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X-ray Pulsars analysed in practical exercises
This page gives you requisite basic information (along with references
included in this cd) about the individual pulsating X-ray sources which
are analysed in the practical exercises, both timing and spectral. This
will fecilitate a better understanding of the physical processes that result
in the particular aspect or property that we are investigating in a given
exercise. It is advisable that you read about each source before starting
the perfunctory running of the software in order to better appreciate
the various steps of the otherwise mundane numerical analysis. For general
reference on the subject you may refer to the following:-
1. M. Van Der Klis 2000, submitted
to ARA&A, astro-ph/0001167
2. Fumiaki Nagase 1989, PASJ,
41, 1
The information and references on the individual source are as follows:-
4U 1728-34
Low-Mass X-ray Binaries (LMXB) are very useful to understand the physics
and dynamics of accretion disks and compact objects. LMXBs are usually
characterized by weak magnetic fields and the accretion disk can reach
regions close to the compact object. The inner part of the disk in LMXBs,
is therefore subjected to a strong gravitational field. This results in
complex power spectra with characteristic frequencies, determined by the
milliseconds dynamical timescales close to the compact object. The kHz
timing structures can be used to probe the physics near the compact object.
The LMXB 4U 1728-34 is a prolific source of sudden thermoneuclear bursts
called type I X-ray bursts, and is one of the best studied LMXBs. The type
I X-ray bursts are usually accompanied by radius expansion of the neutron
star photoshere. 4U 1728-34 has been classified as an atoll source, and
it exhibits most of the behaviors seen in those systems. The LMXBs show
correlations between timing properties and the position of the source in
the X-ray color-color diagram. It was the first neutron star LMXB to show
twin kilohertz quasi-periodic oscillations (kHz QPOs) in the persistent
emission and the first to show oscillations during the type I bursts. The
kHz QPOs and burst oscillations in 4U 1728-34 have been investigated in great
detail using the Rossi X-ray Timing Explorer.
References
1. Tod E. Strohmayer, William Zhang and Jean H. Swank 1997, ApJ, 487,
L77
2. Eric C. Ford and Michiel van der Klis 1998, ApJ, 506, L39
3. Mariano Me´ndez1 and Michiel van der Klis 1999, ApJ, 517, L51
4. Xiang-Dong Li, Subharthi Ray, Jishnu Dey, Mira Dey and Ignazio Bombaci5
1999, 527, L51
5. Steve Van Straaten, Michiel Van Der Klis, Erik Kuulkers AND Mariano
Mendez 2001, ApJ, 551, 907
6. Lucia M. Franco 2001, ApJ, 554, 340
4U 0142+61
Some X-ray pulsars are known to have remarkable similarity
in their properties, which are different from those of other
binary or isolated X-ray pulsars. The properties common to
most of these objects are (1) pulse period in a small range
of 5-12 s, (2) monotonous spin-down, (3) identical X-ray
spectrum consisting of steep power-law and blackbody components,
(4) stable X-ray luminosity for years, (5) faint or
unidentified optical counterpart, and (6) no evidence of orbital
motion. 4U 0142+61 is such an anomalous X-ray pulsar with a pulse
period of 8.7 s. The period history of this pulsar measured
with many satellites and spanning more than 20 years show a
constant spin-down. The energy spectrum can be described well
consisting of a 0.4 keV blackbody and a power law with a
photon index of 3.7. A small radius of a few kilometers of
the blackbody emission zone suggests that it is probably
emitted from the surface of a neutron star.
