Polarisation profiles and rotation measure of PSR J1745-2900 measured at Effelsberg

ATel #5064; K. J.Lee (Max-Planck-Institut fuer Radioastronomie: MPIfR), Ralph Eatough (MPIfR), Ramesh Karuppusamy (MPIfR), David Champion (MPIfR), Evan Keane (Jodrell Bank Centre for Astrophysics: JBCA), Michael Kramer (MPIfR), Dominic Schnitzeler (MPIfR), Aris Noutsos (MPIfR), Bernd Klein (MPIfR), Alex Kraus (MPIfR), Cees Bassa (JBCA), Andrew Lyne (JBCA), Ben Stappers (JBCA), Laura Spitler (MPIfR) , Paulo Freire (MPIfR), Ismael Cognard (CNRS-Orleans), Gregory Desvignes (MPIfR), Patrick Lazarus (MPIfR), Joris Verbiest (MPIfR), Andreas Brunthaler (MPIfR), Heino Falcke (ASTRON, Nijmegen)
on 13 May 2013; 20:01 UT
Credential Certification: Evan Keane (ekean@jb.man.ac.uk)

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We report on improved polarisation measurements, using the Effelsberg telescope, of the newly discovered magnetar, J1745-2900, in the direction of the Galactic Centre (ATel #5020).

We have measured full polarisation (Stokes IQUV) pulse profiles of PSR J1745-2900 at 4.8 and 8.3 GHz. The linear polarisation component is now detected following an increase in our frequency resolution. The degree of linear polarization is nearly 100%. The absence of linear polarisation in our previous measurements (ATel #5058) was due to the Faraday de-polarisation effect. Independent determination of the rotation measure (RM) using 4.8 and 8.3 GHz data suggest a consistent value, RM ~ 66,000 rad/m^2. Thus, as well as having the highest dispersion measure of any known pulsar (ATel #5058), PSR J1745-2900 also has the highest RM by more than an order of magnitude. After the RM correction, the intrinsic polarisation profile of the pulsar can be measured. The high degree of linear polarisation is in agreement with the observations of the other three magnetars seen at radio wavelengths. The polarisation position angle (PA) swing of this pulsar shows an S-shape as expected from a geometrical origin in the so-called rotating vector model. The swing rate is ~ 8 deg/deg (consistent between both frequencies), steeper than previously seen in the other radio magnetars. A detailed analysis will be published elsewhere.

The very high degree of polarisation reported here, together with the extremely high dispersion measure, stable spin-down properties and flat spectrum conclusively show that the source discovered by NuStar is a bona fide radio magnetar, located at the Galactic Centre. The source reported by Burgay et al. (ATel #5035) has a variable period, a low dispersion measure inconsistent with the Galactic Centre and is detectable at low radio frequencies where the magnetar appears to be undetectable due to multi-path scattering. For these reasons it appears that these two sources are unrelated.

In the linked polarisation profile plots red = linear, and blue = circular. Note also that the polarisation angle at the individual frequencies is not absolutely calibrated (i.e. a yet to be determined offset needs to be added).

Polarisation Profile Plots