Support ATel At Patreon

[ Previous | Next | ADS ]

Significant Flattening of Swift J1818.0-1607's Spectral Index via Dual Radio Frequency Observations with the Deep Space Network

ATel #13898; Walid A. Majid (JPL, Caltech), Aaron B. Pearlman (Caltech), Thomas A. Prince (Caltech, JPL), Charles J. Naudet (JPL), Karishma Bansal (JPL)
on 27 Jul 2020; 21:26 UT
Credential Certification: Aaron B. Pearlman (aaron.b.pearlman@caltech.edu)

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

Referred to by ATel #: 13966, 13997, 14001, 14005

Swift J1818.0-1607 is a recently discovered pulsar, initially detected with Swift's BAT instrument on 2020 March 12 (GCN circular #27373). A 1.36 s rotational period was discovered in a follow-up observation with NICER (ATel #13551), suggesting that the source is a new magnetar. A series of subsequent radio observations, carried out at several wavelengths, have confirmed the presence of periodic pulsed radio emission. The pulsar's rotational frequency derivative of -3.74(1) x 10^-11 s^-2 implies a surface dipolar magnetic field of 3 x 10^14 G and a characteristic age of ~300 years (ATel #13588). Further timing observations with NICER yielded a spin-down luminosity of 1.1 x 10^36 erg s^-1 (ATel #13588), close to that of PSR J1119-6127 [1], a high magnetic field rotation-powered pulsar.

We have carried out a series of radio observations of Swift J1818.0-1607 since its initial discovery using the Deep Space Network (DSN) 70 m diameter radio telescope (DSS-63) in Madrid, Spain at center frequencies of 2.3 GHz (S-band) and 8.4 GHz (X-band). Using data from March and April 2020, we reported the first detection of pulsed radio emission at 8.4 GHz and measured a spectral index of -1.9(2) between 2.3 and 8.4 GHz during a radio observation on 2020 April 8. This measurement indicated that the pulsar's radio spectrum had flattened since our earlier observation on 2020 March 26, from which we placed an upper limit of -2.2 on the spectral index (ATel #13649).

Here, we report results from a radio observation of Swift J1818.0-1607, performed simultaneously at S-band and X-band on 2020 July 15 starting at 19:46:39 UTC, which spanned ~2.1 hours. Strong pulsed radio emission was detected at both frequency bands after folding the data modulo the pulsar's measured rotational frequency of 0.7331469(2) Hz. The mean flux density at S-band and X-band were 0.7(1) mJy and 1.0(2) mJy, respectively. From these measurements, we obtain a spectral index of +0.3(2) between these two frequency bands, assuming that the flux density varies according to a power law. This indicates that the pulsar's radio spectrum has inverted. These latest measurements represent a significant flattening of the spectral index, a hallmark feature of the radio-loud magnetars, and firmly establishes Swift J1818.0-1607 as the fifth member of this rare class of magnetars in the Galaxy. Another notable feature of the pulsed emission during the July 15 observation is the large temporal fluctuations in Swift J1818.0-1607's flux density, shown in the pulse profiles provided at the bottom of this ATel. Additional high frequency radio observations, particularly at frequencies above ~10 GHz, are encouraged to determine the extent and nature of Swift J1818.0-1607's broadband pulsed radio emission.

We thank the DSN team for their rapid response in scheduling this observation.

References:

[1] Archibald et al. ApJ, 869, 180 (2018).

Pulse Profiles of Swift J1818.0-1607 (Deep Space Network, DSS-63, 2.3 and 8.4 GHz, 2020 July 15)