A Month of Monitoring the New Magnetar Swift J1555.2-5402 during an X-Ray Outburst
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 × 1014 G, 2.0 kyr, and 2.1 × 1034 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 7σ upper limits of the flux density are 0.043 and 0.026 mJy at the S and X bands, respectively.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2021
- DOI:
- arXiv:
- arXiv:2108.02939
- Bibcode:
- 2021ApJ...920L...4E
- Keywords:
-
- Magnetars;
- Magnetic fields;
- Neutron stars;
- Pulsars;
- Soft gamma-ray repeaters;
- X-ray transient sources;
- 992;
- 994;
- 1108;
- 1306;
- 1471;
- 1852;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 26 pages, 7 figures (3 figures in the appendix), 10 tables (9 tables in the appendix). Submitted to ApJL