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Detection of Radio Pulsations from Swift J1818.0-1607 Simultaneously at 8.3 and 31.9 GHz with the Deep Space Network

ATel #13966; Aaron B. Pearlman (Caltech), Walid A. Majid (JPL, Caltech), Thomas A. Prince (Caltech, JPL), Charles J. Naudet (JPL), Karishma Bansal (JPL), Shinji Horiuchi (CSIRO)
on 24 Aug 2020; 18:41 UT
Credential Certification: Aaron B. Pearlman (aaron.b.pearlman@caltech.edu)

Subjects: Radio, X-ray, Neutron Star, Soft Gamma-ray Repeater, Star, Transient, Pulsar, Young Stellar Object, Magnetar

Referred to by ATel #: 13997, 14001

Swift J1818.0-1607 is a new radio magnetar that was recently discovered using the BAT instrument on board the Neil Gehrels Swift Observatory after an X-ray burst was detected from the source on 2020 March 12 (GCN Circular #27373). Subsequent X-ray and radio observations have revealed detectable pulsations from the magnetar at a rotation period of 1.36 s across a wide range of wavelengths (e.g., see ATels #13551, #13553, #13554, #13559, #13560, #13562, #13569, #13575, #13577, #13580, #13587, #13588, #13649, #13898, [1], and [2]). Measurements of the magnetar's spin frequency and rotational frequency derivative (see ATels #13588 and #13898) imply a young, spin-inferred characteristic age of ~310 years, assuming a braking index of n = 3, and a surface dipolar magnetic field of ~3.1 x 10^14 G, which is comparable to that of other Galactic magnetars [3].

We recently reported that Swift J1818.0-1607's radio spectrum had significantly flattened, based on simultaneous 2.3 and 8.4 GHz observations of the magnetar carried out with the Deep Space Network's 70-m diameter radio telescope, DSS-63 (see ATels #13649 and #13898). The spectral index between these two frequency bands changed from -1.9(2) on 2020 April 8 to +0.3(2) on 2020 July 15. The inverted radio spectrum reported in ATel #13898 is a hallmark characteristic of Galactic radio magnetars, which now solidifies Swift J1818.0-1607 as the newest member of this rare class of pulsars.

Here, we report results from a radio observation of Swift J1818.0-1607 with DSS-35, one of the 34-m Deep Space Network radio telescopes located in Canberra, Australia, following the spectral flattening recently observed from the source. Dual circular polarization data were recorded simultaneously at 8.3 GHz (X-band) and 31.9 GHz (Ka-band) in digital polyphase filterbanks on 2020 August 10, starting at 10:31:00 UTC, with a time and frequency resolution of roughly 512 microseconds and 1 MHz, respectively. The duration of the observation was ~1.6 hours. Pulsed radio emission was detected at both frequency bands after folding the data modulo the magnetar's measured rotational frequency of 0.73309715(8) Hz, which we obtained from a phase-coherent pulsar timing solution. Folded pulse profiles, showing the temporal variability of the radio emission during our observation, are provided at the bottom of this ATel.

The magnetar's mean flux density at 8.3 and 31.9 GHz is 0.7(1) mJy and 0.41(8) mJy, respectively. Assuming that the flux density varies according to a power law, we find that the spectral index is -0.4(2) between these two frequency bands. This spectral index measurement suggests that the magnetar's radio spectrum may have a spectral turnover at high frequencies. The apparent steepening of the radio spectrum toward high frequencies is unlikely to be attributed to variability in the magnetar's X-band flux density, as the X-band flux density reported here is consistent with our previous measurement in ATel #13898, performed roughly 25.6 days earlier. We encourage additional radio observations across a wide range of wavelengths to further characterize the time- and frequency-variability of Swift J1818.0-1607's pulsed radio emission.

We thank the DSN team and the CDSCC staff for their rapid response in scheduling and carrying out this observation.

References:

[1] Esposito et al. ApJL, 896, L30 (2020).

[2] Lower et al. ApJL, 896, L37 (2020).

[3] Kaspi et al. ARA&A, 55, 1, 261 (2017).

Pulse Profiles of Swift J1818.0-1607 (Deep Space Network, DSS-35, 8.3 and 31.9 GHz, 2020 August 10)