1 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 Fermi/Gamma-ray Burst Monitor detection of SGR J1550–5418 Yuki Kaneko and Ersin G¨ o˘ g¨ u¸s Sabancı University, Orhanlı-Tuzla, ˙ Istanbul 34956, Turkey Space Science Office, VP62, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA Chryssa Kouveliotou Jonathan Granot Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK Enrico Ramirez-Ruiz Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA On behalf of GBM Magnetar Team SGR J1550–5418 exhibited three active bursting episodes in 2008 October and in 2009 January and March, emitting hundreds of typical Soft Gamma Repeater (SGR) bursts in soft gamma-rays. The second episode was especially intense, and our untriggered burst search on Fermi /GBM data (8 − 1000 keV) revealed ∼ 450 bursts emitted over 24 hours during the peak of this activity. Using the GBM data, we identified a ∼ 150-s-long enhanced persistent emission during 2009 January 22 that exhibited intriguing timing and spectral properties: (i) clear pulsations up to ∼ 110 keV at the spin period of the neutron star, (ii) an additional (to a power-law) blackbody component required for the enhanced emission spectra with kT ∼ 17 keV, (iii) pulsed fraction that is strongly energy dependent and highest in the 50 − 74 keV energy band. A total isotropic-equivalent energy emitted during this enhanced emission is estimated to be 2 . 9 × 10 $^{40}$( D/ 5 kpc) 2 erg. The estimated area of the blackbody emitting region of ≈ 0 . 046( D/ 5 kpc) 2 km 2 is the smallest “hot spot” ever measured for a magnetar and most likely corresponds to the size of magnetically-confined plasma near the neutron star surface. 1E 1547.0–5408 was observed with the X-ray Multi- Mirror Mission (XMM-Newton) in 2004 as a magne- tar candidate, selected for its galactic plane location and its relatively soft magnetar-like spectrum as seen with the Advanced Satellite for Cosmology and Astro- physics (ASCA) during their Galactic plane survey [1]. Although no period was detected in the original and follow-up XMM observations, its positional coinci- dence with an extended galactic radio source G327.24- 0.13 (possibly a supernova remnant) also suggested that 1E 1547.0–5408 is a magnetar [2]. The subse- quent discovery in radio observations of a spin period of 2.07 s and a period derivative of 2.3 × 10 − 11 s s − 1 led to an estimated dipole surface field of B ∼ 2 . 2 × 10 14 G and confirmed the source’s magnetar nature; the source was also renamed as PSR J1550–5418 [3]. Its period makes 1E 1547.0–5408 the fastest rotating magnetar; the source is also one of the only two that emit in radio wavelengths [4, 5]. The distance of the source has been estimated to be ∼ 4 − 9 kpc using var- ious methods [2, 3], including the most recent esti- mate of 4 − 5 kpc [6]. Throughout this paper we use D$_{5}$ = D/ 5 kpc as the source distance measure. On 2008 October 3, 1E 1547.0–5408 entered an episode of X-ray activity, emitting several typical SGR-like bursts over the next 7 days. During this We discovered an enhancement of the persistent emission lasting ∼ 150 s by examining the data from the first GBM trigger on January 22. Closer inspec- tion of this enhancement in various energy ranges re- vealed periodic oscillations with a period consistent with the spin period of SGR J1550–5418. Here we present a detailed temporal and spectral analysis of this enhanced emission. In § II, we briefly describe our observations and the GBM instrument and data types. We present our temporal analysis results in § III, and our spectral studies in § IV. Finally we dis- cuss the physical implications of our discovery in § V. Complete descriptions of our analysis and discussion are presented in Kaneko et al. (2010) [11]. The source entered a second period of extremely high X-ray burst activity on 2009 January 22 [8]. Dur- ing the first 24 hours of this bursting episode, the Fermi /GBM triggered on the source 41 times: the number of triggers was limited only by the instru- ment’s capability and did not reflect the actual num- ber of bursts emitted by the source. In fact, our on- ground search for untriggered events revealed a total of ∼ 450 bursts during this 24 hour period: an un- usually high burst frequency from a single source [9]. Based on this SGR-like behaviour, we renamed the source as SGR J1550–5418 [10]. period, 22 short duration bursts were observed with the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope [7]. I. INTRODUCTION arXiv:1001.2644v1 [astro-ph.HE] 15 Jan 2010 eConf C091122