The High-frequency Radio Emission of the Galactic Center Magnetar SGR J1745-29 during a Transitional Period

The origin of the high-frequency radio emission detected from several magnetars is poorly understood. In this paper, we report the ~40 GHz properties of SGR J1745-29 measured using Jansky Very Large Array (JVLA) and Robert C. Byrd Green Bank Telescope (GBT) observations between 2013 October 26 and 2014 May 31. Our analysis of a Q-band (45GHz)GBT observation on 2014 April 10 resulted in the earliest detection of pulsed radio emission at high frequencies (>20GHz); we found that the average pulse has a singly peaked profile with width ~75ms (~2% of the 3.764 s pulse period) and an average pulsed flux density of ~100 mJy. We also detected very bright, short (<10 ms) single pulses during ~70% of this neutron star's rotations, and the peak flux densities of these bright pulses follow the same log-normal distribution as measured at 8.5 GHz. Additionally, our analysis of contemporaneous JVLA observations suggest that its 41/44 GHz flux density varied between ~1-4mJy during this period, with a ~ 2 change observed on ~20 minute timescales during a JVLA observation on 2014 May 10. Such a drastic change over short timescales is inconsistent with the radio emission resulting from a shock powered by the magnetar's supersonic motion through the surrounding medium, but consistent with pulsed emission generated in its magnetosphere.