The Magnificent Seven (neutron stars)
Encyclopedia
The Magnificent Seven is the informal name of a group of isolated young cooling neutron stars near to Earth (within 200 to 500 parsec
s). These objects are also known under the names XDINS (X-ray Dim Isolated Neutron Stars) and XTINS (X-ray Thermal Neutron Stars).
, 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 J185635-3754, 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.
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 repeater
s, anomalous X-ray pulsar
s, rotating radio transient
s, 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.
Data for the table were partly taken from Kaplan (2008), partly from a yet-unpublished review by R. Turolla, and partly from other sources. Temperature estimates vary slightly in different publications. The source RX J0720.4-3125 is variable in temperature and pulsed fraction.
(EOS) of matter at supra-nuclear densities. The most direct way of constraining the EOS is to measure simultaneously the neutron star mass and radius. If a neutron star emits blackbody radiation from its surface of radius
at homogeneous temperature 
, the received flux at distance 
is:
So, if distance
is known and 
can be determined by spectral analysis, the previous relation immediately yields the star radius. Reality is somewhat more complicated, but this oversimplified analysis catches the essence of what is needed in order to measure the neutron star radius: distance, flux and surface temperature. Observing the star thermal emission is therefore crucial. Among all thermally emitting neutrons stars the Magnificent Seven are the only ones with a purely blackbody spectrum. Their clean thermal emission, unmarred by contamination from magnetospheric
activity, a surrounding nebula
or supernova remnant
, makes these sources ideal targets for such a study: the Magnificent Seven are the perfect neutron stars.
Despite many attempts, no radio emission is detected from these sources. The preliminary results from latest deep search with the GBT telescope are presented by Kondratiev et al. There are claims that some signal was detected at very low frequencies, but these results are not very certain and require confirmation.
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s). These objects are also known under the names XDINS (X-ray Dim Isolated Neutron Stars) and XTINS (X-ray Thermal Neutron Stars).
History
The first to fit this classification was RX J1856.5-3754RX J1856.5-3754
RX J1856.5-3754 is a nearby neutron star in Corona Australis. It is believed to have been created by a supernova explosion of its companion star about one million years ago and is moving 108 km/s across the sky...
, 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 J185635-3754, 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
ROSAT
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...
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 bodyBlack body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
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
Light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
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
XMM-Newton
The XMM-Newton is an orbiting X-ray observatory launched by ESA in December 1999 on a Ariane 5 rocket...
. 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 repeater
Soft gamma repeater
A soft gamma repeater is an astronomical object which emits large bursts of gamma-rays and X-rays at irregular intervals. It is conjectured that they are a type of magnetar or, alternatively, neutron stars with fossil disks around them....
s, anomalous X-ray pulsar
Anomalous X-ray pulsar
Anomalous X-ray Pulsars are now widely believed to be magnetars—young, isolated, highly magnetized neutron stars. These energetic X-ray pulsars are characterized by slow rotation periods of ~2–12 seconds and large magnetic fields of ~1013–1015 gauss . There are currently 9 known and 1 candidate...
s, rotating radio transient
Rotating radio transient
Rotating radio transients are sources of short, moderately bright, radio pulses, which were first discovered in 2006. RRATs are thought to be pulsars, i.e. rotating magnetised neutron stars which emit more sporadically and/or with higher pulse-to-pulse variability than the bulk of the known pulsars...
s, 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
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
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
Gould Belt
The Gould Belt is a partial ring of stars in the Milky Way galaxy, about 3000 light years across, tilted toward the galactic plane by about 16 to 20 degrees. It contains many spectral class O- and B-type stars, and may represent the local spiral arm to which the Sun belongs—currently the Sun is...
, 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.
Physical characteristics
| Source, RX J | Spin Periods, s | Amplitude/2 | Temperature, eV | Absorption line energy, eV |
|---|---|---|---|---|
| 1856.5−3754 | 7.06 | 1.5% | 60-62 | no |
| 0720.4−3125 | 8.39 | 11% | 85-87 | 270 |
| 1605.3+3249 (RBS 1556) |
??? | - | 93-96 | 450 |
| 1308.6+2127 (RBS 1223) |
10.31 | 18% | 102 | 300 |
| 2143.0+0654 (RBS 1774) |
9.44 | 4% | 102-104 | 700 |
| 0806.4−4123 | 11.37 | 6% | 92 | 460 |
| 0420.0−5022 | 3.45 | 13% | 45 | 330 |
Data for the table were partly taken from Kaplan (2008), partly from a yet-unpublished review by R. Turolla, and partly from other sources. Temperature estimates vary slightly in different publications. The source RX J0720.4-3125 is variable in temperature and pulsed fraction.
Research
The seven objects seem to be the best laboratory to study neutron star atmospheres and, probably, internal structure. The holy grail of neutron star astrophysics is the determination of the equation of stateEquation of state
In physics and thermodynamics, an equation of state is a relation between state variables. More specifically, an equation of state is a thermodynamic equation describing the state of matter under a given set of physical conditions...
(EOS) of matter at supra-nuclear densities. The most direct way of constraining the EOS is to measure simultaneously the neutron star mass and radius. If a neutron star emits blackbody radiation from its surface of radius
at homogeneous temperature , the received flux at distance is:So, if distance
is known and can be determined by spectral analysis, the previous relation immediately yields the star radius. Reality is somewhat more complicated, but this oversimplified analysis catches the essence of what is needed in order to measure the neutron star radius: distance, flux and surface temperature. Observing the star thermal emission is therefore crucial. Among all thermally emitting neutrons stars the Magnificent Seven are the only ones with a purely blackbody spectrum. Their clean thermal emission, unmarred by contamination from magnetosphericMagnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
activity, a surrounding nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
or supernova remnant
Supernova remnant
A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...
, makes these sources ideal targets for such a study: the Magnificent Seven are the perfect neutron stars.
Despite many attempts, no radio emission is detected from these sources. The preliminary results from latest deep search with the GBT telescope are presented by Kondratiev et al. There are claims that some signal was detected at very low frequencies, but these results are not very certain and require confirmation.