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The soft gamma-ray repeater Swift J1555.2-5402 was discovered by means of a12-ms duration short burst detected with Swift BAT on 2021 June 3. Then 1.6hours after the first burst detection, NICER started daily monitoring of thisX-ray source for a month. The absorbed 2-10 keV flux stays nearly constant ataround 4e-11 erg/s/cm2 during the monitoring timespan, showing only a slightgradual decline. A 3.86-s periodicity is detected, and the time derivative ofthis period is measured to be 3.05(7)e-11 s/s. The soft X-ray pulse shows asingle sinusoidal shape with a root-mean-square pulsed fraction that increasesas a function of energy from 15% at 1.5 keV to 39% at 7 keV. The equatorialsurface magnetic field, characteristic age, and spin-down luminosity arederived under the dipole field approximation to be 3.5e+14 G, 2.0 kyr, and2.1e+34 erg/s, respectively. An absorbed blackbody with a temperature of 1.1keV approximates the soft X-ray spectrum. Assuming a source distance of 10 kpc,the peak X-ray luminosity is ~8.5e+35 erg/s in the 2--10 keV band. During theperiod of observations, we detect 5 and 37 short bursts with Swift/BAT andNICER, respectively. Based on these observational properties, especially theinferred strong magnetic field, this new source is classified as a magnetar. Wealso coordinated hard X-ray and radio observations with NuSTAR, DSN, and VERA.A hard X-ray power-law component that extends up to at least 40 keV is detectedat 3-sigma significance. The 10-60 keV flux, which is dominated by thepower-law component, is ~9e-12 erg/s/cm2 with a photon index of ~1.2. Thepulsed fraction has a sharp cutoff above 10 keV, down to ~10% in the hard-tailcomponent band. No radio pulsations are detected during the DSN nor VERAobservations. We place 7{\sigma} upper limits of 0.043mJy and 0.026 mJy on theflux density at S-band and X-band, respectively.

A month of monitoring the new magnetar Swift J1555.2-5402 during an X-ray outburst