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arXiv · 2108.02939

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

Teruaki Enoto·Mason Ng·Chin-ping Hu·Tolga Guver·Gaurava K. Jaisawal·Brendan O'Connor·Ersin Gogus·Amy Lien·Shota Kisaka·Zorawar Wadiasingh·Walid A. Majid·Aaron B. Pearlman·Zaven Arzoumanian·Karishma Bansal·Harsha Blumer·Deepto Chakrabarty·Keith Gendreau·Wynn C. G. Ho·Chryssa Kouveliotou·Paul S. Ray·Tod E. Strohmayer·George Younes·David M. Palmer·Takanori Sakamoto·Takuya Akahori·Sujin Eie
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Abstract

The soft gamma-ray repeater Swift J1555.2-5402 was discovered by means of a 12-ms duration short burst detected with Swift BAT on 2021 June 3. Then 1.6 hours after the first burst detection, NICER started daily monitoring of this X-ray source for a month. The absorbed 2-10 keV flux stays nearly constant at around 4e-11 erg/s/cm2 during the monitoring timespan, showing only a slight gradual decline. A 3.86-s periodicity is detected, and the time derivative of this period is measured to be 3.05(7)e-11 s/s. The soft X-ray pulse shows a single sinusoidal shape with a root-mean-square 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 3.5e+14 G, 2.0 kyr, and 2.1e+34 erg/s, respectively. An absorbed blackbody with a temperature of 1.1 keV 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 the period of observations, we detect 5 and 37 short bursts with Swift/BAT and NICER, respectively. Based on these observational properties, especially the inferred strong magnetic field, this new source is classified as a magnetar. We also 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 detected at 3-sigma significance. The 10-60 keV flux, which is dominated by the power-law component, is ~9e-12 erg/s/cm2 with a photon index of ~1.2. The pulsed fraction has a sharp cutoff above 10 keV, down to ~10% in the hard-tail component band. No radio pulsations are detected during the DSN nor VERA observations. We place 7σ upper limits of 0.043mJy and 0.026 mJy on the flux density at S-band and X-band, respectively.

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Teruaki Enoto, Mason Ng, Chin-ping Hu, Tolga Guver, Gaurava K. Jaisawal, Brendan O'Connor, Ersin Gogus, Amy Lien, Shota Kisaka, Zorawar Wadiasingh, Walid A. Majid, Aaron B. Pearlman, Zaven Arzoumanian, Karishma Bansal, Harsha Blumer, Deepto Chakrabarty, Keith Gendreau, Wynn C. G. Ho, Chryssa Kouveliotou, Paul S. Ray, Tod E. Strohmayer, George Younes, David M. Palmer, Takanori Sakamoto, Takuya Akahori, Sujin Eie. 2021-08-06. A month of monitoring the new magnetar Swift J1555.2-5402 during an X-ray outburst. https://doi.org/10.3847/2041-8213%2Fac2665

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