X-ray pulsars in SMC
K1 RXTE Observations of X-ray pulsars in SMC
SMC X-2 — outburst in 2000 — LX > 1038 erg s-1 —
P = 2.37 s|
||Authors: R.H.D. Corbet, F.E. Marshall, M.J. Coe, S. Laycock, G. Handler|
||Journal-ref: ApJ 548 (2001) L41 [astro-ph/0011418 ]|
||Title: The Discovery of an Outburst and Pulsed X-ray Flux from SMC X-2 from RXTE Observations|
|Abstract: Rossi X-ray Timing Explorer All Sky Monitor observations of
SMC X-2 show that the source experienced an outburst in January to April 2000 reaching a peak
luminosity of greater than LX = 1038 erg s-1.
RXTE Proportional Counter Array observations during this outburst reveal the presence of pulsations with a
2.37s period. However, optical photometry of the optical counterpart showed the
source to be still significantly fainter than it was more than half a year
after the outburst in the 1970s when SMC X-2 was discovered.
More X-ray Pulsars Found Crowding Neighbouring Galaxy
[5 April 2001] Astronomers have found two, and possibly three, new X-ray pulsars spinning in the
Small Magellanic Cloud (SMC) — the Milky Way's diminutive galactic neighbor and orbital partner.
This brings the total number of pulsars in the SMC to 25 (or 26) — a far higher concentration of pulsars
than in our own galaxy. The pulsars could have been created during a burst of star formation a few million years
ago when the two galaxies were at their closest.
The X-ray Pulsar Signature: As a pulsar spins on its axis, beams of X-rays sweep out from its two
magnetic poles, like the beam of a lighthouse. As the beams sweep past us, we see rapid brightenings
in the X-ray flux one after another, producing a distinct signal.
The 1.0 metre telescope of the South African Astronomical Observatory (SAAO) has been used to identify the
optical counterparts to many X-ray pulsars.
Silas Laycock along with Robin Corbet and others at NASA's Goddard Space Flight Center in Maryland, uncovered
the pulsars with the Rossi X-ray Timing Explorer satellite.
"The discovery of so many X-ray pulsars has been a surprise to most astronomers," Laycock said in a
prepared statement. A few years ago, astronomers were aware of only one pulsar in the SMC. This current rate
of discovery suggests the SMC harbors far more pulsars than have been discovered to date.
"There seems to be 10-times the concentration of X-ray pulsars there compared to the Milky Way," he said.
What is an X-ray pulsar?
An X-ray pulsar is a type of spinning neutron
star — the core remains of a star once several times more massive than our Sun. Such a massive star, upon
depleting its nuclear fuel, violently expels much of its
mass into space in an event called a supernova explosion. The remaining ember, which still packs more
than a Sun's worth of mass into a sphere about 16 kilometers wide, becomes the neutron star.
X-ray pulsars occur in binary star systems. The compact pulsar orbits around a young and unstable
hydrogen-burning star, which is surrounded by a large disk of hydrogen that provides a reservoir of fuel to
power the X-ray pulsar. The tiny pulsar becomes visible and "pulses" in X-ray radiation when the orbit brings
the two stars close to each other.
Gas from the "normal" star's equatorial disk spills over onto the pulsar, attracted by the latter's strong
gravity. The pulsar's enormous magnetic field then channels the gas toward the magnetic poles of the pulsar
where the gas attains speeds up to 20 percent that of light and heats to temperatures far hotter than when it
was part of the healthy star. This gas now glows predominantly in the X-ray band.
Laycock said that binary stars and supernovas with eccentric orbits must be quite common in the Small
An unusual galaxy
The SMC is about 65 kpc away and is the second-closest galaxy to the
Milky Way, visible to the naked eye in the Southern Hemisphere. Its total mass is only one-fiftieth that of
our galaxy's, and yet it seems to contain at least 10 times more X-ray pulsar systems than the Milky Way.
In the SMC astronomers have detected a very high proportion of hot young stars, along with the
still-glowing remains of many recent supernova explosions. These are just two of the clues that provide
a "smoking gun" in the hunt for what produces the SMC's pulsars, suggesting a burst of new-star formation as
recent as about 5 million years ago.
"X-ray pulsars of the kind we are finding in the SMC have a very limited lifetime," said Corbet, of the
Universities Space Research Association. "That we are discovering so many may mean that they all formed at about
the same time."
The role of tidal forces
Such a large star birth rate may well be related to
the fierce gravitational interactions that have taken place between the SMC and Milky Way Galaxy. The SMC
slowly orbits the Milky Way in an elliptical path and, a few million years ago, the two galaxies were at their
Corbet suspects that the Milky Way caused large tidal forces to occur in the SMC, which resulted in the birth
of numerous bright, massive stars. These stars then later exploded to form the X-ray pulsars we see today.
The SMC represents an exceptional target for astronomers to study X-ray pulsar systems. All the
objects are at essentially the same distance and are situated far above the gas- and dust-strewn plane of the
Milky Way, hence providing an excellent testbed for investigating evolutionary theories.
Trying to observe such systems in the Milky Way is fraught with difficulties arising from the extremely
variable interstellar absorption effects in different directions.
Identification of the massive young binary companions was obtained by using the 39-inch (1-meter) telescope
at the South African Astronomical Observatory.
