Polarimetric Monitoring of Relativistic Jets in AGN The method is based on the correlation (and relative localization) of prominent multi spectral range flares along the spectrum from the radio to the γ-ray domains Extreme polarization flares (detected on VLBI images) seem to happen exactly at the time of bright γ-ray flares on some cases, which provides a tool for identification and localization. Robustness of results is based on observational evidence, not on modeling AO 0235+164 total flux light curves AO 0235+164 polarization curves Agudo et al. (2011). See also Marscher et al. (2008, Nature; 2010), Jorstad et al. (2010; 2013), Agudo et al. (2011b)
Needs to Study Relativistic jets in AGN with SKA The nature of relativistic jets in AGN is intimately related to the magnetic fields that thread them, provide them their structure and synchrotron & IC emission, and allow them to be formed. A large fraction of the current problems about relativistic jets in AGN are related to the properties of the innermost regions of such jets, close to the supermassive black holes that produce them, where the strongest Faraday rotation takes place (e.g. Zavala & Taylor 2004, Asada et al. 2008, Gómez et al. 2011, Agudo et al. 2012). Therefore, studying these problems requires angular resolutions of the order of a milliarcsecond or less to access the inner parsec scale region of jets VLBI Synchrotron self-absorption, which reduces its effect with increasing frequency (e.g. Lobanov 1998, Hada et al. 2011), prevents to observe the innermost regions of relativistic jets, where these are still being formed and collimated (if observations are performed at frequencies much lower than 15-20 GHz) High Frequencies
Needs to Study Relativistic jets in AGN with SKA AGN jets are abundant and powerful sources of linearly polarized radiation all over the sky. and up to cosmological distances Ideal background sources to perform Faraday rotation studies of intervening systems But, they display very large RM produced in their cores and inner jets (see above) Zavala & Taylor (2004) Indeed, Taylor et al. (2009) report RM >100 radm^2 for a fraction of sources far from the Galactic plane (measurements from NVSS data) Taylor et al. (2009) A problem for accurate studies of intervening systems if the contribution of the background is not disentangled For such high RM, it is perhaps desirable to measure at high frequencies Introduction Current Challenges Current Polarization Studies Requirements for SKA1 New Science with SKA1 Further Considerations
What New Science Will SKA1 Add for Relativistic Jets in AGN? High sensitivity + high dynamic range + high angular resolution (with VLBI) will allow for eliminating the current sensitivity limit on polarization studies of AGN jets to: Observe the first relativistic jets, and their magnetic properties, from the first AGN even from the epoch of reionization For the first time, circular polarization studies over hundreds of sources (currently limited to a few tens), which would provide reliable statistical studies of jet physics, in particular relativistic jet composition Deep images of jet polarization resolved in the jet transverse direction (important for magnetic field structure analysis) Make detailed studies of thousands of jets from powerful blazars (BL Lacs and quasars) And dozens (perhaps hundreds of) weak radio AGN (Seyferts & LINERs) That would allow to understand the long standing problem of the radio-loud/radio-quiet dichotomy in AGN To observe many examples of weak counter-jets in a large number of objects, hence allowing for unprecedented jet parameters computation All this from the southern hemisphere! Where no parallel observatory or array to those currently available to observe the northern sky is currently available.
Further Considerations on VLBI and High Frequency Requirements VLBI Capability: On which regard to the SKA1, this can be done by connecting a phased SKA array (core) with other radio stations or arrays. This is probably more easily implemented on SKA1-mid (better potential for UV coverage with existing/ planned arrays) If the phased SKA1 array would be included on a VLBI interferometer, SKA would dominate the sensitivity of the entire VLBI array. Therefore, most compact objects seen with the SKA would be detected in VLBI mode involving the SKA phased array Note, VLBI also useful to resolve ambiguities because of source confusion. Frequency coverage up to ~15-22 GHz: Probably more easily implemented on SKA1-mid by employing Wide Band Single Pixel Feeds. Can provide continuous frequency coverage from ~2 to 15 GHz on a single receiver feed. Makes receiver set extremely cheap with regard to several standard receiver boxes for the same (or even smaller) frequency range.