References
1. G. L. Israel, S. Mereghetti and L.
Stella 1994, ApJ, 433, L25
2. N. E. White, L. A. Angelini,
K. Ebisawa, Y. Tanaka and P. Ghosh 1996, ApJ, 463, L83
3. Colleen A. Wilson, Stefan
Dieters, Mark H. Finger, G. Matthew Scott and Jan Van Paradijs 1999, ApJ,
513, 464
4. B. Paul, M. Kawasaki,
T. Dotani and F. Nagase 2000, ApJ, 537, 319
5. Feryal Ozel, Dimitrios Psaltis
and Victoria M. Kaspi 2001, ApJ, 563, 255
GX 1+4
The name of this source indicates its closeness to the centre
of our galaxy. GX 1+4 is the most enigmatic of all binary X-ray
pulsars. The pulse period is in the range of 110-130 s. In the
1970s, it showed the fastest spin-up among the binary X-ray
pulsars with a time scale of only about 100 years. In 1987,
the pulsar was found to be spinning down and it continues to
do so except for a brief period in 1994-1995, which was
accompanied by a large increase in the X-ray flux. The neutron
star has a M6 III star as its companion. The pulsar has a hard
energy spectrum and it has been a good target for both soft and
hard X-ray observations. The spectral cut-off usually seen in
most high mass binary X-ray pulsars is not detected in GX 1+4
and therefore, it is the brightest pulsar in hard X-rays
(20-100 keV). The X-ray spectrum shows high and variable
absorption in the low energy band by local circumstellar
material. X-ray fluoroscence line emission at 6.4 keV from
iron is very prominent in the spectrum. The strength of the
line is correlated with the degree of absorption in the low
energies, indicating that the circumstellar material is excited
by the hard X-ray emission, which in turn produces the line
emission.
References
1. Taro Kotani, Tadayasu Dotani,
Fumiaki Nagase, John G. Greenhill, Steven H. Pravdo et al. 1999, ApJ,
510, 369
2. Wei Cui 1997, ApJ, 1997,
482, L163
3. Deepto Chakrabarty, Lars
Bildsten, Mark H. Finger, John M. Grunsfeld, Danny T. Koh et al. 1997, ApJ,
481, L101
4. A. R. Rao, B. Paul, V.
R. Chitnis, P. C. Agrawal and R. K. Manchanda 1994, A&A, 289, L43
5. Tadayasu Dotani, Tsuneo
Kii, Fumiaki Nagase, Kazuo Makishima, Takaya Ohashi et al. 1989, PASJ, 41,
427
Vela X-1
Vela X-1 is a 283-s pulsar orbiting a B0.5 Ib supergiant in a
period of 8.96 days. The massive early-type star has substantial
mass loss through a strong wind. A fraction of the stellar wind
gets swept up by the neutron star, which generates a strong
X-ray continuum. The companion star has a mass of 23 solar mass
and radius of 34 solar radius. The neutron star is only at a
distance of about 0.6 solar radii from the surface of the
supergiant.
Since the discovery of the X-ray pulsations in Vela X-1, its
slightly eccentric (e=0.09) orbit has been determined from
Doppler-shift measurements of pulse-arrival times. From the radial-velocity
variations of the B supergiant companion the neutron stars mass was estimated
to be about 1.8 solar mass.
References
1. Norbert S. Schulz, Claude R.
Canizares, Julia C. Lee and Masao Sako 2002, Apj, 564, L21
2. Bram Boroson,Richard
McCray, Timothy Kallman and Fumiaki Nagase 1996, ApJ, 465, 940
3. Fumiaki Nagase, Gregory
Zylstra, Takashi Sonobe, Taro Kotani, Hajime Inou et al. 1994, ApJ, 436,
L1
4. J. E. Deeter, P. E. Boynton,
F. K. Lamb and G. Zylstra 1989, Apj, 336, 376
5. P. E. Boynton, J. E.
Deeter, F. K. Lamb and G. Zylstra 1986,
Apj, 307, 545
6. J. E. Deeter, P.
E. Boynton, F. K. Lamb and G. Zylstra 1987, Apj, 314, 634
XTEJ 1946+274
This transient X-ray source was discovered in 1998 September and the presence
of a pulsar immediate revealed. From the very early
observations the pulsar was found to be spinning up, a property
that is commonly seen in transient pulsars. The energy spectrum
consists of a complex continuum, with a weak iron emission line
at 6.6 keV and a column density of 1.6 10^22 cm-2. There is also
an 80 d periodicity in the X-ray light curve, probably caused by
the neutron star's motion along an eccentric orbit. The periodic
X-ray modulation observed in XTE J1946+274 at the orbital period
of the binary is known to be present in many other X-ray pulsars
with a Be-star as companion. Observations of this pulsar with the
Indian X-ray Astronomy Experiment (IXAE), pulse periods and its
local derivatives were obtained. These observations suggest that
the orbit of this Be X-ray binary is eccentric. A cyclotron
resonance scattering feature (or "cyclotron line") has also been
detected at 35 keV, which implies a magnetic field strength of
3.1 × 10^12 G of the neutron star.