Past X-ray pulsars in the SMC were detected by the Rossi Explorer, the Italian-Dutch BeppoSAX Observatory
and the Japan-U.S. ASCA Observatory.
| — |
||Authors: S. Laycock, R. H. D. Corbet, M. J. Coe, F. E. Marshall, C. Markwardt, J. Lochner|
||Journal-ref: ApJS 161 (2005) 96 [astro-ph/0406420 ]|
||Title: Long Term Behavior of X-ray Pulsars in the Small Magellanic Cloud|
Results of a 4 year monitoring campaign of the SMC using the Rossi X-ray
timing Explorer (RXTE) are presented. This large dataset makes possible
detailed investigation of a significant sample of SMC X-ray binaries. 8 new
X-ray pulsars were discovered and a total of 20 different systems were
detected. Spectral and timing parameters were obtained for 18. In the case of
10 pulsars, repeated outbursts were observed, allowing determination of
candidate orbital periods for these systems. We also discuss the spatial and
pulse period distributions of the SMC pulsars.
K1.2 X-ray sources in SMC
| — |
||Authors: F. Haberl, W. Pietsch|
||Ref: "X-rays from Nearby Galaxies" (2007) [0712.2720 ]|
||Title: X-ray source populations in the Magellanic Clouds|
Early X-ray surveys of the Magellanic Clouds (MCs) were performed with the imaging instruments of the
Einstein, ASCA and ROSAT satellites revealing discrete X-ray sources and large-scale diffuse emission.
Large samples of supernova remnants, high and low mass X-ray binaries and super-soft X-ray sources could
be studied in detail.
Today, the major X-ray observatories XMM-Newton and Chandra with their advanced angular and spectral
resolution and extended energy coverage are ideally suited for detailed population studies of the
X-ray sources in these galaxies and to draw conclusions on our own Galaxy.
We summarize our knowledge about the X-ray source populations in the MCs from
past missions and present first results from systematic studies of the Small
Magellanic Cloud (SMC) using the growing number of archival XMM-Newton observations.
The study of X-ray source populations and diffuse X-ray emission in nearby galaxies is of major importance in
understanding the X-ray output of more distant galaxies as well as learning about processes that occur on
interstellar scales within our own Galaxy.
The MCs, satellites of the Milky Way, show different chemical compositions, are irregular in shape, and
are heavily interacting with the Milky Way. This influences the process of star formation
and the study of stellar populations in the MCs is particularly rewarding. Their proximity makes the MCs the
ideal galaxies for X-ray studies.
Previous X-ray surveys of the MCs, which were performed with the imaging instruments of the Einstein, ASCA
and ROSAT satellites, revealed discrete X-ray sources and large-scale diffuse emission. The early
Einstein observations unveiled more than one hundred point-like sources in the Large Magellanic Cloud (LMC;
Long et al., 1981; Wang et al., 1991) and seventy in the SMC (Wang & Wu, 1992). ASCA found more than 100
sources and detected coherent pulsations from 17 sources in the SMC (Yokogawa et al., 2003). In particular the
high sensitivity and the large field of view of the ROSAT PSPC provided the most comprehensive catalogues of
discrete X-ray sources in the directions of the LMC
(758 in an area of ~59 square degrees; Haberl & Pietsch, 1999)
and the SMC (517 in an area of ~18 square degrees; Haberl et al., 2000).
Together with ROSAT HRI observations this yielded about 1000 and 550 X-ray sources in the
areas of the LMC and SMC, respectively (Sasaki et al., 2000a,b). A spectral analysis of the emission from the
hot thin plasma in the interstellar medium (ISM) using ROSAT PSPC data of the MCs revealed temperatures
between 1 and 10 MK (Sasaki et al., 2002).
2. Source populations
About fifty HMXB pulsars are now known in the SMC together with many more candidates which exhibit similar
X-ray properties, but yet without detected pulsations which reveal the spin period of a neutron star
(e.g., Haberl & Sasaki, 2000; Haberl & Pietsch, 2004).
Many of the Be-HMXBs in the SMC were discovered during outburst (often exceeding
1037 erg s-1 in X-ray luminosity), in particular with ASCA
(Yokogawa et al., 2003) and RXTE (Laycock et al., 2005).
|HMXB pulsars in the SMC|
Image credit: Haberl & Pietsch (2007)
Comparison of the pulse period distribution of HMXB pulsars in the Milky Way
and the SMC.
Despite the large difference in galaxy mass, nearly as many HMXB pulsars are known (status 2006) in
the Milky Way (53) and
the SMC (46).
In both galaxies most of the pulsars are found with spin periods between 100 s and 1000 s.
The fraction of pulsars with spin periods between 1 s and 100 s is slightly higher in the SMC
as compared to the Milky Way.
In nearly all cases in the SMC the mass donor star is a Be star. When the binary orbit is wide and
eccentric, the passage of the neutron star close to the disc results in X-ray outbursts and a transient
behaviour of the Be-HMXBs.
Luminous super-soft X-ray sources were discovered with the Einstein observatory (CAL 83 and CAL 87) and
were established as a new class of X-ray binaries after the ROSAT discoveries of five new such objects in the
SMC (Kahabka et al., 1994; Greiner, 1996). SSSs exhibit very soft X-ray spectra (characteristic temperatures kT
of a few tens of eV) and show a variety of intensity variations on different time scales (little variations,
slow exponential decay over years, transient outbursts, off-states).
