The Magnificent Seven

The Magnificent Seven is the informal name of a group of isolated young cooling neutron stars at a distance of 120 to 500 parsecs from Earth. These objects are also known under the names XDINS (X-ray Dim Isolated Neutron Stars) or simply XINS. History The first to fit this classification was RX J1856.5-3754, which was discovered by Walter et al. in 1992, and confirmed as a neutron star in 1996. The term Magnificent Seven was initially applied to the sources RX J1856.5-3754, RBS1556, RBS1223, RX J0806.4-4132, RX J0720.4-3125, RX J0420.0-5022 and MS 0317.7-6647. However, it was soon shown that MS 0317.7-6647 is, in fact, not a neutron star. Then in 2001 a new object fitting this classification was discovered: 1RXS J214303.7+065419/RBS 1774. Since 2001, no new good candidates have appeared. All seven sources were discovered by the ROSAT satellite. Characteristics All seven are recognized to be relatively close by (less than a few hundred parsecs), middle-age (several hundred thousand years) isolated neutron stars emitting soft X-rays due to cooling. The cooling is confirmed by the black body shapes of their spectra. Typical temperatures are about 50–100 electronvolts (57.5–115 kilokelvins (see Electron temperature); for comparison, the Sun's corona has a temperature of about 5 megakelvins). At least six out of the seven show spin periods in the range of approximately 3 to 12 seconds. The light curve shapes are quasisinusoidal and single-peaked. However, RX J1308.6+2127 displays a double-peaked light curve, and in RX J0420.0-5022 there is some evidence for a skewness in the pulse profile, with a slower rise and faster decline. Rather counter-intuitively, the spectrum of both RX J0720.4-3125 and RX J1308.6+2127 becomes harder at pulse minimum.