The Magnificent Seven (neutron Stars) - Characteristics

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 (eV). 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 period derivatives are 7 10−14 s/s and 10−13 s/s, respectively. The derived dipolar field is 2–3 1013 G and the spin-down ages are 2 and 1.5 million years.

For a long time the Seven were considered to be steady sources, to the point that RX J0720.4-3125 was included among the calibration sources for the EPIC and RGS instruments on board the orbital X-ray telescope XMM-Newton. The continuous monitoring revealed however that the source underwent conspicuous changes in the period 2001-2003. In particular, while the total flux stayed more or less constant, the blackbody temperature steadily increased, going from ~86 to over 90 eV. This was accompanied by a change of the pulse profile, with an increase of the pulsed fraction. More recently this trend seems to have reversed. Starting from 2004, the temperature decreased, and there are hints that the overall evolution may be cyclic, with a period of ~10 years.

The Magnificent Seven represent a large class of young neutron stars with many properties different from normal radio pulsars. There are other types of young isolated neutrons stars which are different from standard radio pulsars, such as soft gamma repeaters, anomalous X-ray pulsars, rotating radio transients, and central compact objects in supernova remnants. Some of them can be related to the Magnificent Seven.

Some of the seven have very weak optical counterparts. For the brightest one (RX J1856-3754), the trigonometric parallax and proper motion are known. The distance to the sources is about 161 parsecs. Similar data is obtained for the second brightest object RX J0720.43125. The distance is ~330 parsecs. Projected velocities are ~280 kilometers per second (km/s) and ~115 km/s, respectively. These data allow astronomers to reconstruct the stars' trajectory and so identify the site of their birth. Distance estimates to other sources can be found in Posselt et al. (2007)

Population synthesis studies show that the Magnificent Seven are related to the Gould Belt, a local group of stars with an age of ~30–50 million years formed by massive stars. Reconstruction of trajectories of neutron stars confirmed this conclusion. In the solar vicinity, these neutron stars outnumber radio pulsars of the same age. This means that the Magnificent Seven-like objects may be one of the most typical young neutron stars with a galactic birth rate larger than that of normal radio pulsars.

XMM-Newton's observations made it possible to detect wide absorption features in spectra of several of the Magnificent Seven. Although their origin is not clear yet (see Haber (2006) for references and more detailed description of the results), it is almost certain that the star strong magnetic field plays a fundamental role in their formation. Absorption features may then provide a powerful diagnostics for the strength of the surface field. At present, two main explanations for their origin have been suggested: either proton cyclotron resonances or atomic transitions in light elements. Interestingly, for the two sources in which a spin-down measure is available, the values of B obtained from spin-down assuming magnetodipolar braking are in reasonable agreement with those inferred from the line energy. Once the nature of the lines has been settled and if an independent measurement of the magnetic field is available (e.g. through spin-down), a measure of the gravitational redshift will be possible, paving the way to the simultaneous determination of both the star mass and radius.