The most popular model for SSS involves nuclear burning on the surface of an accreting white dwarf (WD)
which can explain the observed luminosities. The WDs in SSS indicate an older population consistent with their
distribution mainly in the outer parts of the MCs. After the detection in the MCs, SSSs were also found in
other local group galaxies.
Interestingly the majority of SSSs in M31 was identified with optical novae which enter a SSS state some
time after optical outburst with onset and duration of the SSS state varying strongly from source to source.
The MCs are sufficiently close to detect SSSs at low luminosities which allows us to address the question if
permanently low-luminosity (too low for the high accretion rates inferred from the models) SSSs exist,
or if they are highly variable which could point to unstable nuclear burning. XMM-Newton observations of
faint SSSs in MC fields show that this class is composed of very different objects. Symbiotic stars,
central stars of planetary nebulae and even a Be star were identified as optical counterparts.
Be/WD systems are predicted to be more numerous than Be/neutron star binaries and the fact that we have
so far not discovered any clear Be/WD case needs to be explained by binary evolution models.
Haberl, F., Filipovi´c, M. D., Pietsch, W., & Kahabka, P. 2000, A&AS, 142, 41
Haberl, F. & Pietsch, W. 1999, A&AS, 139, 277
Haberl, F. & Sasaki, M. 2000, A&A 359, 573
Haberl, F. & Pietsch, W. 2004, A&A 414, 667
Haberl, F. & Pietsch, W. 2005, A&A 438, 211
Kahabka, P., Pietsch,W., & Hasinger, G. 1994, A&A 288, 538 (5 SSSs in SMC)
Super-soft X-ray sources in the fields of the Magellanic Clouds
Laycock, S., Corbet, R. H. D., Coe, M. J., et al. 2005, ApJS, 161, 96
Long, K. S., Helfand, D. J., & Grabelsky, D. A. 1981, ApJ 248, 925
Sasaki, M., Haberl, F., & Pietsch, W. 2002, A&A 392, 103
Wang, Q., Hamilton, T., Helfand, D. J., & Wu, X. 1991, ApJ 374, 475
Yokogawa, J., Imanishi, K., Tsujimoto, M., Koyama, K., & Nishiuchi, M. 2003, PASJ, 55, 161
X-ray outburst of the 6.85 s pulsar XTE J0103-728
XTE J0103-728 — P = 6.85 s
— LX(0.2-10 keV) = 2 × 1037 erg s-1|
||Authors: F. Haberl, W. Pietsch|
||Journal-ref: A&A (2008) [0801.4679 ]|
||Title: XMM-Newton observations of the Small Magellanic Cloud:
X-ray outburst of the 6.85 s pulsar XTE J0103-728|
A bright X-ray transient was seen during an XMM-Newton observation in the direction of the Small Magellanic Cloud
(SMC) in October 2006.
The EPIC data allow us to accurately locate the source and to investigate its temporal and spectral behaviour.
X-ray spectra covering 0.2-10 keV and pulse profiles in different energy bands were extracted from the EPIC data.
The detection of 6.85 s pulsations in the EPIC-PN data unambiguously identifies the transient with XTE J0103-728, discovered
as 6.85 s pulsar by RXTE. The X-ray light curve during the XMM-Newton observation shows flaring activity of
the source with intensity changes by a factor of two within 10 minutes.
Modelling of pulse-phase averaged spectra with a simple absorbed power-law indicates systematic residuals
which can be accounted for by a second emission component. For models implying blackbody emission,
thermal plasma emission or emission from the accretion disk (disk-blackbody), the latter yields physically
sensible parameters. The photon index of the power-law of ~0.4 indicates a relatively hard spectrum.
The X-ray luminosity was
LX(0.2-10 keV) = 2 × 1037 erg s-1
with a contribution of ~3% from the disk-blackbody component.
A likely origin for the excess emission is reprocessing of hard X-rays from
the neutron star by optically thick material near the inner edge of an accretion disk.
From a timing analysis we determine the pulse
period to 6.85401 s indicating an average spin-down of ~0.0017 s per year since the discovery of
XTE J0103-728 in May 2003.
The X-ray properties and the identification with a Be star confirm XTE J0103-728 as Be/X-ray binary transient
in the SMC.
The transient pulsar XTE J0103-728 with a period of 6.8482±0.0007 s was discovered during RXTE observations of
the Small Magellanic Cloud (SMC). The pulsations were detected on 2003, Apr. 29, May 7, May 15 and May 19 but not
on Apr. 24 and May 28 (Corbet et al. 2003), suggesting an X-ray outburst which lasted about three to four weeks.
This behaviour is characteristic for Be/X-ray binaries undergoing a type II outburst which is caused by
the ejection of matter from the Be star and enhanced accretion onto a compact object, in most
cases a neutron star (Negueruela 1998).
Shorter outbursts (type I) with a duration of a few days are often separated by the orbital
period of the binary system. They generally occur close to the time of periastron passage of the neutron star
when the neutron star approaches the circumstellar disc of the Be star (see e.g. Okazaki & Negueruela 2001).
Be/X-ray binaries form the major class of High Mass X-ray Binaries (HMXBs). In the remaining systems - the
supergiant HMXBs - the compact object accretes matter from the fast stellar wind of an early type O or B
supergiant star. In the SMC more than 60 Be-HMXBs are known, while only one HMXB is
established as supergiant system (SMCX-1).
Recent reviews of the optical and X-ray properties of these systems can be found
in Coe et al. (2005) and Haberl & Pietsch (2004), respectively.
