Detection of pulsed radio emission from new magnetar Swift J1818.0-1607

ATel #13553; Ramesh Karuppusamy (Max-Planck-Institut fuer Radioastronomie, MPIfR), Gregory Desvignes (LESIA-Paris Observatory & MPIfR), Michael Kramer (MPIfR), Nataliya Porayko (MPIfR), David Champion (MPIfR), Pablo Torne (IRAM), Ben Stappers (JBCA, University of Manchester), Alexander van der Horst (The George Washington University), Chryssa Kouveliotou (The George Washington University), Brendan O'Connor (The George Washington University)
on 14 Mar 2020; 22:18 UT
Credential Certification: Gregory Desvignes (gdesvignes.astro@gmail.com)

Tweet

Following the Swift/BAT detection of a short burst from the new soft gamma-ray repeater (SGR) candidate, Swift J1818.0-1607, (GCN circular 27373), Enoto et al. (ATel 13551) reported the detection of coherent pulsations with a spin period of 1.36 s. They suggested that Swift J1818.0-1607 is a new magnetar with a spin period of 1.36 s, which would be the second shortest among the known magnetars (see PSR J1846-0258 in Gavriil, F. et al., Science, 319, 1802, 2008).

Using the 100-m Effelsberg radio telescope of the MPIfR we observed this source on March 14, 06:37 UT, for 2000 s at 1370 MHz in both baseband and incoherent dedispersion mode. Resulting from this observation we can report the detection of strong radio pulses with a barycentric period of 1363.490(3) ms.

The folded radio pulses produce a simple single-peaked radio profile with a significance of about 200-sigma. Given the observing set-up using an effective bandwidth of 250 MHz, this corresponds to an estimated flux density of 0.8(2) mJy at 1370 MHz. Using the coherently dedispersed data, we measure a duty cycle of about ~7% at a 10% intensity level. The dispersion measure is determined to be 706(4) cm^-3 pc. This corresponds to a distance in the range from 4.8 to 8.1 kpc, as determined by the YMW16 and NE2001 electron density models, respectively. The large inferred distance is consistent with pulse smearing due to interstellar scattering visible at the lower parts of the observing bandwidth.

We obtain a barycentric spin frequency of 0.733412(2) Hz, using the incoherently dedispersed data in order to avoid confusion with scattering. This value can be compared with a spin frequency of 0.733417(4) Hz measured by Enoto et al. at 01:38 UT on March 13 after barycentering the NICER event data. We conclude that the radio and X-ray measured spin frequencies are consistent. Based on these measurements, we expect the spin-down in frequency to be less than 10^-10 s^-2. Further observations with the 100-m Effelsberg telescope are underway to study the evolution of the spin frequency.

We measure a very high degree of polarisation in the pulse profile. It is dominated by nearly 100% linearly polarised emission, which exhibits a distinct swing of its position angle, and a small degree of circular polarisation, which changes handedness at the pulse centre. The Rotation Measure is determined to be +1442(3) rad m^-2. The very high degree of polarisation is consistent with that seen from other radio-loud magnetars (e.g. ATEL #5064), lending support to the source's identification as a magnetar.

The polarisation profile is available here:
ftp://ftp.mpifr-bonn.mpg.de/outgoing/ramesh/1818-1607/1818.png

Based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut fuer Radioastronomie) at Effelsberg