Diagnosing Magnetic Field Geometry in Blazar Jets Using Multi-Frequency, Centimeter-Band Polarimetry and Radiative Transfer Modeling

Round 1

Reviewer 1 Report

This paper presents a very interesting comparison of numerical radiative transfer simulations to the UMRAO multi-frequency polarimetry data set for the blazar OT 081. The 25 year time span of the UMRAO program is unprecedented and represents a treasure trove of polarization data.

In particular, the authors explore the effect that shock propagation through varying magnetic field topologies (i.e., in the presence/absence of a helical field & with varying contributions of a helical field superimposed onto a turbulent component; Figs.5 & 6) has on the time evolution of the EVPAs at 4.8, 8.0, & 14.5 GHz. The 'simulated' multi-frequency EVPA light-curves in which a helical field is superimposed on a turbulent jet component best match the types of EVPA variations seen in the UMRAO data. This represents an important contribution to the scholarship and this article merits publication.

My one major criticism:

The details of the actual inner workings of the radiative transfer model are hard to follow and should be expanded in Section 3. In particular,

(1) Is this a plasma cell/zone based model? Radiative transfer (and in particular the inclusion of Faraday effects such as rotation and mode conversion) implies the propagation of synchrotron emission through distinct regions of magnetized plasma. I strongly feel that a schematic diagram of the basic model setup would greatly help your readers understanding of the nature of the modelling performed.

(2) Your model's total flux density light-curves (e.g., Fig.2), are (I'm assuming) in units of [Jy]. This implies that assumptions about the jet's magnetic field strength [Gauss], the number density of synchrotron emitting electrons [cm^-3] and the length scale [cm] through the jet/plasma have been made. These values are, however, not present in any of your Tables. Are the parameters I've mentioned here a part of your modeling? If assumptions have been made about these values they should be listed as well.

Minor revisions: 

Fig.1) Make the figure larger - you have lots of unused white space (*applies to all figures). The label "UMRAO Jul 31, 2012" seems not applicable since the x-axis is in time (from 1980-2010).

End of sentence on line 115 *citation needed

The sentence on lines 169-171 *Have you've mixed up 'outer' & 'inner' here; MOJAVE (@15 GHz) probes inner/optically thin regions of the jet whereas UMRAO (@4, 8, & 15 GHz) probes outer/optically thick regions...correct me if I'm wrong.

I feel to make more direct comparisons to the observations, adopting vertical axis limits that match the lower UMRAO panels (especially for the EVPAs) would be helpful. Where the models 'disagree' is also as interesting and informative as where the models 'agree'.

Fig.2) In the lower panels you have the label "UMRAO Data" and the color-coded labels for frequency (i.e., 4, 8, & 15 GHz). In the upper panels you should have a "Model Data" label and I'd also reproduce the color coded frequency labels (*applies to all figures - especially later in the paper where you can't toggle down to the observed data).

Fig.5) & Fig.6) Expressing the field topology as "an ordered multiple of 0.05" etc. is a little hard to follow. You help your reader along in the figure captions by summarizing what this means, i.e. "negligible helical B field". Consider adding the summary sentence as a plot label above each of the four series of panels in figures 5 & 6. This will make it easier to grasp the interesting comparisons you are making here!

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper presents detailed numerical modeling of 3-frequency radio monitoring of flux and polarization of the blazar OT081, whose behavior is representative of other blazars monitoring for several decades by the University of Michigan Radio Astronomy Observatory. The basis for the model is a propagating shock wave in a relativistic jet, which has become the standard scenario for interpreting variations in structure, flux, and polarization. The paper describes the model and applies it to several outbursts of flux in OT081. The study is an excellent example of application of an idealized physical model to interpretation of the data. The general trends are fit quite well; only details that would require a more complex model deviate from the data. I recommend publication after the authors consider my comments and suggestions, given below.

Abstract:

Penultimate sentence: it is unclear what "by energy density" means - is it from a former version and should be deleted? "the ratio ... of the ... magnetic field" seems sufficient without it.

line 88: "thermal" is inappropriate, since the energy distribution is non-thermal; it could just be noted that the Lorentz factor here is E/mc**2.

Line 117: Not all angles of obliquity correspond to shocks, since the velocity component along the shock normal needs to be supersonic. Perhaps change
"any direction" to " a wide range of directions"

Lines 123-126: I do not think that it is true that the computations ignore retarded time effects, since the aberration of the angle of the shock front is largely a retarded time effect. I assume that what is meant is that the change in the physical parameters is treated as occurring simultaneously across the shock, which is a reasonable approximation if the time-scale of change in the parameters is longer than the light-travel time across the line of sight. I suggest that a succinct way of indicating this would be appropriate.

Lines 129-132: The sentence is long and convoluted; I recommend rewording it.

Lines 161-162: I am surprised not to see the energy density of the relativistic electrons and ratio of magnetic to electron energy densities in the underlying flow listed as a key parameter. How are they set?

Lines 171-174 & 260: I am puzzled by the role of the "fiducial `thermal' Lorentz factor." (Again, the word "thermal" is inappropriate, although at least it is placed in
quotes here.) The value of the Lorentz factor whose critical frequency is 8 GHz is determined by the magnetic field, Doppler factor, and redshift. Why should it be set?

Lines 174-175: the word "high" is relative to 1, but it is much lower than the values of 1000-10,000 often used by modelers of SEDs of VHE-emitting BL Lac objects such as OT081. I recommend just writing "to a value of ~50".

Lines 177-179: Aren't the observed rise and decay times of the flares also constraints?

Line 213: The meaning/purpose of "(order multiple)" is unclear. Maybe "see text" would be better.

Table 2: It is not clear what "during spread" refers to in the last column of row 2

Line 251: "EVPA light curves" would probably be better just as "EVPA curves"

Line 289: Any Q vs. U behavior upstream of the 14.5 GHz emission site won't be seen in the light curves. I think that what is meant is "in the upstream end of the centimeter-band emission site."

Line 291: "affect" should be "effect"

Line 296: "the effect" is written twice

Line 443: Redundant, ungrammatical wording: "orientation of the EVPAs relative to the flow direction" should simply be "EVPAs".

Line 445: Is "in a transparent, non-relativistic flow" a necessary qualification to the statement? I don't think that aberration can result in an apparently parallel-to-flow field being far from parallel in the plasma frame. See Lister et al. (1998, ApJ, 504, 702

Line 473: "(panel d)" should not be in parentheses

Line 502: "more that" should be "more than"

Ref. 18: "Marscher" is misspelled as "Marsher"

Author Response

Please see the attachment.

Author Response File: Author Response.pdf