Coe, M. J., Edge, W. R. T., Galache, J. L., & McBride, V. A. 2005, MNRAS, 356, 502
Haberl, F. & Pietsch, W. 2004, A&A 414, 667
Haberl, F. & Pietsch, W. 2005, A&A 438, 211
Negueruela, I. 1998, A&A 338, 505
Okazaki, A. T. & Negueruela, I. 2001, A&A 377, 161
13 supernova remnants (SNR) in the Small Magellanic Cloud
| — |
||Authors: K.J. van der Heyden, J.A.M. Bleeker, J.S. Kaastra|
||Journal-ref: A&A 421 (2004) 1031-1043 [astro-ph/0309030 ]|
||Title: Synoptic study of the SMC SNRs using XMM-Newton|
|Abstract: We present a detailed X-ray spectral analysis of 13 supernova
remnants (SNR) in the Small Magellanic Cloud (SMC). We apply both
single-temperature non-equilibrium ionisation models and models based on the
Sedov similarity solution, where applicable. We also present detailed X-ray
images of individual SNRs, which reveal a range of different morphological features.
Eight remnants, viz DEM S 32, IKT 2, HFPK 419, IKT 6, IKT 16, IKT 18
and IKT 23, are consistent with being in their Sedov evolutionary phase. IKT 6
and IKT 23 both have a clear shell like morphology with oxygen-rich X-ray emitting material in the centre.
We draw attention to similarities between these two remnants and the well studied, oxygen-rich remnant
IKT 22 (SNR 0102-72.3) and propose that they are more evolved versions of IKT 22.
IKT 4, IKT 5, DEM S 128 and IKT 5 are evolved remnants which are in, or in the
process of entering, the radiative cooling stage. We argue that the X-ray
emission from these four remnants is most likely from the ejecta remains of type Ia SNe.
Our modeling allow us to derive estimates for physical
parameters, such as densities, ages, masses and initial explosion energies.
Our results indicate that the average SMC hydrogen density is a factor of ~6
lower as compared to the Large Magellanic Cloud. This has obvious implications
for the evolution and luminosities of the SMC SNRs.
We also estimate the average SMC gas phase abundances for the elements O, Ne, Mg, Si and Fe.
K3 SNR 0102-72.3 — A Young, Oxygen-rich Supernova Remnant
The young SNR 1E 0102.2-7219: testing dust formation in primordial galaxies
Stanimirovic, S., Bolatto, A. D., Sandstrom, K., Leroy, A. K., Simon, J. D., Gaensler,
B. M., Shah, R. Y., Jackson, J. M.,
ApJ 632 (2005) L103 [astro-ph/0509786 ]
- Common Name: 1E 0102.2-7219
- Distance: 60kpc(distance to SMC, Westerlund, 1990)
- Center of X-ray emission (J2000): ( 01 04 02.0, -72 01 52.7 )
- X-ray size: 0.72x0.75
- Description: Small, bright, circular shell
| — |
||Authors: Gaetz, T. J.; Butt, Yousaf M.; Edgar, Richard J.; Eriksen, Kristoffer A.; Plucinsky,
Paul P.; Schlegel, Eric M.; Smith, Randall K.|
||Journal-ref: ApJ 534 (2000) L47-L50 [astro-ph/0003355 ]|
||Title: Chandra X-Ray Observatory Arcsecond Imaging of the Young, Oxygen-rich Supernova Remnant 1E 0102.2-7219|
|Abstract: We present observations of the young, oxygen-rich
supernova remnant 1E 0102.2-7219 taken by the Chandra X-Ray Observatory during
its orbital activation and checkout phase. The boundary of the blast-wave shock
is clearly seen for the first time, allowing the diameter of the remnant and the
mean blast-wave velocity to be determined accurately. The prominent X-ray bright
ring of material may be the result of the reverse shock encountering ejecta; the
radial variation of O VII versus O VIII emission indicates an ionizing shock
propagating inward, possibly through a strong density gradient in the ejecta. We
compare the X-ray emission with Australia Telescope Compact Array 6 cm radio
observations (Amy & Ball) and with archival Hubble Space Telescope [O III]
observations. The ring of radio emission is predominantly inward of the outer
blast wave, which is consistent with an interpretation of synchrotron radiation
originating behind the blast wave but outward of the bright X-ray ring of
emission. Many (but not all) of the prominent optical filaments are seen to
correspond to X-ray bright regions. We obtain an upper limit of
~9×1033 ergs s-1 (3 sigma) on any potential pulsar X-ray
emission from the central region.
| — |
||Authors: M. Sasaki, T.F.X. Stadlbauer, F. Haberl, M.D. Filipovi'c, P.J. Bennie|
||Journal-ref: A&A(2001) [astro-ph/0011117 ]|
||Title: XMM-Newton EPIC Observation of SMC SNR 0102-72.3|
|Abstract: Results from observations of the young oxygen-rich supernova
remnant SNR 0102-72.3 in the Small Magellanic Cloud during the calibration
phase of the XMM-Newton Observatory are presented. Both EPIC-PN and MOS
observations show a ringlike structure with a radius of ~15'' already known
from Einstein, ROSAT and Chandra observations. Spectra of the entire SNR as
well as parts in the eastern half were analyzed confirming shocked hot plasma
in non-uniform ionization stages as the origin of the X-ray emission. The
spectra differ in the northeastern and the southeastern part of the X-ray
ring, showing emission line features of different strength. The temperature in
the northeastern part is significantly higher than in the southeast, reflected
by the lines of higher ionization stages and the harder continuum. Comparison
to radio data shows the forward shock of the blast wave dominating in the
northern part of the SNR, while the southern emission is most likely produced
by the recently formed reverse shock in the ejecta. In the case of the overall
spectrum of SNR 0102-72.3, the two-temperature non-equilibrium ionization
model is more consistent with the data in comparison to the single
plane-parallel shock model. The structure of SNR 0102-72.3 is complex due to
variations in shock propagation leading to spatially differing X-ray spectra.
