Extreme Physics at Extreme Baselines Andrei Lobanov, MPIfR Bonn - PowerPoint PPT Presentation

Extreme Physics at Extreme Baselines Andrei Lobanov, MPIfR Bonn

VLBI View of AGN Jets  SVLBI and mmVLBI are best direct probes of physics of central engine in AGN: T b , polarization, magnetic field. Poynting flux Kinetic flux dominated dominated Launching region RA, 22GHz VLBI, 215GHz VLBI, 86GHz VLBA, 43GHz 50 µ as in M87 5 R S 2

Uncharted Territory ...  RadioAstron has extended interferometric baselines to uv- spacings of up to 15 G λ . EHT reaches up to ~8 G λ . Both venture into truly uncharted domains.  Narrow range of PA covered by RA space baselines: may be problematic for analysis and even detection. Gómez et al. 2015; talk by Jose Luis tomorrow

... and its Maps  The jets are strongly resolved in RA images. What you see are the brighter „threads“ inside the flow. What are their properties? Polarization, spectrum, and T b should tell this. Vega Garcia et al. 2016; talks by Laura, Manel, and Tuomas

(Brightness) Temperature  „A temperature is a comparative objective measure of ... hot and cold“ (Wikipedia).  ... a microscopic measure of kinetic (thermal) energy stored.  Brightness temperature is a black body temperature needed to emulate what you see from you favorite (not so black) body – e.g., in the Planck regime: or in the Rayleigh-Taylor regime

Getting to that I ν  You want to have I ν , but really measure S over an area Ω .  If you don‘t care about the extent of your region, you need to care about the resolution limit of your instrument. Then  Otherwise, you need to image or model the structure of interest, before you can estimate T b . Take, for instance (as everybody does) an elliptical gaussian:

What If I Can‘t Make That Image? 1) Tough luck, except if you are doing interferometry. 2) Reality of life, if you are doing tough interferometry.  Tough interferometry: -- space, mm-, submm- VLBI -- optical interferometry (often) -- snapshot observations (e.g., geodetic VLBI)  Interferometrist‘s luck: V ( q ) = F I ( r ) Measure S (proxied by V ) and θ (proxied by q -1 ) with every single visibility.  Use it or lose it!

Feeling Lucky  ... must be very easy: 𝐽 𝜉 𝑑 2 𝑇 𝜇 2 𝑈 𝑐 = 2𝑙 𝜉 2 = 2𝑙 Ω . -- Make a single measurement of V on a baseline B . -- Take the proxies 𝑇 → 𝑊 and 𝜄 → 1/ 𝑟 ( Ω → 𝜌 � ) 𝑟 2 -- Recall that 𝑟 = 𝐶 ⁄ 𝜇 And you’ll get 𝐽 𝜉 𝑑 2 𝑊 𝐶 2 𝑈 𝑐 = 2𝑙 𝜉 2 = 2𝜌 𝑙 We’re done! Let’s go and have some party!

That Darn V ( q )= V q exp(- i φ q )  ... comes in different shapes, and is also noisy ( σ q )  Hence you need to know something about V ( q ) at least at two different values of q . For instance, V 0 = V ( q)| q =0 .  Then you can use 𝑊 𝑟 < 𝑊 0 and 𝑊 𝑟 + 𝜏 𝑟 ≤ 𝑊 0 to constrain 𝑈 𝑐 .

Let‘s Commit a Treason  by taking V 0 for a ride... and dropping it underway.  All you need for this is to assume a shape, for instance, a Gaussian, or a disk, or a shell – whatever comes close to the physical reality of your target.  For example, for a Gaussian, one has: 𝑟 𝐶 2 𝑊 and you see now why 𝑈 𝑐 = 2𝜌 𝑙 was a bad idea. ...Well, let‘s also see if we can get rid of V 0

How Low Can One Fall...  while picking one‘s favorite value of V 0 ?  Indeed, there is always a minimum of T b , realized for some V 0 > V q , since 𝑈 𝑐 → ∞ for 𝑊 0 → 𝑊 𝑟 and 𝑊 0 → ∞ .  It‘s at 𝑊 0 = 𝑓 𝑊 𝑟 for the Gaussian.  So, for a given V q , you cannot get brightness temper- ature smaller than no matter how hard you may try. (And we‘re halfway there.)

