Studying Galactic interstellar turbulence through fluctuations in
synchrotron emission: First LOFAR Galactic foreground detection
ArXiv 1308.2804 (2013)
Authors:
M Iacobelli, M Haverkorn, E Orrú, RF Pizzo, J Anderson, R Beck, MR Bell, A Bonafede, K Chyzy, R-J Dettmar, TA Enßlin, G Heald, C Horellou, A Horneffer, W Jurusik, H Junklewitz, M Kuniyoshi, DD Mulcahy, R Paladino, W Reich, A Scaife, C Sobey, C Sotomayor-Beltran, A Alexov, A Asgekar, IM Avruch, ME Bell, I van Bemmel, MJ Bentum, G Bernardi, P Best, L Birzan, F Breitling, J Broderick, WN Brouw, M Bruggen, HR Butcher, B Ciardi, JE Conway, F de Gasperin, E de Geus, S Duscha, J Eisloffel, D Engels, H Falcke, RA Fallows, C Ferrari, W Frieswijk, MA Garrett, J Griessmeier, AW Gunst, JP Hamaker, TE Hassall, JWT Hessels, M Hoeft, J Horandel, V Jelic, A Karastergiou, VI Kondratiev, LVE Koopmans, M Kramer, G Kuper, J van Leeuwen, G Macario, G Mann, JP McKean, H Munk, M Pandey-Pommier, AG Polatidis, H Röttgering, D Schwarz, J Sluman, O Smirnov, BW Stappers, M Steinmetz, M Tagger, Y Tang, C Tasse, C Toribio, R Vermeulen, C Vocks, C Vogt, RJ van Weeren, MW Wise, O Wucknitz, S Yatawatta, P Zarka, A Zensus
Abstract:
The characteristic outer scale of turbulence and the ratio of the random to
ordered components of the magnetic field are key parameters to characterise
magnetic turbulence in the interstellar gas, which affects the propagation of
cosmic rays within the Galaxy. We provide new constraints to those two
parameters. We use the LOw Frequency ARray (LOFAR) to image the diffuse
continuum emission in the Fan region at (l,b) (137.0,+7.0) at 80"x70"
resolution in the range [146,174] MHz. We detect multi-scale fluctuations in
the Galactic synchrotron emission and compute their power spectrum. Applying
theoretical estimates and derivations from the literature for the first time,
we derive the outer scale of turbulence and the ratio of random to ordered
magnetic field from the characteristics of these fluctuations . We obtain the
deepest image of the Fan region to date and find diffuse continuum emission
within the primary beam. The power spectrum of the foreground synchrotron
fluctuations displays a power law behaviour for scales between 100 and 8 arcmin
with a slope of (-1.84+/-0.19). We find an upper limit of about 20 pc for the
outer scale of the magnetic interstellar turbulence toward the Fan region. We
also find a variation of the ratio of random to ordered field as a function of
Galactic coordinates, supporting different turbulent regimes. We use power
spectra fluctuations from LOFAR as well as earlier GMRT and WSRT observations
to constrain the outer scale of turbulence of the Galactic synchrotron
foreground, finding a range of plausible values of 10-20 pc. Then, we use this
information to deduce lower limits of the ratio of ordered to random magnetic
field strength. These are found to be 0.3, 0.3, and 0.5 for the LOFAR, WSRT and
GMRT fields considered respectively. Both these constraints are in agreement
with previous estimates.
