Read more about these very peculiar objects: https://en.wikipedia.org/wiki/Magnetar
Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter and have a mass 2-3 times that of the Sun. The density of the interior of a magnetar is such that a thimble full of its substance would have a mass of over 100 million tons. Magnetars are differentiated from other neutron stars by having even stronger magnetic fields, and rotating comparatively slowly, with most magnetars completing a rotation once every one to ten seconds, compared to less than one second for a typical neutron star. This magnetic field gives rise to very strong and characteristic bursts of X-rays and gamma rays. The active life of a magnetar is short. Their strong magnetic fields decay after about 10,000 years, after which activity and strong X-ray emission cease. Given the number of magnetars observable today, one estimate puts the number of inactive magnetars in the Milky Way at 30 million or more.