Constraining the Nature of the 18 min Periodic Radio Transient GLEAM-X J162759.5-523504.3 via Multiwavelength Observations and Magneto-thermal Simulations

Abstract

We observed the periodic radio transient GLEAM-X J162759.5-523504.3 (GLEAM-X J1627) using the Chandra X-ray Observatory for about 30 ks on 2022 January 22–23, simultaneously with radio observations from the Murchison Widefield Array, MeerKAT, and the Australia Telescope Compact Array. Its radio emission and 18 min periodicity led the source to be tentatively interpreted as an extreme magnetar or a peculiar highly magnetic white dwarf. The source was not detected in the 0.3–8 keV energy range with a 3σ upper limit on the count rate of 3 × 10−4 counts s−1. No radio emission was detected during our X-ray observations either. Furthermore, we studied the field around GLEAM-X J1627 using archival European Southern Observatory and DECam Plane Survey data, as well as recent Southern African Large Telescope observations. Many sources are present close to the position of GLEAM-X J1627, but only two within the 2″ radio position uncertainty. Depending on the assumed spectral distribution, the upper limits converted to an X-ray luminosity of L X < 6.5 × 1029 erg s−1 for a blackbody with temperature kT = 0.3 keV, or L X < 9 × 1029 erg s−1 for a power law with photon index Γ = 2 (assuming a 1.3 kpc distance). Furthermore, we performed magneto-thermal simulations for neutron stars considering crust- and core-dominated field configurations. Based on our multiband limits, we conclude that (i) in the magnetar scenario, the X-ray upper limits suggest that GLEAM-X J1627 should be older than ∼1 Myr, unless it has a core-dominated magnetic field or has experienced fast cooling; (ii) in the white dwarf scenario, we can rule out most binary systems, a hot sub-dwarf, and a hot magnetic isolated white dwarf (T ≳ 10.000 K), while a cold isolated white dwarf is still compatible with our limits.

Discussions on the Nature of GLEAM-X J162759.5–523504.3

“…If the fallback disk is no longer active and GLEAM-X J1627 can now be treated as an isolated magnetar, then only the latter effect is possible. Whether the fallback disk is active or not is not known at present (Gencali et al 2022;Rea et al 2022;Ronchi et al 2022).…”

“…Optical observations may help to discriminate between the white dwarf and the neutron star models (Hurley-Walker et al 2022;Rea et al 2022). Since neutron stars can sustain smaller periods, future observations of more radio transients with smaller periods may also help to clarify whether they are neutron stars or white dwarfs.…”

Evolutionary implications of a magnetar interpretation for GLEAM-X J162759.5-523504.3

“…Based on the greatest distance allowed by the dispersion measure, dmax = 1.8 kpc, this gives LX 7 × 10 31 erg s −1 for the X-ray luminosity (Hurley-Walker et al 2022). A followup search conducted by Rea et al (2022) implies an even tighter upper-limit for this dmax, LX 2 × 10 30 erg s −1 . In this section, we review models of thermal regulation in magnetars as a means to predict the surface temperature as a function of field strength (Sec.…”

“…These figures illustrate that there is generally an upper limit for the core field strength implied by the absence of X-rays. For example, even if we assume a tiny X-ray efficiency of 0.1% (i.e., LX 10 −3 L ∞ s ), the Chandra observations of GLEAM-X J1627, which translate into an upperlimit of LX ∼ 10 30 erg s −1 (Rea et al 2022), require core field strengths of B 3.1 × 10 14 G for ρ = ρnuc and B 5.5 × 10 14 G for ρ = 4ρnuc, as shown by the dashed, vertical lines in Fig. 5.…”

“…Based on the greatest distance allowed by the dispersion measure, dmax = 1.8 kpc, this gives LX 7 × 10 31 erg s −1 (Hurley- . A deeper search using the Chandra X-ray Observatory was carried out by Rea et al (2022), who placed the even stricter upper limit LX 2 × 10 30 erg s −1 , with the exact bound depending on assumptions about the spectral shape. The latter upper limit would generally be expected of persistent, thermal emissions from a Myr-old magnetar (Thompson & Duncan 1996;Turolla et al 2011;Olausen & Kaspi 2014).…”