K4.1 The mass of the neutron star in SMC X-1
| — |
||Authors: A.K.F. Val Baker, A.J. Norton, H. Quaintrell|
||Journal-ref: A&A 441 (2005) 685 [astro-ph/0507206 ]|
||Title: The mass of the neutron star in SMC X-1|
We present new optical spectroscopy of the eclipsing binary pulsar
Sk 160/SMC X-1. From the He I absorption lines, taking heating corrections
into account, we determine the radial velocity semi-amplitude of Sk 160 to be
21.8 ± 1.8 km/s. Assuming Sk 160 fills its Roche-lobe, the inclination angle
of the system is i=65.3 deg ± 1.3 deg and in this case we obtain
limits for the mass of the neutron star as
Mx = 1.21 ± 0.10 M and for
Sk 160 as
Mo = 16.6 ± 0.4 M.
However if we assume that the inclination
angle is i=90 deg, then the ratio of the radius of Sk 160 to the radius of its
Roche-lobe is beta = 0.79 ± 0.02, and
the lower limits for the masses of the
two stars are
Mx = 0.91 ± 0.08 M and
Mo = 12.5 ± 0.1 M.
show that the HeII 4686A emission line tracks the motion of the neutron star,
but with a radial velocity amplitude somewhat less than that of the neutron
star itself. We suggest that this emission may arise from a hotspot where
material accreting via Roche lobe overflow impacts the outer edge of an
Eclipsing X-ray pulsars offer a means of directly measuring neutron star masses. However, only 7 such systems
are currently known and the neutron star masses in each case are not determined to high accuracy. If the
situation can be improved, the equation of state for nuclear matter may be constrained, so testing theories that
The optical counterpart to SMC X-1 is the B0 I supergiant, Sk 160. The X-ray source has a pulse period of 0.72 s
and exhibits an eclipse duration of 0.610 ± 0.019 d in the 3.892 d orbit.
The X-ray emission from SMC X-1 has also been found to exhibit a long quasi-stable super-orbital period of
50–60 days, believed to be a result of obscuration of the neutron star by a precessing accretion disk.
K4.2 The X-ray pulsar SMC X-1
| — |
||Authors: : S. Naik, B. Paul|
||Journal-ref: A&A 418 (2004) 655-661 [astro-ph/0402096 ]|
||Title: BeppoSAX observations of the accretion-powered X-ray pulsar SMC X-1|
We present here results obtained from three BeppoSAX observations of the accretion-powered X-ray pulsar SMC X-1
carried out during the declining phases of its 40--60 days long super-orbital period.
Image credit: S. Naik & B. Paul
Fig. 3. The LECS, MECS, and PDS pulse profiles of SMC X-1 during the high state of 40–60 days super-orbital
period (1997 March 02 BeppoSAX observation) are shown here for different energy bands with 8 phase bins (top
panel) and 16 phase bins (other panels) per pulse. Two pulses in each panel are shown for clarity.
Timing analysis of the data clearly shows a continuing spin-up of the neutron star. Energy-resolved timing
analysis shows that the pulse-profile of SMC X-1 is single peaked at energies
less than 1.0 keV whereas an additional peak, the amplitude of which increases
with energy within the MECS range, is present at higher energies.
Broad-band pulse-phase-averaged spectroscopy of the BeppoSAX data, which is done for the
first time since its discovery, shows that the energy spectrum in the 0.1--80
keV energy band has three components, a soft excess that can be modeled as a
thermal black-body, a hard power-law component with a high-energy exponential
cutoff and a narrow and weak iron emission line at 6.4 keV.
Pulse-phase resolved spectroscopy indicates a pulsating nature of the soft spectral
component, as seen in a few other binary X-ray pulsars, with a certain phase
offset with respect to the hard power-law component. Dissimilar shape and phase
of the soft and hard X-ray pulse profiles suggest a different origin of the soft and hard components.
The bright, eclipsing, accretion-powered binary X-ray pulsar SMC X-1 was first detected during a rocket flight
(Price et al. 1971). The discovery of X-ray eclipses with the Uhuru satellite established the binary nature of
The pulsar, with a pulse period of 0.71 s, is orbiting a B0I super-giant (Sk 160) of
mass ~ 19 M
with an orbital period of ~ 3.89 days.
Since its discovery, observations with various X-ray observatories clearly show a
steady spin-up of the neutron star in the binary system.
This makes SMC X-1 an exceptional X-ray pulsar in which no spin-down episode has been observed.
An observed decay in the orbital period with a time scale of 3 × 106 yr is interpreted as due to tidal
interaction between the neutron star and the binary companion. The latter is presumed to
be in the hydrogen shell burning phase of its evolution.
