A Month of Monitoring the New Magnetar Swift J1555.2-5402 during an X-Ray Outburst

The soft gamma-ray repeater Swift J1555.2-5402 was discovered by means of a short burst detected with Swift BAT on 2021 June 3. Then, 1.6 hr after the burst, the Neutron star Interior Composition Explorer (NICER) started daily monitoring of this target for a month. The absorbed 2-10 keV flux stayed nearly constant at around 4 x 10(-11) erg s(-1) cm(-2) during the monitoring, showing only a slight gradual decline. An absorbed blackbody with a temperature of 1.1 keV approximates the soft X-ray spectrum. A 3.86 s periodicity is detected, and the period derivative is measured to be 3.05(7) x 10(-11) s s(-1). The soft X-ray pulse shows a single sinusoidal shape with an rms pulsed fraction that increases as a function of energy from 15% at 1.5 keV to 39% at 7 keV. The equatorial surface magnetic field, characteristic age, and spin-down luminosity are derived under the dipole field approximation to be 3.5 x 10(14) G, 2.0 kyr, and 2.1 x 10(34) erg s(-1), respectively. We detect 5 and 45 bursts with Swift/BAT and NICER, respectively. Based on these properties, this new source is classified as a magnetar. A hard X-ray power-law component that extends up to at least 40 keV is detected with the Nuclear Spectroscopic Telescope Array (NuSTAR). The 10-60 keV flux is similar to 9 x 10(-12) erg s(-1) cm(-2) with a photon index of similar to 1.2. The pulsed fraction has a sharp cutoff at around 10 keV with an upper limit (less than or similar to 10%) in the hard-tail band. No radio pulsations are detected during the DSN or VERA observations. The 7 sigma upper limits of the flux density are 0.043 and 0.026 mJy at the S and X bands, respectively.

Automated summary

The soft gamma-ray repeater Swift J1555.2-5402 was discovered through short burst detection by Swift BAT and daily monitoring by NICER, showing a periodicity and pulsed behavior characteristic of magnetars. Derived properties including magnetic field strength, characteristic age, and spin-down luminosity, along with detection of a hard X-ray power-law component by NuSTAR, classify this new source as a magnetar.