1. Markus Böttcher North-West University Potchefstroom South Africa
SALT Spectropolarimetry and Self-Consistent SED and Polarization Modeling of Blazars
Markus Böttcher
North-West University
Potchefstroom
South Africa
Collaborators: Hester Schutte (NWU)
Brian van Soelen, Richard J. Britto1,
Johannes P. Marais (UFS)
David Buckley (SAAO)
1For the Fermi-LAT Collaboration
Supported by the South African Research Chairs Initiative (SARChI) of the Department of Science and Technology and the National Research Foundation of South Africa.

2. Multiwavelength SEDs 3C279 Disk
Often ambiguity in accretion-disk (or SSC?) contribution to the optical spectrum
Disk
(Bottacini et al. 2016)

3. Synchrotron Polarization
For synchrotron radiation from a power-law distribution of electrons with ne (g) ~ g-p → Fn ~ n-a with a = (p-1)/2
= fB,order 𝑝+1 𝑝+7/3 sy
= fB,order 𝛼+1 𝛼+5/3
PPL
p = 2 → P = fB,order * 69 %
p = 3 → P = fB,order * 75 %

4. Compton cross section is polarization-dependent:
Compton Polarization
Compton cross section is polarization-dependent:
e = hn/(mec2):
Thomson regime: e ≈ e’
ds/dW = 0 if e∙e’ = 0
Scattering preferentially in the plane perpendicular to e.
Preferred polarization direction is preserved; polarization degree reduced to ~ ½ of target-photon polarization. p e k p’ e’ k’
=> PSSC ~ Psy/2

5. External-Compton Scattering
External Compton only relevant for scattering by relativistic electrons
Relativistic aberration => approx. axisymmetric rad. field in co-moving frame of e-
=> External-Compton emission ~ Unpolarized:
e- (g)
1/g

6. Multiwavelength Polarization
3C279
Optical Spectropolarimetry
Polarization Degree P Sy
SSC
Total
Disk EC
(Bottacini et al. 2016)

7. Multiwavelength Polarization
3C279
Optical Spectropolarimetry
Wavelength l
Polarization Degree P Sy
SSC
Total
Disk EC
(Bottacini et al. 2016)

8. Multiwavelength Polarization
3C279
Optical Spectropolarimetry
Wavelength l
Polarization Degree P Sy
SSC
Total
Disk EC
(Bottacini et al. 2016)

9. The Southern African Large Telescope (SALT)
Johannesburg
Potchefstroom (NWU)
Carnarvon
(MeerKAT → SKA1)
Sutherland (SALT)
Cape Town

10. The Southern African Large Telescope (SALT)
11m fixed-altitude telescope (largest single optical telescope in the southern hemisphere)
Funded by international consortium (SA, USA, Germany, Poland, India, UK, New Zealand)
Robert-Stobie Spectrograph (RSS): Low – medium resolution (R ~ 5000 – 9000) spectrograph with polarimetry capabilities

11. Spectropolarimetry Observations of Flaring Blazars
SALT Large Programme: Spectroscopy / Spectropolarimetry Follow-up of High-Energy Transients (PI: D. Buckley [SAAO])
Spectropolarimetry Observations of Flaring Blazars
Preliminary
In the first ~ 1.5 years:
~ 15 ToO observations
8 flaring blazars
(incl. IceCube-EHE )

12. Preliminary Example: 4C +01.02 (PKS B0106+013) FSRQ at z = 2.1
Large g-ray
flare in July
2016
SALT spectropol. obs. in July and Nov. 2016
Preliminary
(R. J. Britto)

13. Preliminary 4C +01.02 (PKS B0106+013) Lya C IV 1549 (z = 2.1)
Significant (and time-variable) optical polarization
Decreasing towards shorter wavelength
=> Addition of unpolarized
component (accretion
disk).
Preliminary
(B. v. Soelen, R. J. Britto)

14. 4C +01.02 Previous SED modeling (Ghisellini et al. 2011)
Would predict unpolarized optical emission!
= 14
MBH = 5x109 M0
Ld = 7.5x1046 erg/s
LB/Le = 15
(Ghisellini et al. 2011)

15. Preliminary Preliminary 4C +01.02
Quasi-simultaneous SED + spectropolarimetry modeling:
Preliminary
MBH = 4*108 M0
Ld = 1046 erg/s
fB,order = 0.2
MBH = 5x109 M0 would produce too cold disk
-> fails to produce frequency-dependence of polarization.
Preliminary
Follow-up observations in
Aug (low state):
Very low polarization.

16. Summary
Successful SALT Large Programme for spectroscopy / spectropolarimetry follow-up of transients
7 flaring blazars followed-up in 2016/2017
Spectropolarimetry allows one to disentangle different radiation components (synchrotron vs. disk vs. SSC)
4C (z = 2.1) successfully modelled with synchrotron-dominated optical emission; small disk contribution at
the blue end. Constrain BH mass to ~ 4*108 M0
Supported by the South African Research Chairs Initiative (SARChI) of the Department of Science and Technology and the National Research Foundation of South Africa.