search

zblz / naima

Derivation of non-thermal particle distributions through MCMC spectral fitting
http://naima.readthedocs.io
BSD 3-Clause "New" or "Revised" License
45 stars 54 forks source link

Doppler boosting #161

Open mireianievas opened 7 years ago

mireianievas commented 7 years ago

Hi,

Looking at the code and the references for the Aharonian radiative models, Just looking into the examples (particularly the SSC one) I'm not sure how doppler boosting and syncrotron self absorption are implemented or if it can be included in an easy way. I'm not a theoritician, so maybe I'm missing something basic :) .

Mireia

zblz commented 7 years ago

Hi Mireia,

Doppler boosting is not applied on any of naima's models. It could in principle be done externally by using the emitter frame of reference to compute the emission with naima (i.e., if there is an external photon field contribution it would have to be boosted appropriately) and they transform to the observer frame of reference.

It might be a nice feature to add.

kbruegge commented 5 years ago

Would also be interested in that. Maybe @asandrock can help out a bit here.

asandrock commented 5 years ago

For processes without external photon fields, such as synchrotron-self Compton models, this is rather easy, since it basically amounts to powers of the Doppler factor which scale the output when boosting to the rest frame of the black hole (if you think about jets in black hole binaries or AGN).

The problem with external photon fields is that naima assumes isotropic photon and electron distributions; when you think about external photon fields in jetted sources, either they are anisotropic from the beginning (e.g. the accretion disk) or even if they are approximately isotropic in the galaxy reference frame (e.g. the broad line radiation in a blazar for emission regions within the broad line region), the electron distribution boosted to the galaxy frame is not, and vice versa the photon distribution boosted to the jet frame is anisotropic even if is isotropic in the black hole rest frame.

There is a method due to Georganopoulos et al. [ApJ 561 111 (2001)] which I think would be comparatively easy to implement for isotropic external radiation fields. It rescales the energies and normalization of the electron distribution to the black hole rest frame by powers of the Doppler factor, and the Compton scattering kernel remains the same as for isotropic internal fields. For anisotropic external fields, you have to integrate over more dimensions which can be numerically tricky.

zblz commented 5 years ago

@asandrock - These sort of considerations where the reasons I did not implement Doppler boosting in naima initially. It's hard to come up with a general enough interface that is useful for any situation.

naima does support anisotropic IC, so that could be used on the transformed photon distribution.

It would be good if we could get an example of using naima with the Georganopoulos et al approach in the examples directory.