A super-orbital period of 40–60 days in SMC X-1, analogous to the well known X-ray pulsars
Her X-1 and LMC X-4, was suggested by Gruber & Rothschild (1984) and was
confirmed by recent observations with the RXTE/ASM
and CGRO/BATSE. Varying obscuration by a precessing accretion disk provides a good explanation for
the long term quasi-periodic intensity variations.
K4.3 SMC X-1, LMC X-4 and Cen X-3
HMXB — eclipsing X-ray pulsar|
||Authors: A. van der Meer, L. Kaper, M. H. van Kerkwijk, M. H. M. Heemskerk, E.P.J. van den Heuvel|
||Journal-ref: A&A (2007) [0707.2802 ]|
||Title: Determination of the mass of the neutron star in SMC X-1, LMC X-4 and Cen X-3 with VLT/UVES|
the results of a spectroscopic monitoring campaign of the OB-star companions
to the eclipsing X-ray pulsars SMC X-1, LMC X-4 and Cen X-3. High-resolution
optical spectra obtained with UVES on the ESO Very Large Telescope are used to
determine the radial-velocity orbit of the OB (super)giants with high precision.
The excellent quality of the spectra provides the opportunity to
measure the radial-velocity curve based on individual lines, and to study the
effect of possible distortions of the line profiles due to e.g. X-ray heating
on the derived radial-velocity amplitude. Several spectral lines show
intrinsic variations with orbital phase. The magnitude of these variations
depends on line strength, and thus provides a criterion to select lines that
do not suffer from distortions. The undistorted lines show a larger
radial-velocity amplitude than the distorted lines, consistent with model predictions.
Application of our line-selection criteria results in a mean
KOpt = 20.2 ± 1.1, 35.1 ± 1.5, and 27.5 ± 2.3 km/s, for the
OB companion to SMC X-1, LMC X-4 and Cen X-3, respectively.
Adding information on the projected rotational velocity of
the OB companion (derived from our spectra), the duration of X-ray eclipse and
orbital parameters of the X-ray pulsar (obtained from literature), we arrive
at a neutron star mass of
M = 1.06 ± 0.1, 1.25 ± 0.1 and 1.34 ± 0.15 M for
SMC X-1, LMC X-4 and Cen X-3, respectively.
The mass of SMC X-1 is near the minimum mass (~1 M)
expected for a neutron star produced in a supernova.
We discuss the implications of the
measured mass distribution on the neutron-star formation mechanism, in
relation to the evolutionary history of the massive binaries.
A neutron star is the compact remnant of a massive star
(M ~ 8 M) with a central density that
can be as high as 5 to 10 times the density of an atomic nucleus. The global structure of a
neutron star depends on the equation of state (EOS) under these extreme conditions, i.e. the relation between
pressure and density in the neutron star interior (e.g. Lattimer & Prakash 2004).
Given an EOS, a mass-radius relation for the neutron star and a corresponding maximum neutron-star mass can
be derived. The “stiffness” of the EOS depends e.g. on how many bosons are present in matter of such a high
density. As bosons do not contribute to the fermi pressure, their presence will tend to “soften” the EOS.
For a soft EOS, the maximum neutron-star mass will
be low (e.g. < 1.55 M for the EOS applied by
Brown & Bethe (1994)); for a higher mass, the object would collapse into a black hole.
The accurate measurement of neutron-star masses is therefore important for our understanding of the EOS of
matter at supra-nuclear densities. Currently, the most massive neutron star in an X-ray binary is the X-ray
pulsar Vela X-1 (Barziv et al. 2001; Quaintrell et al. 2003) with a mass of
1.86±0.16 M. The
millisecond radio pulsar J0751+1807 likely has an even higher
mass: 2.1 ± 0.2 M (Nice et al. 2005).
Both results are in favor of a stiff EOS (see also Srinivasan 2001).
Neutron stars also have
a minimum mass limit. The minimum stable neutron-star mass
is about 0.1 M, although a more realistic minimum
stems from a neutron star’s origin in a supernova. Lepton-rich proto neutron stars
are unbound if their masses are less than about 1 M
(Lattimer & Prakash 2004).
Another issue is the neutron-star mass distribution: the detailed supernova mass ejection mechanism
accompanying the formation of the neutron star is not understood, but it is likely that the many neutrinos that
are produced during the formation of the (proto-) neutron star in the centre of the collapsing
star play an important role (e.g. Burrows 2000).
Timmes et al. (1996) present model calculations from which they conclude that
Type II supernovae (massive, single stars) will give a bimodal
neutron-star mass distribution, with peaks at
1.28 and 1.73M,
while Type Ib supernovae (such as produced by stars in binaries,
which are stripped of their envelopes) will produce neutron stars
within a small range around 1.32 M.
Despite the fact that it is in a binary, the massive neutron star in Vela X-1 may belong to
the second peak in this mass distribution.
Neutron stars are detected either as radio pulsars, single or in
a binary with a white dwarf or neutron star companion, or as X-ray
sources in binaries with a (normal) low-mass (LMXB) or a high-mass companion star (HMXB).
Presently, all accurate mass determinations have been for neutron stars that were almost certainly formed
in Type Ib supernovae and that have accreted little since.
Exceptions are J1909-3744, a pulsar (+ white dwarf) with
a mass of 1.438±0.024 M (Jacoby et al. 2005),
and the massive neutron star in J0751+1807, which may have originated from an LMXB.
The most accurate masses have been derived for the binary
radio pulsars. Until recently, all of these were consistent
with a small mass range near 1.35 M
(Thorsett & Chakrabarty 1999).
