Ultra Wideband radio observations of magnetar Swift J1555.2-5402 at Parkes

ATel #14680; M. Burgay, M. Pilia, A. Possenti (INAF - OACagliari), A. Borghese, F. Coti-Zelati, N. Rea (ICE-CSIC) P. Esposito (IUSS Pavia), G. L. Israel (INAF - OARoma), E. Keane (National University of Ireland), on behalf of a larger collaboration
on 6 Jun 2021; 21:26 UT
Credential Certification: Marta Burgay (burgay@oa-cagliari.inaf.it)

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We report on a search for radio pulsations from the newly discovered magnetar Swift J1555.2-5402 (GCN #30120), for which a periodicity of ~3.861 s was detected by NICER (ATel #14674) and a spin period derivative was derived combining Swift/XRT, NuSTAR and NICER data (ATel #14679). The source was observed using the Ultra Wideband Low-frequency receiver (UWL) of the 64-m Parkes Radio telescope on June 4th from 8:08 UT for 2.16 hrs, and on June 5th from 10:17 UT for 3.16 hr. Data were acquired with the Medusa backend over a 3.3 GHz bandwidth centred at 2.4 GHz, using a frequency resolution of 1 MHz and a sampling time of 0.128 ms. The time-series was 2-bit digitized. Coarser data, recorded over smaller frequency bands, were also acquired using the DFB backend, for a prompt and quicker analysis. In particular, 256 MHz of data centred at a frequency of 1369 MHz were acquired on June 4th, with a frequency resolution of 0.5 MHz, while 1024 MHz centred at 3100 MHz were recorded the following day, with a frequency resolution of 2 MHz. The DFB time series were 2-bit sampled every 0.256 ms.

All the data were folded using the X-ray ephemeris from Israel et al. 2021 (ATel #14679) and searched over a period range spanning +/- 0.3 ms around the nominal spin period at the time of the radio observations, and over a dispersion measure range from 0 to 1500 pc/cm3. UWL data were also split into three sub-bands, from 0.7 to 1 GHz, from 1 to 2 GHz and from 2 to 4 GHz, and folded independently. No periodic pulsations at the X-ray spin period were found in either dataset. Adopting a threshold signal-to-noise ratio (S/N) of 7 and a duty cycle of 10%, and taking into account the difference in sky and system temperature at different frequencies, this translates into the following flux density upper limits for the three aforementioned sub-bands: 0.13 mJy (at 850 MHz), 0.027 mJy (at 1.5 GHz), 0.015 mJy (at 3 GHz), for the longer observation of June 5th.

A preliminary search for single pulses in the DFB data was also carried out and resulted in no good candidates above a S/N of 7, corresponding to a peak flux density upper limit of 0.6 Jy and 0.2 Jy respectively for the 1.3 and 3.1. GHz datasets, for a pulse of duration 1 ms. A more thorough search for single de-dispersed pulses over the full UWL bandwidth, and for simultaneous X-ray flares, is underway and will be reported separately.

We would like to thank the Parkes Observatory director for the prompt scheduling of our NAPA proposal P1083.