On the X-ray efficiency of the white dwarf pulsar candidate ZTF J190132.9+145808.7

Publications of the Astronomical Society of Japan, 2024, 76(4), 702–707 https://doi.org/10.1093/pasj/psae041 Advance access publication date: 2024 May 23 On the X-ray efficiency of the white dwarf pulsar candidate ZTF J190132.9+145808.7 Aya BAMBA ,1 ,2 ,3 ,∗ Yukikatsu TERADA,4 Kazumi KASHIYAMA and Tadayuki TAKAHASHI1 ,6 ,5 ,6 Shota KISAKA,7 Takahiro MINAMI,1 ,6 1 Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Research Center for the Early Universe, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 3 Trans-Scale Quantum Science Institute, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 4 Graduate School of Science and Engineering, Saitama University, 255 Shimo-Ohkubo, Sakura, Saitama 338-8570, Japan 5 Astronomical Institute, Tohoku University, Aoba-ku, Sendai, Miyagi 980-8578, Japan 6 Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan 7 Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan 2 E-mail: Abstract Strongly magnetized, rapidly rotating massive white dwarfs (WDs) emerge as potential outcomes of double degenerate mergers. These WDs can act as sources of non-thermal emission and cosmic rays, gethering attention as WD pulsars. In this context, we studied the X-ray emissions from ZTF J190132.9+145808.7 (hereafter ZTF J1901+14), a notable massive isolated WD in the Galaxy, using the Chandra X-ray obser vator y. Our results showed 3.5σ level evidence of X-ray signals, although it is marginal. Under the assumption of a photon index of 2, we derived its intrinsic flux to be 2.3 (0.9–4.7) × 10−15 erg cm−2 s−1 and luminosity 4.6 (2.0–9.5) × 1026 erg s−1 for a 0.5–7 keV band in the 90% confidence range, given its distance of 41 pc. We derived the X-ray efficiency (η) concerning the spin-down luminosity to be 0.012 (0.0022–0.074), a value comparable to that of ordinary neutron star pulsars. The inferred X-ray luminosity may be compatible with curvature radiation from sub-TeV electrons accelerated within open magnetic fields in the magnetosphere of ZTF J1901+14. Conducting more extensive X-ray observations is crucial to confirm whether ZTF J1901+14-like isolated WDs are also significant sources of X-rays and sub-TeV electron cosmic rays, similar to other WD pulsars in accreting systems. Keywords: acceleration of particles — equation of state — magnetic fields — white dwarfs — X-rays: individual (ZTF J190132.9+145808.7) 1 Introduction Low-mass stars like our Sun evolve into white dwarfs (WDs) at the end of their lifetimes. One-third of stellar objects are believed to be white dwarfs. These objects are significant not only as major constituents of the Galaxy but also as key entities for understanding the physics under high-density environments, the mechanism of SN Ia explosions, and more. Additionally, WDs close to the Chandrasekhar limit are pivotal for the substantial production of neutronized species like 58 Ni and 55 Mn through efficient electron capture processes (Iwamoto et al. 1999; Seitenzahl et al. 2013). Massive and rapidly rotating WDs are of particular interest because they can form through WD–WD mergers (Dan et al. 2014). As a result of such mergers, these massive WDs possess a smaller radius and higher density, consistent with their equations of state (MR1/3 ≈ const., where M and R denote mass and radius respectively; Schwab 2021). A more rapid rotation period is anticipated due to the conservation of angular momentum, although the extent of this conservation remains a topic of study (Schwab 2021). Dipole magnetic fields of such WDs are believed to be intensified by potent dynamo mechanisms during mergers (Tout et al. 2008; García-Berro et al. 2012; Das & Mukhopadhyay 2012). Identifying and quantifying such massive and rapidly rotating WDs are crucial steps in understanding WD–WD merger rates and their associated nucleosynthesis processes. While massive WDs typically exhibit high-temperature colors in the optical band, most have spin periods around 104 s or longer. Recent deep optical surveys have identified several potential remnants of WD–WD mergers with rapid spin periods. One notable candidate identified by the Zwicky Transient Facility is ZTF J190132.9+145808.7 (hereafter ZTF J1901+14). This WD is near the Chandrasekhar limit with a mass between 1.327–1.365 M , a measured radius comparable to the Moon, and a notably short spin period of 416 s (Caiazzo et al. 2021). We propose an innovative approach to investigate these magnetic and rapidly rotating WDs. They are expected to emit nonthermal X-rays as a result of their spin-down, akin to isolated neutron stars, leading to their nickname “white dwarf pulsars.” Such WDs are anticipated to release hard pulsating X-rays, presenting a novel method for their identification. Ostriker et al. (1970) originally proposed this idea, and Sousa et al. (2022) also mentioned this possibility. Observationally, the first such X-ray emission was reported from an accreting magnetized WD, AE Aqr (Terada et al. 2008), which has an incredibly fast rotation period of 33 s. This was followed by discoveries in AR Sco (Buckley et al. 2017; Takata et al. 2018) with a period of 118 s and J191213.72−441045.1 (Pelisoli et al. 2023) with a period of 5.30 min. Their rapid spin, significant magnetic field, and larger radius compared Received: 2023 October 4; Accepted: 2024 April 23 © The Author(s) 2024. Published by Oxford University Press on behalf of the Astronomical Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. ∗ Publications of the Astronomical Society of Japan (2024), Vol. 76, No. 4 703 Table 1. Properties of WD pulsar candidates. Distance d (pc) Mass (M ) Radius R (km) Period P (s) Magnetic field B (MG) Dipole moment μ‡ Spin-down energy Ė ‡ Spin-down flux Ė /4πd 2 References§ ZTF J1901+14 J032900.79−212309.24* J070753.00+561200.25* J221141.80+113604.5* EUVE J0317−855 41 1.327–1.365 2140 416 600–900 0.93–1.39 7.9–17.8 3.4–7.7 (1) 59 1.344 2366† 558 50–100 0.10–0.21 0.03–0.12 0.006–0.02 (2) 87 1.291 2978† 3780 No data No data No data No data (2) 69 1.27 3194† 70 15 0.08 68 10.4 (3) 27 1.34±0.3 2417† 725 450 1 1 1 (4), (5) * SDSS name. Estimated with the best-fitting value and Nauenberg (1972). ‡ Normalized to Ė of EUVE J0317−855. § References: (1) Caiazzo et al. (2021), (2) Kilic et al. (2023), (3) Kilic et al. (2021), (4) Kawka et al. (2007), (5) Harayama et al. (2013). † 2 Target selection and observations Our primary goal is to detec (...truncated)