2. Eclipsing high-mass X-ray binaries
Five high-mass X-ray binaries are known to host an eclipsing X-ray pulsar:
Vela X-1, 4U 1538-52, SMC X-1, LMC X-4 and Cen X-3.
The eclipse provides an important constraint on
the orbital inclination i, an essential parameter for the mass determination.
For the eclipsing X-ray source 4U 1700-37 with
the O6.5 Iaf+ companion HD 153919 (Jones et al. 1973;Mason
et al. 1976) no X-ray pulsations have been detected, although
the compact object most likely is a neutron star.
The absence of X-ray pulsations prohibits the accurate determination of the orbital parameters of
the neutron star, and thus its mass.
Recent analyses of the radial-velocity curve of the wind-fed system Vela X-1
(Barziv et al. 2001; Quaintrell et al. 2003) with its B0.5 Ib companion
have shown that the neutron star in this system has a mass of
1.86 ± 0.16 M.
Such a high neutron-star mass provides an important constraint on the EOS at supra-nuclear density.
2.1. SMC X-1
The B0 supergiant Sk 160 (V = 13.3 mag) is the companion to the eclipsing X-ray pulsar SMC X-1,
located in the “wing” of the Small Magellanic Cloud at a distance of 60.6 kpc. The spin
period of the pulsar is 0.71 s and the orbital period is 3.89 d,
which is decaying on a timescale of 3 Myr due to tidal interaction. A super-orbital, though not strictly
periodic variation of ~ 60 d is present in the system, most likely due to a precessing tilted accretion disc.
The mass of the optical companion is around 16.7
2.2. LMC X-4
After the first detection of LMC X-4 by the Uhuru satellite (Giacconi et al. 1972), the binary nature of its
optical counterpart was confirmed by Chevalier & Ilovaisky (1977). The V = 14.0 mag O8 III companion
is in a 1.41 d orbit, which is decaying on a timescale of ~ 500 kyr.
The optical light curve shows ellipsoidal variations and a super-orbital period of ~ 30 d due to a precessing
accretion disc. The X-ray light curve includes regular eclipses as well as a pronounced flux
modulation of a factor ~ 60 with a period of 30.5 d. This long-term variation is attributed to the precessing
accretion disc. Kelley et al. (1983) discovered the 13.5 s X-ray pulsations of LMC X-4.
Kelley et al. (1983) obtain
MX = 1.47 ± 0.4 M
and Mopt = 15.8 ± 2.2 M.
2.3. Cen X-3
Cen X-3 was discovered by Chodil et al. (1967) and became the first detected binary X-ray pulsar
(Giacconi et al. 1971). The V = 13.3 mag optical counterpart V779 Cen was identified by Krzeminski (1974),
an O6-7 II-III star in a 2.09 d circular orbit with the 4.84 s X-ray pulsar.
The optical light curve indicates the likely presence of an accretion disc, but no strong evidence is found for
X-ray heating. The X-ray light curve includes episodes of high and low X-ray flux with a characteristic
timescale of 120–165 d.
The resulting neutron-star mass is
MX = 1.21 ± 0.21 M
and the mass of the O-type companion
Mopt = 20.5 ± 0.7 M.
Barziv, O., Kaper, L., Van Kerkwijk, M.H., Telting, J.H., Van Paradijs, J. 2001, A&A 377, 925
Brown, G. E. & Bethe, H. A. 1994, ApJ 423, 659
A Scenario for a Large Number of Low-Mass Black Holes in the Galaxy
Bethe, H. A. & Brown, G. E., 1998, ApJ 506, 780
Evolution of Binary Compact Objects That Merge
Bethe, H. A.; Brown, G. E.; Lee, C. -H. [astro-ph/0510379 ]
Evolution and Merging of Binaries with Compact Objects
Burrows, A. 2000, Nature, 403, 727 Supernova explosions in the Universe
Jacoby, B. A., Hotan, A., Bailes, M., Ord, S., & Kulkarni, S. R. 2005, ApJ 629, L113
Lattimer, J. M. & Prakash, M. 2004, Science, 304, 536 (Neutron Stars)
Nice, D., Splaver, E., Stairs, I., et al. 2005, ApJ 634, 1242 (PSR J0751+1807)
Quaintrell, H., Norton, A. J., Ash, T. D. C., et al. 2003, A&A 401, 313
Srinivasan, G. 2001, in Black Holes in Binaries and Galactic Nuclei, 45
Stairs, I. H. 2004, Science 304, 547
Pulsars in Binary Systems: Probing Binary Stellar Evolution and General Relativity
Thorsett, S. E. & Chakrabarty, D. 1999, ApJ 512, 288
Timmes, F. X., Woosley, S. E., & Weaver, T. A. 1996, ApJ 457, 834
K4.4 Neutron Star Masses
Neutron Star Mass — m = 1.35 ± 0.04 M|
||Authors: S. E. Thorsett, Deepto Chakrabarty |
||Journal-ref: ApJ 512 (1999) 288 [astro-ph/9803260 ]|
||Title: Neutron Star Mass Measurements. I. Radio Pulsars|
Abstract: There are now about fifty known radio pulsars in binary systems, including at least five in double
neutron star binaries. In some cases, the stellar masses can be directly determined from measurements of
relativistic orbital effects.
In others, only an indirect or statistical estimate of the masses is possible.
