Astronomy & Astrophysics manuscript no. ms1909 c ESO 2019 September 5, 2019 Spatially resolved X-ray study of supernova remnants that host magnetars: Implication of their fossil field origin Ping Zhou1; 2 Jacco Vink1; 3; 4, Samar Safi-Harb5, and Marco Miceli6; 7 1 Anton Pannekoek Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands; e-mail: [email protected] 2 School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China 3 GRAPPA, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands 4 SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 5 Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB R3T 2N2, Canada 6 Dipartimento di Fisica e Chimica E. Segrè, Università degli Studi di Palermo, Palermo, Italy 7 INAF-Osservatorio Astronomico di Palermo, Palermo, Italy Received June 1, 2019; accepted July 15, 2019 ABSTRACT Magnetars are regarded as the most magnetized neutron stars in the Universe. Aiming to unveil what kinds of stars and supernovae can create magnetars, we have performed a state-of-the-art spatially resolved spectroscopic X-ray study of the supernova remnants (SNRs) Kes 73, RCW 103, and N49, which host magnetars 1E 1841−045, 1E 161348−5055, and SGR 0526−66, respectively. The three SNRs are O- and Ne-enhanced and are evolving in the interstellar medium with densities of > 1–2 cm−3. The metal composition and dense environment indicate that the progenitor stars are not very massive. The progenitor masses of the three magnetars are constrained to be < 20 M (11–15 M for Kes 73, . 13 M for RCW 103, and ∼ 13–17 M for N49). Our study suggests that magnetars are not necessarily made from very massive stars, but originate from stars that span a large mass range. The explosion energies of the three SNRs range from 1050 erg to ∼ 2 × 1051 erg, further refuting that the SNRs are energized by rapidly rotating (millisecond) pulsars. We report that RCW 103 is produced by a weak supernova explosion with significant fallback, as such an explosion explains the low 50 −2 −3 explosion energy (∼ 10 erg), small observed metal masses (MO ∼ 4 × 10 M and MNe ∼ 6 × 10 M ), and sub-solar abundances of heavier elements such as Si and S. Our study supports the fossil field origin as an important channel to produce magnetars, given the normal mass range (MZAMS < 20 M ) of the progenitor stars, the low-to-normal explosion energy of the SNRs, and the fact that the fraction of SNRs hosting magnetars is consistent with the magnetic OB stars with high fields. Key words. ISM: individual objects (Kes 73, RCW 103, N49)— ISM: supernova remnants — nuclear reactions, nucleosynthesis, abundances — Pulsars: general — Stars: magnetars 1. Introduction sources usually detected in X-ray and soft γ-ray bands. In recent years, the extremely slowly rotating pulsar 1E 161348−5055 Stars with mass & 8 M end their lives with core-collapse (CC) (P = 6:67 hr) in RCW 103 is also considered to be a magnetar, supernova (SN) explosions (see Smartt 2009, for a review). Two because some of its X-ray characteristics (e.g., X-ray outburst) products are left after the explosion: a compact object (a neutron are typical of magnetars (De Luca et al. 2006; Li 2007; D’Aì star, or a black hole for the very massive stars) and a supernova et al. 2016; Rea et al. 2016; Xu & Li 2019). remnant (SNR). Both products are important sources relevant to numerous physical processes. Since the two objects share a com- The origin of the high magnetic fields of magnetars is still mon progenitor and are born in a single explosion, studying them an open question. There are two popular hypotheses: (1) a dy- together will result in a better mutual understanding of these ob- namo model involving rapid initial spinning of the neutron star jects and their origin. (Thompson & Duncan 1993), (2) a fossil field model involving Magnetars are regarded as a group of neutron stars with ex- a progenitor star with strong magnetic fields (Ferrario & Wick- arXiv:1909.01922v1 [astro-ph.HE] 4 Sep 2019 tremely high magnetic fields (typically 1014–1015 G, see Kaspi ramasinghe 2006; Vink & Kuiper 2006; Vink 2008; Hu & Lou & Beloborodov 2017, for a recent review and see references 2009). The dynamo model predicts that magnetars are born with therein). To date, around 30 magnetars and magnetar candi- rapidly rotating proto-neutron stars (on the order of millisecond), dates have been found in the Milky Way, Large Magellanic which can power energetic SN explosions (or release most of the Cloud (LMC), and Small Magellanic Cloud (Olausen & Kaspi energy through gravitational waves, Dall’Osso