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IRAM 30m telescope detection of the magnetar Swift J1818.0-1607 between 86 and 154 GHz

ATel #14001; Pablo Torne (Instituto de Radioastronomia Milimetrica, IRAM), Kuo Liu (Max-Planck-Institute for Radioastronomy, MPIfR), Ismael Cognard (CNRS & Station Astronomy De Nancay), Gregory Desvignes (LESIA-Paris Observatory & MPIfR), Ramesh Karuppusamy (MPIfR), Michael Kramer (MPIfR), Gabriel Paubert (IRAM), Andrew Lyne (JBCA, University of Manchester), Kaustubh Rajwade (JBCA, University of Manchester), Ben Stappers (JBCA, University of Manchester), Ralph Eatough (NAOC-Beijing), Salvador Sanchez (IRAM), Juan Macias-Perez (LPSC-Grenoble), Bilal Ladjelate (IRAM), Stefano Berta (IRAM), Miguel Sanchez-Portal (IRAM), Santiago Navarro (IRAM), Angel Bongiovanni (IRAM), Carsten Kramer (IRAM), and Karl Schuster (IRAM)
on 8 Sep 2020; 22:09 UT
Credential Certification: Pablo Torne (torne@iram.es)

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

Referred to by ATel #: 14005

Swift J1818.0-1607 is a new pulsar discovered through a soft gamma-ray burst on 2020 March 12 (GCN Circular #27373). A rapid follow up at X-ray and radio wavelengths resulted in early parameter estimation confirming the magnetar nature of the star (see e.g., ATels #13551, #13553, #13559). The initial detections derived a steep radio spectral index (ATels #13560, #13562, #13575), measured to -1.9(2) on 2020 April 8 (ATel #13649). The steep spectral index contrasted with the typical flat or slightly inverted radio spectrum of the other known radio magnetars. However, in 2020 July 15, Atel #13898 reported a significant flattening of the radio spectrum of Swift J1818.0-1607 using simultaneous observations at 2.3 and 8.4 GHz, obtaining a new spectral index of +0.3(2). This spectral flattening was confirmed up to 31.9 GHz (ATel #13966), although with a slightly steeper spectral index of -0.4(2). ATel #13997 reports a spectral index of -0.97(1) between 4 and 22 GHz just 11 hours after the observations of ATel #13966, supporting that in addition to the flattening tendency, there may be a short-term variability of J1818.0-1607's spectrum.

Here, we present detections of the magnetar Swift J1818.0-1607 at simultaneously-observed frequencies of 86, 101, 138 and 154 GHz. The observations were carried out on 2020 August 15 with the Eight Mixer Receiver (EMIR) at the IRAM 30-m Radio Telescope in Granada, Spain, between 18:25 and 21:55 UTC for a total on-source integration time of 2.17 hr. The receiver outputs were connected to a multi-channel broadband continuum backend sampling 8 GHz of bandwidth per frequency band every 100 us. The measured averaged flux density at the folding period of 0.7330382(6) Hz (derived from a lower-frequency ephemeris from Effelsberg observations) at the different frequency bands is S86=1.32(7), S101=1.30(8), S138=0.47(5) and S154=0.74(7) mJy. Assuming that the spectrum is described by a single power law, these values yield an averaged spectral index in the short millimetre band of -1.4(4). The IRAM 30-m observations overlap partly in time with L-band observations from the Nancay Radio Telescope in France at 1.47 GHz, measuring a flux density S1.47=2.01(1) mJy. Including the Nancay data, and assuming again a single power law to describe the radio spectrum, the fit between 1.47 and 154 GHz results in a spectral index -0.15(2).

The difference in spectral index measured between 86 and 154 GHz and when including a simultaneous L-band flux density measurement could be related with a proposed turnover at high frequencies (ATel #13966). An alternative explanation includes a short-term frequency-dependent variability of the magnetar intensity. These detections up to 154 GHz confirm in any case the frequency extent of the spectral flattening and shows that this new magnetar is also detectable in the short millimetre band.

Link to IRAM 30-m pulse profiles of Swift J1818.0-1607 on 2020 August 15