A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

Abstract

Neutron star-neutron star mergers are known to be associated with short gammaray bursts 1-4 . If the neutron star equation of state is sufficiently stiff, at least some of such mergers will leave behind a supramassive or even a stable neutron star that spins rapidly with a strong magnetic field (i.e., a magnetar) 5-8 . Such a magnetar signature may have been observed as the X-ray plateau following a good fraction (up to 50%) of short gamma-ray bursts 9, 10 , and it has been expected that one may observe short gamma-ray burst-less X-ray transients powered by double neutron star mergers 11, 12 . A fast X-ray transient (CDF-S XT1) was recently found to be associated with a faint host galaxy whose redshift is unknown 13 . Its X-ray and host-galaxy properties allow several possible explanations including a short gamma-ray burst seen off axis, a low-luminosity gamma-ray burst at high redshift, or a tidal disruption event involving an intermediate mass black hole and a white dwarf 13 . Here we report a second X-ray transient, CDF-S XT2, that is associated with a galaxy at redshift z = 0.738 14 .The light curve is fully consistent with being powered by a millisecond magnetar. More intriguingly, CDF-S XT2 lies in the outskirts of its star-forming host galaxy with a moderate offset from the galaxy center, as short bursts often do 15,16 . The estimated event rate density of similar X-ray transients, when corrected to the local value, is consistent with the double neutron star merger rate density inferred from the detection of GW170817 1 .Upon the completion of the deepest X-ray survey to date, the 7 Ms Chandra Deep Field-

Magnetars from Neutron Star–White Dwarf Mergers: Application to Fast Radio Bursts

“…Additionally, during the field amplification, the spin-down torque N dip responds to the magnetic dipole radiation luminosity which follows the evolution of the angular velocity of the shell, as displayed in Figure 2. This Poynting flux could generate a high energy (X-ray/γ-ray) transient lasting hundreds to thousands of seconds via magnetic dissipation with brightness up to L dip,peak ∼ 10 46 erg s −1 , which is likely similar to an X-ray transient source named CDF-S XT2 discovered by Xue et al (2019). Besides the differential rotation, Duncan & Thompson (1992) and Thompson & Duncan (1993) suggested that a key parameter for the success of α − ω dynamo is the Rossby number R O relevant to convection (the ratio of the rotation period to the convective overturn time).…”

Magnetar formation through a convective dynamo in protoneutron stars

“…Because of the rapid rotation, we predict that neutron stars formed in mergers have largescale magnetic fields of magnetar strength generated by a strong field convective dynamo. This will be tested with dedicated numerical simulations and future multimessenger observations (33).…”

Modelling double neutron stars: radio and gravitational waves

“…The post-merger remnant may be a rapidly rotating NS with a high magnetic field (a magnetar), visible in X-ray and gamma rays (e.g. Xue et al 2019). In addition, DNS post merger remnants may also be a source of Fast Radio Bursts (FRBs, e.g.…”