Polarimetric detection of the magnetar Swift J1818.0-1607 from 4 to 22 GHz with the Effelsberg 100-m Telescope
ATel #13997; Kuo Liu (Max-Planck-Institute for Radiastronomy, MPIfR), Ramesh Karuppusamy (MPIfR), Ismael Cognard (CNRS & Station Astronomy De Nançay), Gregory Desvignes (LESIA-Paris Observatory & MPIfR), Michael Kramer (MPIfR), Andrew Lyne (JBCA, University of Manchester), Kaustubh Rajwade (JBCA, University of Manchester), Ben Stappers (JBCA, University of Manchester), Pablo Torne (Instituto de Radioastronomia Milimetrica, IRAM)
on 8 Sep 2020; 12:26 UT
Credential Certification: Kuo Liu (kliu.psr@gmail.com)
Subjects: Radio, Neutron Star, Pulsar, Magnetar
Referred to by ATel #: 14001
Swift J1818.0-1607 is a newly discovered magnetar that has been seen to produce both X-ray (ATel #13551, [1]) and radio pulsations (ATel #13553, ATel #13554). Initial follow-up observations after the discovery showed that this magnetar had a steep radio spectrum (ATel #13580, [2]). However, more recent observations strongly suggested that its spectrum has flattened significantly (ATel #13898, ATel #13966).
Here, we report on radio polarimetric detections of Swift J1818.0-1607 from C to K band, using the Effelsberg 100-m Telescope. The observations were carried out at C-X band (6 GHz), Ku band (15.5 GHz), and at K-band (20 GHz), each with 4-GHz bandwidth. The exact starting time (UTC) and duration of the observations are 2020-08-10--21:33 and 20 mins, 2020-08-10--23:19 and 21 mins, and 2020-08-13--20:15 and 48 mins, respectively. The observations on 2020-08-10 were approximately 11 hours after those reported in ATel #13966. The polarization was calibrated based on a 2-min noise-diode exposure preceding the pulsar scans. The rotation measure value (+1442(3) rad m^-2) reported in ATel #13553 was used to correct for the Faraday rotation in the linear polarizations. The flux density was calibrated using on/off scans on planetary nebula NGC 7027.
Detection of the pulse profile has been achieved at all three frequency bands, after folding the data with a pulsar ephemerides obtained from our regular monitoring campaign. The pulse profiles all exhibit significant linear and circular polarizations, seemingly a natural extension from those observed at lower frequencies (ATel #13553, [2]). The measured flux densities are 1.85(1) mJy at C-X band, 0.74(2) mJy at Ku-band on 2020-08-10, and 0.92(2) mJy at K-band on 2020-08-13. Fitting a power-law to the measurements on 2020-08-10 yields a spectral index of -0.97(1), steeper than that reported in ATel #13966. This may suggest a time-varying spectrum of the source on short timescales.
The polarization profiles are available at the following links:
ftp://ftp.mpifr-bonn.mpg.de:/outgoing/pulsar/J1818-1607/CX.png
ftp://ftp.mpifr-bonn.mpg.de:/outgoing/pulsar/J1818-1607/Ku.png
ftp://ftp.mpifr-bonn.mpg.de:/outgoing/pulsar/J1818-1607/K.png
We thank Alex Kraus for scheduling the observing slots and the Effelsberg team for assisting with the observations.
References:
[1] Esposito et al. ApJL, 896, L30 (2020).
[2] Lower et al. ApJL, 896, L37 (2020).