We review the general problem of mass measurement in radio pulsar binaries, and
critically discuss all current estimates of the masses of radio pulsars and their companions.
We find that significant constraints exist on the masses of
twenty-one radio pulsars, and on five neutron star companions of radio pulsars.
All the measurements are consistent with a remarkably narrow underlying
gaussian mass distribution, m = 1.35 ± 0.04 M.
There is no evidence that extensive mass accretion
(Dm >~ 0.1 M) has
occurred in these systems.
We also show that the observed inclinations of millisecond pulsar
binaries are consistent with a random distribution, and thus find no evidence
for either alignment or counteralignment of millisecond pulsar magnetic fields.
The most precisely measured physical parameter of any pulsar is its spin frequency. The frequencies of the
fastest observed pulsars (PSR B1937+21 at 641.9 Hz and B1957+20 at 622.1 Hz) have already been used to set
constraints on the nuclear equation of state at high densities under the assumption that these pulsars are
near their maximum (breakup) spin frequency. However, the fastest observed spin frequencies may
be limited by complex accretion physics rather than fundamental nuclear and gravitational
physics. A quantity more directly useful for comparison with physical theories is the neutron star mass.
The basis of most neutron star mass estimates is the analysis of binary motion. Soon
after the discovery of the first binary radio pulsar (Hulse and Taylor 1975), it became clear
that the measurement of relativistic orbital effects allowed extremely precise mass estimates.
Indeed, the measurement uncertainties in several cases now exceed in precision our knowledge
of Newton’s constant G, requiring masses to be quoted in solar units GM? rather than
kilograms if full accuracy is to be retained.
Hulse, R. A. and Taylor, J. H. 1975, ApJ 195, L51
K5 INTEGRAL observations
— SMC X-1|
||Authors: V.A. McBride, M.J. Coe, A.J. Bird, A.J. Dean, A.B. Hill, K.E.McGowan, M.P.E. Schurch,
A. Udalski, I. Soszynski, M. Finger, C.A. Wilson, R.H.D. Corbet, I. Negueruela |
||Journal-ref: MNRAS (2007) [0709.0633 ]|
||Title: INTEGRAL observations of the Small Magellanic Cloud|
The first INTEGRAL observations of the Small Magellanic Cloud (carried out in 2003) are
reported in which two sources are clearly detected.
The first source, SMC X-1, shows a hard X-ray eclipse and measurements of its pulse period indicate a
continuation of the long-term spin-up now covering ~30 years.
SMC X-1The second source is likely to be a high mass X-ray binary, and shows a potential
periodicity of 6.8s in the IBIS lightcurve. An exact X-ray or optical
counterpart cannot be designated, but a number of proposed counterparts are
discussed. One of these possible counterparts shows a strong coherent optical
modulation at ~2.7d, which, together with the measured hard X-ray pulse
period, would lead to this INTEGRAL source being classified as the fourth
known high mass Roche lobe overflow system.
X-ray satellite observations have revealed that the Small Magellanic Cloud (SMC) contains an unexpectedly large
number of High Mass X-ray Binaries (HMXB). At the time of writing, ~60 known or probable sources of this type
have been identified in the SMC and they continue to be discovered at a rate of about 2-3 per year, although
only a small fraction of these are active at any one time because of their transient nature.
Unusually (compared to the Milky Way and the LMC) all the X-ray binaries so far discovered in
the SMC are HMXBs, and equally strangely, only one of the
objects is a supergiant system (SMC X-1), all the rest are
Be/X-ray binaries. A review of these systems may be found
in Haberl & Sasaki (2000) and Coe et al. (2005).
Coe, M.J., McGowan, K.E. and Schurch, M.P.E., 2007
Haberl F. & Sasaki M., 2000, A&A 359, 573.
|Literatur zu "SMC pulsars"|
|S. E. Thorsett, D. Chakrabarty||1999|| ApJ 512, 288||
"Neutron Star Mass Measurements. I. Radio Pulsars"
|Gaetz, T. J.; Butt, Yousaf M., et al.||2000||ApJ 534, L47-L50||
"Chandra Arcsecond Imaging of the Young, Oxygen-rich Supernova Remnant 1E 0102.2-7219"
|R.H.D. Corbet, F.E. Marshall, M.J. Coe, |
S. Laycock, G. Handler
|2001||ApJ 548, L41||
"The Discovery of an Outburst and Pulsed X-ray Flux from SMC X-2 from RXTE Observations"
|S. Naik, B. Paul||2004||A&A 418, 655-61||
"BeppoSAX observations of the accretion-powered X-ray pulsar SMC X-1"
|S. Laycock, R. H. D. Corbet, M. J. Coe, |
F. E. Marshall, C. Markwardt, J. Lochner
|2005||ApJS 161, 96||
"Long Term Behavior of X-ray Pulsars in the Small Magellanic Cloud"
|K.J. van der Heyden, J.A.M. Bleeker, J.S. Kaastra||2004||A&A 421, 1031-43||
"Synoptic study of the SMC SNRs using XMM-Newton"
|Stanimirovic, S., Bolatto, A. D., Sandstrom, K., et al.||2005||ApJ 632, L103||
"The young SNR 1E 0102.2-7219: testing dust formation in primordial galaxies "
|A.K.F. Val Baker, A.J. Norton, H. Quaintrell||2005||A&A 441, 685||
"The mass of the neutron star in SMC X-1"
|H. Heintzmann||( Eintrag vom 23.5.2008) ||
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