The Magnificent Seven (neutron stars)

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) and XTINS (X-ray Thermal Neutron Stars).

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.

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.

A coherent timing solution has been recently obtained for RX J0720.4-3125 and RX J1308.6+2127. The periods are changing by 7 × 10⁻¹⁴ seconds per second and 10⁻¹³ s/s, respectively. The derived dipolar field is 2–3 × 10¹³Gauss and the spin-down ages are 2 and 1.5 million years.

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