et al. 2009). This 2014). For historical reasons, these magnetars are categorised as group of neutron stars is expected to be made from very massive anomalous X-ray pulsars and soft gamma-ray repeaters, based stars (Heger et al. 2005). The fossil field hypothesis predicts that on their observational properties. However, the distinction be- magnetars inherit magnetic fields from stars with high magnetic tween the two categories has blurred over the last 10–20 years. fields. Nevertheless, for the fossil field model, there is still a dis- Unlike the classical rotational powered pulsars, this group of pute on whether magnetars originate preferentially from high- pulsars rotates slowly with periods of P ∼ 2–12 s, large pe- mass progenitors (> 20 M , Ferrario & Wickramasinghe 2006, riod derivatives P˙ ∼ 10−13– 10−10 s s−1, and are highly variable 2008) or less massive progenitors (Hu & Lou 2009). Article number, page 1 of 13 A&A proofs: manuscript no. ms1909 Motivated by the questions about the origin of magnetars, Table 1. Observational information of SNRs that host magnetars. we performed a study of a few SNRs that host magnetars. As the SNRs are born together with magnetars, studying them allows us SNRs obs. ID exposure (ks) obs. time PI to learn what progenitor stars and which kinds of explosion can Kes 73 729 29.6 2000-07-23 Slane create this group of pulsars. Therefore, we can use observations 6732 25.2 2006-07-30 Chatterjee of SNRs to test the above two hypotheses. 16950 29.0 2015-06-04 Borkowski 17668 21.2 2015-07-07 Borkowski In order to get the best constraints of the progenitor masses, 17692 23.6 2015-07-08 Borkowski explosion energies, and asymmetries of SNRs, we selected those 17693 23.1 2015-07-09 Borkowski SNRs showing bright, extended X-ray emission. Among the ten RCW 103 11823 63.3 2010-06-01 Garmire SNRs hosting magnetars (nine in Olausen & Kaspi 2014, and 12224 18.1 2010-06-27 Garmire RCW 103), only four SNRs fall into this category. They are 17460 25.1 2015-01-13 Garmire Kes 73, RCW 103, N49 (in the LMC), and CTB 109. CTB 37B N49 10123 28.2 2009-07-18 Park is another SNR hosting a magnetar, but with an X-ray flux one 10806 27.9 2009-09-19 Park order of magnitude fainter and with sub or near-solar abun- 10807 27.3 2009-09-16 Park 10808 30.2 2009-07-31 Park dances (Yamauchi et al. 2008; Nakamura et al. 2009; Blumer et al. 2019). Here we do not consider HB9, as the association Notes. For Kes 73 and RCW 103, the detector was ACIS-I. For N49, between HB9 and the magnetar SGR 0501+4516 remains un- the detector used was ACIS-S. certain. Vink & Kuiper (2006) and Martin et al. (2014) have studied the overall spectral properties of SNRs Kes 73, N49, and CTB 109 and found that their SN explosions are not ener- getic. In this study, with RCW 103 included and CTB 109 ex- 2.2. Adaptive binning method cluded, we constrain the progenitor masses of the magnetars, provide spatial information about various parameters (such as In order to perform spatially resolved X-ray spectroscopy, we abundances, temperature, density), and explore the asymmetries dissected the SNRs into many small regions and extracted the using a state-of-the-art binning method. We exclude the old- spectrum from each region in individual observations. We em- est member CTB 109 from our sample.1 Therefore, our sam- ployed a state-of-the-art adaptive spatial binning method called ple contains Kes 73, RCW 103, and N49, which host magnetars the weighted Voronoi tessellations (WVT) binning algorithm 1E 1841−045, 1E 161348−5055, and SGR 0526−66, respec- (Diehl & Statler 2006), a generalization of the Cappellari & tively. Their ages have been well constrained, and the spectra Copin (2003) Voronoi binning algorithm, to optimize the data of most regions could be well explained with a single thermal usage and spatial resolution. The same method has been used plasma model (see Sect. 3). The distance of Kes 73 is suggested to analyze the X-ray data of SNR W49B and study its progen- to be 7.5–9.8 kpc using the HI observation by Tian & Leahy itor star (Zhou & Vink 2018). The X-ray events taken from the (2008) and 9 kpc using CO observation (Liu et al. 2017). Here event file are adaptively binned to ensure that each bin contains we take the distance of 8.5 kpc for Kes 73. The distance of a similar number of X-ray photons. Therefore, the WVT algo- RCW 103 is taken to be 3.1 kpc according to the HI observa- rithm allows us to obtain spectra across the SNRs with similar tion (Reynoso et al. 2004, the upper limit distance is 4.6 kpc).