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Spin-evolution of the new magnetar J1818.0-1607

ATel #13559; David Champion (Max-Planck-Institut fuer Radioastronomie, MPIfR), Gregory Desvignes (LESIA-Paris Observatory & MPIfR), Fabian Jankowski (JBCA, University of Manchester), Ramesh Karuppusamy (MPIfR), Michael Keith (JBCA), Chryssa Kouveliotou (The George Washington University), Michael Kramer (MPIfR), Andrew Lyne (JBCA), Mitchell B Mickaliger (JBCA), Brendan O'Connor (The George Washington University), Nataliya Porayko (MPIfR), Kaustubh Rajwade (JBCA), Ben Stappers (JBCA), Pablo Torne (IRAM), Alexander van der Horst (The George Washington University), Patrick Weltevrede (JBCA)
on 17 Mar 2020; 18:08 UT
Credential Certification: Gregory Desvignes (gdesvignes.astro@gmail.com)

Subjects: Radio, Neutron Star, Soft Gamma-ray Repeater, Star, Pulsar, Magnetar

Referred to by ATel #: 13560, 13569, 13575, 13577, 13580, 13588, 13649, 13966, 14001

We report on radio timing observations of the magnetar candidate Swift J1818.0-1607 (GCN circular 27373). After the detection of a pulse period of 1.36 s by NICER (ATel #13551), strong radio pulses were detected with the 100-m Effelsberg radio telescope of the MPIfR and the 76-m Lovell Telescope of the University of Manchester, UK (ATels: #13553, #13554).

At Effelsberg, in addition to the 2ks observations on March 14, 06:37 UT at 1370 MHz, we observed at 2.64 GHz (effective bandwidth of 130 MHz after removing radio frequency interference) for 1.8ks on March 15, 07:25 UT. The data from these observations were coherently dedispersed and folded at the dispersion measure, DM, and period reported in ATel #13553. Furthermore, we observed the source with the Effelsberg 100-m radio telescope at 6 GHz (with an effective bandwidth of 4 GHz) for 4.12 ks on March 17, 04:17 UT but did not detect the source, possibly due to an error in the position reported in GCN circular 27373 and the smaller beam size at higher frequencies.

At the Lovell 76-m telescope, the source was observed at 1536 MHz on March 15, 07:25 UT for a total of 10 ks in separate 30-min observations, on March 16, 03:26 UT for 3 ks, and on March 17, 04:01:17 UT for 5 ks, in addition to the March 14 observations (ATel # 13554), with a consistent observational set-up. On March 15, 16 and 17, the data were recorded not only with the DFB backend, but also at a higher time resolution using a ROACH-based backend which coherently dedispersed and folded the data at the DM and period reported in ATel #13553 over a bandwidth of approximately 250 MHz. The data were cleaned to remove strong radio frequency interference, and the times-of-arrivals generated from both backends were cross-checked.

We confirm the earlier determination of the DM and measure a refined value of 701(1) cm^-3 pc (cf. ATels: #13553, #13554). This value may be modified due to possible profile evolution in radio frequency and time, which are currently unaccounted for.

For a reference epoch of MJD 58922.31 and assuming the position referred to in GCN circular 27373, we measure a spin frequency of 0.7334110(2) Hz. Using the combined Lovell and Effelsberg data sets, we measure a spin-down of the source, expressed in a frequency derivative of F1 = -4.39(1) x 10^-11 s^-2, consistent with the upper limit presented in ATel #13353. The uncertainties in F0 and F1 reflect the possible impact of a likely deviation from the assumed source position. The values translate into a period derivative of P1 = 8.16(2) x 10^-11. Based on the current spin-down, and assuming a braking index of 3, we determine a characteristic age of only 265(1) yrs. This value is very similar to that of SGR J1806-20 with the current smallest characteristic age on record (e.g. Woods et al., 2007, ApJ, 654, 470).

This measured spin-down implies a "characteristic" surface dipole magnetic field of 3.4 x 10^14 G and a spin-down luminosity of 1.3 x 10^36 erg s^-1 (assuming a moment of inertia of 10^38 kg m^2). These values confirm the magnetar nature of the detected source.

We intend to continue the timing observations with the Lovell and Effelsberg telescopes.

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