1. The jet of Mrk 501 from millions of Schwarzschild radii down to a few hundreds
Marcello Giroletti
INAF Istituto di Radioastronomia
and
G. Giovannini, G. Taylor, B. Cotton
M.Perez Torres, P. Edwards, M. Chiaberge

2. Outline Introduction Observations & results
High sensitivity: HSA
High resolution: VSOP1 & Global mm-VLBI Array
Open questions & perspectives
Summary

3. Markarian 501 BL Lac @ z = 0.034, HBL high energy: radio
detected by EGRET and TeV Cherenkov
highly active/variable
SSC models require high (d>10) Doppler factor
radio
S5 GHz = 1 Jy
VLA: two sided
pc scales: one sided-jet, twisting
Albert et al. 2007
Cassaro et al. 1999

4. High Sensitivity Array (HSA)
The High Sensitivity Array:
VLBA (10x25m)
phased VLA (27x25m)
Arecibo (300m)
Green Bank (110m)
Effelsberg (100m)
7x more sensitive than the VLBA alone!
Our observations:
8 hrs, 1.6 GHz (optically thin regime), large recording rate (256 Mbps)
final rms noise: 25 mJy beam-1

5. Increase in sensitivty means…
VLBA snapshot
Deep Space VLBI
Our new data: not only a beautiful image but also rich of information
3000% increase in number of pixels above noise

6. HSA: the jet velocity structure
transverse:
slice at 100 mas from core is limb brightened
two velocity regime?
radial
SED/high energy emission require high velocity jet (Γ=15, θ=4o (δ15) in the g-ray region (< 0.02 pc)
one-sided jet confirms relativistic velocity out to 1” from the core
probably larger viewing angle (q=10o-15o) and slower velocity (Γ=10, δ2.6)

7. HSA: the magnetic field structure
polarized intensity as high as 100 mJy/beam
organized magnetic field
B stratified in the inner jet, perpendicular to jet axis after the bend
from adiabatic model: consistent with polarization, requires high initial velocity
helix: intriguing but inconsistent with predictions

8. high res by long baselines: Space VLBI
compact structures are resolved in higher resolution images
no proper motion over 9 epochs
similar to other TeV blazars
limb brightening clearly revealed by VSOP
interpreted in terms of velocity structure: fast inner spine, slower external shear
intermediate viewing angle
important for theoretical jet models
where is its origin?
need even better resolution!
Giroletti et al. 2004

9. high res by higher frequency: mm-VLBI
VLBA GHz, not
the Global mm VLBI array with larger dishes and longer baselines (VLBA+IRAM telescopes+Eb+On+Mh)
resolution ~0.05 mas
still not very sensitve ( Jy)...
...but enough to detect Mrk501

10. GMVA obs of Mrk 501: results
resolution ~110 x 40 mas
rms ~ 1.5 mJy/beam
core unresolved at 86 GHz
size smaller than 0.03 pc (gaussian fit)
MBH=109 Msun, 1 RS=10-4 pc
radius ~ 300 RS
TB > 4 x 109 K
St=150 mJy, Sc=45 mJy
limb brightening in core region?
10 mJy mas, PA 172o
cleaning, model fitting, self-calibration difficult

11. GMVA obs of Mrk 501: results
steep high frequency spectrum, ~8 GHz
magnetic field in the range 0.01 – 0.03 G
structure within the “core”
starting velocity and acceleration?
constraints from g-ray emission? GLAST, AGILE
jet structure
did we eventually find a "real" jet knot?
orientation, limb brightening seem to be in agreement with VLBA at 43 GHz
Marscher et al. 2002

12. VLBI obs of Mrk 501: open questions
1) is limb brightened structure real?
need better sensitivity and (u,v) coverage
2) is spectrum really optically thin?
need more accurate amplitude calibration and (possibly) resolution
3) what is the magnetic field structure?
need sensitivity and polarimetry
4) what do other blazars look like?
need sensitivity and ease of use

13. VLBI observations: perspectives
sub-parsec scale structure:
new mm-VLBI observations just obtained for Mrk501 (8/5/08)
mm-VLBI observations for 4 more blazars to take place next session (10/08)
VSOP2 (launch 2012) will be the key…
connection to high-energy activity:
waiting for GLAST, AGILE results
Medicina and Noto radio telescopes participated in recent multi-l campaign
…stay with us!

14. An announcement… 23-26 September, Bologna: IX EVN Symposium
registration and call for contributions are open!
30 registered participants to date, 100 expected
AGN will be largely discussed
outlook on connection with GLAST, AGILE, ALMA, VSOP, e-VLA…

15. take home notes mrk 501 interesting source
twisting jet, one vs. two sided structure, TeV source
high resolution and sensitivity essential
HSA: perpendicular B field
VSOP1 successful: limb brightening, resolved components
mm-VLBI detection ok, but open questions remain
VSOP2, GLAST needed
also for many more sources!

16. GMVA obs of Mrk 501: open questions
GMVA (u,v)-coverage
HSA polarization

17. HSA polarization

18. Jet velocity and orientation from data:
High velocity jet with Γ =15 and θ = 4o (δ  15) in the gamma-ray region (< 0.02 mas < 0.02 pc) – from high frequency emission, gamma-ray variability, and SED (SSC models)
In the radio jet region the jet orientation move from 4o to 10o - 15o to explain the limb-brightened structure radio structure and Γ decrease to 10 (δ  2.6) at mas from the core ( pc)
The jet velocity decrease is low: from HSA data we estimate relativistic jets (v > 0.6 c at 1” from the core)
At 5” (10 kpc de-projected) the jet is no more relativistic (Γ= v = 0.2c) as expected from FR I jet velocities
Jet PA not constant for unknown reasons
Observational data are in agreement with a simple adiabatic model if the magnetic field is mostly perpendicular to the jet (a parallel field predicts a too fast velocity decrease with respect to the j cj ratio)