A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger
We discover the first X-ray transient that has no corresponding gamma-ray burst and is powered by a magnetar (i.e., with a magnetic field strength ~1E14-1E15 Gauss) formed during a binary neutron-star merger, which verifies the theoretical prediction that the direct product of a merger of binary neutron stars can be a supramassive (i.e., more massive than typical neutron stars) millisecond magnetar, thereby strongly constraining the basic physics regarding the neutron star equation of state and extreme magnetic field strength.
A neutron star is the compact object formed after a supernova explosion occurring in the late evolutionary stage of a massive star, and it is one of the most mysterious objects in the universe. It is composed of almost all neutrons. It has extreme physical properties such as ultra-high density and super-strong magnetic field. It is an excellent natural laboratory for testing basic physical laws. However, up to now, our understanding about the basic properties of neutron stars (such as the equation of state) is still relatively vague.
Whether the direct product of a binary neutron star merger can be a neutron star, besides the possibility of being a black hole, has not been determined for a long time. Some theoretical studies predict that if the equation of state of neutron stars is sufficiently stiff, that is, the pressure increases sharply with the increase of nuclear density, there should be at least some binary neutron star mergers that would leave behind supramassive millisecond magnetars (see Figure 1) or even stable neutron stars. Because the X-ray radiation powered by a magnetar is basically isotropic, we would expect to see a short-gamma-ray-burst-less X-ray transient with a characteristic light curve, if our viewing angle is off the jet axis. However, such sources have never been discovered.
We find a new X-ray transient (dubbed CDF-S XT2, or XT2 in short) in the 7 Ms Chandra Deep Field-South (7Ms CDF-S), the deepest and most sensitive X-ray survey so far. Its X-ray emission lasts only about 7 hours, and the redshift of its host galaxy is 0.738, lying about 6.6 billion light years away from us. The observational data and theoretical analyses show that: (1) XT2 has no gamma-ray detection. (2) XT2 has a characteristic light curve -- a plateau plus a sharp power-law decay (see Figure 2, Top left), which is in perfect agreement with the theoretical prediction of X-ray emission from post-merger supramassive millisecond magnetars. (3) XT2 is located at the outskirts of its host galaxy (see Figure 2, Top right), which is consistent with the scenario of binary neutron stars being "kicked out" to the galaxy outskirts by the asymmetric counterforce of supernova explosion; furthermore, the probability that XT2 originates from the merger of binary compact stars based on the physical properties of its host galaxy is very high. (4) When the estimated event rate density of transients similar to XT2 is revised to the value in the local universe, it agrees with the value robustly derived from the first gravitational-wave detection (i.e., GW170817) of a binary neutron-star merger, which further supports an origin of binary neutron-star merger for XT2. All the above arguments unanimously indicate that XT2 is very likely the first detected X-ray transient that is powered by a magnetar as the aftermath of a binary neutron-star merger, with no corresponding short gamma-ray burst.
The discovery of XT2 verifies the previous theoretical prediction that the direct product of a binary neutron-star merger can be a supramassive millisecond magnetar, places strong constraints upon the basic physics regarding neutron star equation of state (i.e., must be stiff enough) and ultra-high magnetic field strength (i.e., greater than 1E14-1E15 Gauss), eliminates a number of soft nuclear models of equation of state, deepens our understanding of the basic properties of neutron stars, and provides a new perspective to study binary neutron-star mergers and neutron stars themselves.
See our full paper at Nature.
Figure 1: Artist’s impression of a magnetar as the aftermath of a binary neutron-star merger (courtesy of Guoyan Wang and Cong He).
Figure 2: Some key properties of XT2, including X-ray light curve (Top left), X-ray images at different time frames (Bottom), and offset to the host-galaxy center (Top right).