Detection of single pulses at 1.5 GHz from the new magnetar Swift J1818.0-1607 from Sardinia Radio Telescope L/P-band observations
A. Ridolfi, M. Burgay, A. Corongiu, D. Perrodin, M. Pilia, A. Possenti (INAF-OAC), P. Esposito (IUSS),  G. L. Israel (INAF-OAR), A. Borghese, F. Coti Zelati, N. Rea (ICE-CSIC, IEEC), on behalf of a larger collaboration
Swift J1818.0-1607 is a new magnetar that was recently discovered by Swift after its Burst Alert Telescope detected a short burst on March 12, 2020 (GCN circular 27373). Subsequent NICER observations led to the detection of 1.36-second coherent pulsations in the 2-10 keV X-ray band (ATel #13551)

In the following days, several radio telescopes looked at Swift J1818.0-1607. Pulsations from the source with a 1.36-second periodicity were first detected in the L band (1.4 GHz) part of the radio spectrum by the Effelsberg telescope (ATel #13553), and later confirmed in the same band by the Lovell (ATel #13554) and the MeerKAT telescopes (ATel #13562). The new magnetar was also clearly detected in the S (2 GHz) and C (4-8 GHz) bands by the Green Bank Telescope (ATel #13560, #13575).

We used the Sardinia Radio Telescope (SRT) to observe Swift J1818.0-1607 simultaneously in the P (0.34 GHz) and L (1.5 GHz) radio frequency bands. The source was observed for 1 hour, starting on March 19, 2020 at 05:05 UTC.

In the L band, we recorded the total intensity signal using the Digital Filterbank Mark III in incoherent search mode, with a time resolution of 100 us. After radio frequency interference excision, we obtained an effective usable bandwidth of ~500 MHz, centered at 1548 MHz and divided into 1-MHz wide channels. After de-dispersing the band with a dispersion measure (DM) of 701 pc cm-3, we folded using the nominal position of the source reported in the GCN circular 27373 and the spin parameters measured by Champion et al. (ATel #13559). This led to a clear detection of the magnetar with a signal-to-noise ratio (S/N) of ~16. We extracted topocentric times-of-arrival to obtain local measurements of the DM and of the spin frequency. We measured a DM of 700.8(6) pc cm-3, which is compatible with the earlier estimates. The measured spin frequency (F0) for the epoch MJD 58927.23 is 0.7333920(2) Hz. The latter is also compatible, within 1 sigma, with the value of 0.7333923(5) Hz predicted for the epoch using the spin-down parameters reported by Champion et al. (ATel #13559).

We also carried out a search for single pulses using PRESTO. The data were down-sampled by a factor of four and de-dispersed at our best DM of 700.8 pc cm-3. We then ran the python script "single_pulse_search.py" with a S/N threshold of 8 and a maximum width of 0.1 seconds. We found a total of 41 single pulses emitted from the magnetar. The widths of the single pulses range from 12 to 25 ms approximately, and their S/N (calculated using the PSRCHIVE's "pdmp" routine over each single rotation) range between 10 and 25. We see no pulse sub-structure within the time resolution of our folded profiles (made of 1024 bins, each corresponding to ~1.3 ms), at odds with the multiple peaks seen at higher frequencies by Gajjar et al (ATel #13575).

In the P band, we used the ROACH backend to record baseband data using a central frequency of 336 MHz, and a total bandwidth of 80 MHz. These data were coherently de-dispersed at the DM value measured from the L-band data and folded with our best spin parameters. We report no detection of the magnetar in the P band. We ascribe our non-detection to scattering due to the interstellar medium. Indeed, clear evidence of scattering was already seen in the lower part of the L band by the other telescopes. In particular, Lower et al. (ATel #13562) measured a characteristic scattering time scale of 44 ms at 1 GHz. This translates to a scattering time scale of ~3.45 seconds at 336 MHz, which means that the pulsations are very likely completely smeared in the P band.

Plots of the SRT L-band detection of Swift J1818.0-1607 and a few prominent single pulses can be found at the link below.

We thank the Director of INAF-OAC, Emilio Molinari, and the SRT Head of Operations, Sergio Poppi, for approving our DDT proposal and promptly scheduling the observation.
radio,x-ray,gamma ray,magnetar,neutron star,pulsar,soft gamma-ray repeater,star,transient