How High Can One Climb...  while going away from 𝑊 0 = 𝑓 𝑊 𝑟 ?  Possible answers: 𝑊 0 → ∞ (yahoo!) 𝑊 0 = 𝑇 tot (not good) 𝑊 0 = 𝑊 𝑟 + 𝜏 𝑟 (perhaps, the better one)  Then, you‘d get:

A One-Slide Recap  A single visibility amplitude, V q , and its error, σ q , measured at a spatial frequency, q , are sufficient for obtaining estimates of the minimum brightness temperature, and an upper limit of a brightness temperature under the assumption that the structure is resolved at the spatial frequency of the measurement.  Specific expressions for other patterns of brightness distributions can also be derived (see A&A, 574, 84).

Brightness Temperature Runs  ... well within the ( T b,min , T b,lim ) bracket, at all q > 200M λ

And It Does That Even Better  ... for a jet dominated object (NGC 1052)

Prove It For The Masses!  Test on MOJAVE data ( T b from elliptical Gaussian model fits of the compact cores; Kovalev+2005)  Compare with T b,lim from 1% of the longest baselines

Doctoring the Proof  What if T b is determined by transverse size of the jet?

Can We Fail w ith Model Fit?  Yes, we can! Too low T b estimates may result from (unjustifiably) taking too much of the extended structure on board.

Trying It at Gigalambdas  ... only makes things better. Just look at direct comparison with 3mm VLBI data (Lee+2008), made at B > 2 G λ

What Do We Get from RadioAstron?  Most of the AGN imaged with RA show 𝑈 𝑐 , 𝑛𝑛𝑛 ≥ 10 13 K and 𝑈 𝑐 , 𝑚𝑛𝑛 ≥ 10 14 K  Both are well above the IC limit on brightness temperature. 0642+449 BL Lac Lobanov et al. 2015 Gómez et al. 2015

New Physics or Old Calibration Errors?  Seemingly, calibration errors should have yielded too low T b as well, but they didn‘t – more systematic analysis is in Yuri‘s talk on the RadioAstron AGN survey.  Let‘s see what can we do to get those high T b values: T b ~ 10 12 K T b >> 10 12 K e - e + e - p + Emitting particles: Emission: incoherent coherent Particle distribution: power law -> monoenergetic Physical conditions: ~ equilibrium continued injection Geometrical conditions: outside of jet cone inside of jet cone  Eery... the right column could very well describe... a pulsar! or, generically, a highly magnetized object. If so, we may expect high T b to be accompanied by high magnetic field.

What if You Crank Up the B?  Taking a look at a „normal“ IC-loss dominated plasma in a strong magnetic field gives: 𝑈 𝑐 , 𝑛𝑛𝑛 ~ 7 × 10 9 K 𝐶 3 / 4 G  This, of course, implies a sky-rocketing 𝜉 𝑛 ∝ 𝐶 1 / 2 .  However, the rogue 𝜉 𝑛 can be kept low if the plasma particle density 𝑂 0 ∝ 𝐶 −7 / 2 .  This is actualy pretty feasible for: – a „runaway“ TEMZ cell; – a BZ beam inside of BP jet; – a truly „indigenous“ pair creation (for 𝐶 > 10 13 G )

Bottom Lines  A measurement of visibility amplitude, V q , alone is sufficient to derive an estimate of the minimum brightness temperature, T b,min .  A measurement of V q and its error σ q provides an estimate of limiting brightness temperature, T b,lim , under the requirement of the structure to be marginally resolved at the spatial frequency of measurement.  The range ( T b,min , T b,lim ) provides a good bracket for Tb when measurements are done at q >200 M λ .  In some cases (elongated or overly resolved structures), T b,lim is a better estimate of the maximum brightness temperature.

B s and T b s in AGN  The RA estimates of T b,min imply B > 10 5 G.  Good evidence for B~ 10 3 —10 4 G in the nuclear region (recall presentation by Anne-Kathrin).  Perhaps even stronger fields are implied by RM > 10 8 rad/m 2 measured with ALMA (Marti-Vidal+ 2015).  Even higher magnetic fields can be expected for exotic objects such as magnetized rotators (Kardashev 1995) or gravastars (Mazur & Mottola 2001).  The quest for high T b must therefore continue!