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Swift J1818.0-1607: NuSTAR and INTEGRAL observations of the new magnetar

ATel #13569; A. Borghese, F. Coti Zelati, N. Rea (ICE-CSIC, IEEC), G. L. Israel (INAF), P. Esposito (IUSS), D. Gotz (CEA), V. Savchenko (U. Geneve), A. Tiengo (IUSS), S. Mereghetti, F. Pintore, A. Ridolfi, G. Rodriguez (INAF), D. Vigano (ICE-CSIC, IEEC), on behalf of a larger collaboration
on 19 Mar 2020; 17:44 UT
Credential Certification: Nanda Rea (rea@ice.csic.es)

Subjects: X-ray, Transient, Pulsar, Magnetar

Referred to by ATel #: 13575, 13580, 13588, 13966

We observed the new radio-emitting magnetar Swift J1818.0-1607 (GCN 27373, Atels #13551, #13553, #13554, #13559, #13560, #13562) with NuSTAR and XMM-Newton simultaneously on 2020-03-15, as part of our pre-approved monitoring campaign on new magnetar outbursts. The quick-look NuSTAR data reveal emission up to about 15 keV. The spectrum is well described by an absorbed blackbody plus power law model with blackbody temperature of kT~1 keV and power law photon index of Gamma~3.1 (the absorption column density was fixed to nH=1.2e23 cm^-2, i.e. the value derived from the joint fit of the Swift XRT data). The observed flux on March 15th was ~1.7e-11 erg/s/cm^2 over the 3-15 keV energy band. We detect the spin period of Swift J1818.0-1607 at P = 1.3634995(5) s (1sigma c.l.).

Furthermore, between 2020-03-13 and 2020-03-16, INTEGRAL observed the source three times, but its soft X-ray spectrum did not allow a detection in the hard X-ray band. We derived the following 3sigma upper limits on the observed flux with ISGRI: <1.8e-11 erg/s/cm^2 (28-40 keV) and <3.5e-11 erg/s/cm^2 (40-80 keV).

This new radio magnetar has the largest rotational power of the class (1.3e36 erg/s; Atel #13559), exceeding by a factor of ~10 its current X-ray luminosity during outburst. Its X-ray efficiency and young spin-down age point toward a neutron star born with characteristics in between the high-B pulsar PSR 1846-0258 and the very active magnetar SGR 1806-20.

We thank the NuSTAR, XMM-Newton and INTEGRAL teams for the prompt reaction after our trigger, and for the large effort in planning these and our future observations as simultaneous as possible. We will report on the XMM-Newton data and future X-ray observing schedule as soon as we receive the processed data.