The spin-down rate of the newly discovered magnetar Swift J1555.2-5402

ATel #14679; G. L. Israel (INAF-OAR) , F. Coti Zelati, A. Borghese, R. Sathyaprakash, N. Rea (ICE-CSIC), M. Imbrogno (INAF-OAR), P. Esposito (IUSS Pavia), S. Campana (INAF-OAB), M Burgay, A. Possenti, M. Pilia (INAF-OAC), C. Kouveliotou, B. O'Connor (GWU), E. Gogus (Sabanci University) on behalf of a larger collaboration
on 6 Jun 2021; 14:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nanda Rea (rea@ice.csic.es)

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Swift J1555.2-5402 was discovered with Swift/BAT on June 3rd, 2021, via the detection of a short X-ray burst similar to those emitted by magnetars (Palmer et al., GCN #30120). The same day, a periodic modulation of its X-ray flux with P = 3.8611(1) s was measured using NICER data, which confirmed its magnetar nature (Coti Zelati et al., ATEL #14674). Further NICER and Swift observations showed more bursts from the source, and a luminosity and spectrum typical of magnetars in outburst (Enoto et al., GCN #30126; Evans, ATEL #14675).

We have been monitoring Swift J1555.2-5402 using Swift/XRT in windowed timing mode with daily cadence, and a NuSTAR DDT performed between June 5th and 6th, for a total of ~40 ks. By applying a phase-fitting technique on the data available so far (including ~7 ks of Swift/XRT data, ~38 ks of preliminary NuSTAR data, and ~3 ks of data acquired by NICER on June 3rd), we derived the following timing solution: period P = 3.861006(2) s and period derivative Pdot = (6.0+/-1.2)E-11 s/s (at a reference epoch T0 = 59368.0 MJD; the uncertainties in parenthesis are at 1 sigma).

From the above values of P and Pdot, we infer the following preliminary estimates for Swift J1555.2-5402: a surface dipolar magnetic field at the equator (using the vacuum dipole formula) of B ~ 5e14 G, a spin-down luminosity of ~4e34 erg/s, and a characteristic age of ~1 kyr.

Note that care should be exerted in over interpreting these preliminary estimates, as the period derivative still has a large uncertainty, and it is typically observed to be highly variable in magnetars, especially during the early phases of an outburst.

We shall continue monitoring the source at different wave-bands in the next weeks.