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

• Published in: Astrophysical journal. Letters Vol. 920; no. 1; p. L4
• Main Authors: Enoto, Teruaki, Ng, Mason, Hu, Chin-Ping, Güver, Tolga, Jaisawal, Gaurava K., O’Connor, Brendan, Göğüş, Ersin, Lien, Amy, Kisaka, Shota, Wadiasingh, Zorawar, Majid, Walid A., Pearlman, Aaron B., Arzoumanian, Zaven, Bansal, Karishma, Blumer, Harsha, Chakrabarty, Deepto, Gendreau, Keith, Ho, Wynn C. G., Kouveliotou, Chryssa, Ray, Paul S., Strohmayer, Tod E., Younes, George, Palmer, David M., Sakamoto, Takanori, Akahori, Takuya, Eie, Sujin
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.10.2021; IOP Publishing
• Subjects: Blackbody; Dipoles; Fluctuations; Flux density; Gamma rays; Luminosity; Magnetars; Magnetic fields; Magnetic properties; Monitoring; Neutron stars; Pulsars; Soft gamma-ray repeaters; Soft x rays; Spectroscopic telescopes; X ray spectra; X-ray transient sources; X-rays
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ac2665

Abstract 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 × 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) × 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 × 10 14 G, 2.0 kyr, and 2.1 × 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 ∼9 × 10 −12 erg s −1 cm −2 with a photon index of ∼1.2. The pulsed fraction has a sharp cutoff at around 10 keV with an upper limit (≲10%) in the hard-tail band. No radio pulsations are detected during the DSN or VERA observations. The 7 σ upper limits of the flux density are 0.043 and 0.026 mJy at the S and X bands, respectively.