References
1. W. A. Heindl, W. Coburn,
D. E. Gruber, R. E. Rothschild and I. Kreykenbohm et al. 2001, ApJ, 563,
L35
2. B. Paul, P. C.
Agrawal, K. Mukerjee, A. R. Rao et al. 2001, A&A, 370, 529
4U 1907+09
The X-ray source 4U 1907+09 was first detected with the UHURU satellite
as a variable X-ray source. From the temporal analysis of data from a major
X-ray flare observed in 1980, a periodicity of about 8.3 day was detected
which is inferred as the binary period of the source. The binary modulation
in the X-ray flux took the form of a double X-ray flare, with a strong primary
flare followed by a smaller shorter secondary flare slightly less than half
a cycle later. The occurrence of two X-ray outbursts separated by 0.45 in
phase in every binary cycle of 8.38 days and the longer pulse period of 4U
1907+09 indicate the binary companion to be a Be star. X-ray pulsations in
4U 1907+09 with a period of 437.5 s was discovered using the data from Tenma
satellite. Pulse arrival time analysis showed the X-ray pulsar to have a mildly
elliptical
orbit (e ~ 0.2) with a period consistent with the refined X-ray photometric
period of 8.3765 day.
A secondary outburst was detected during the 1996 August observations of
the source with the Indian X-ray Astronomy Experiment (IXAE). The timing analysis
of the outburst data shows the presence of 14.4 s transient oscillations which
is different from the oscillations of 18.2 s as detected by RXTE during the
flare of 1996 February. These quasi-periodic oscillations (QPOs) in pulsars
represent characteristic properties of an accretion disk around a neutron
star and hence provide evidence for the formation of a transient accretion
disk around the neutron star in 4U 1907+09.
References
1. J. J. int'Zand, T. E. Strohmayer
and A. Baykal 1997, ApJ, 479, L47
2. J. J. int'Zand, T.
E. Strohmayer and A. Baykal 1998, ApJ, 496, 386
3. K. Mukerjee, P. C.
Agrawal, B. Paul, A. R. Rao, J. S. Yadav et al. 2001, ApJ, 548, 368
4. Altan Baykal, Cagdas
Inam, P M. Ali Alpar, Jean in't Zand and Tod Strohmayer 2001, MNRAS, 327,
1269
CEN X-3
Cen X-3 was the first X-ray source discovered to be a pulsator in a
binary system. Being one of the brightest accreting X-ray pulsars, it is
one of the six out of about 35 X-ray pulsars known today in which the observation
of X-ray eclipses has permitted the determination of all orbital and stellar
parameters. The inclination of the orbital plane and the mass of the neutron
star have been estimated to be i > 63 degree and M = 1.07 +/- 0.6
Solar mass. The optical counterpart of the pulsating X-ray eclipsing binary
Cen X-3 has been identified as an early type star (O6.5III star) of radius
R ~ 12 solar radius and mass M ~ 17-19 solar mass. The distance of the system
is estimated to be ~ 8 kpc. An archetypal massive X-ray binary, Cen X-3 contains
a pulsar of period 4.8 s with a bianry orbital period of 2.1 days. The monitoring
of the pulse period by various X-ray astronomy missions over the past years
has revealed a trend of secular spin-up with wavy fluctuations on a time
scale of a few years. Among the massive X-ray binaries, only Cen X-3 is known
to exhibit a finite rate of change of orbital period. Although a strong
stellar wind emanates from the massive early-type companion to supply the
neutron star with accretion fuel, the presence of an accretion disk fed by
Roche-lobe overflow
is inferred from optical light curves. The secular spin-up trend of the neutron
star and the large X-ray luminosity (~ 10^{38} ergs/s) imply the presence
of an accretion disk, as does the detection of QPO from the source. Furthermore,
there are indications that the stellar wind is too fast to be accreted at
the rate required to power the X-ray source. If, indeed, a disk supplies
most of the accretion fuel, the wind and other, closer circumstellar matter
should still play an important role in the system as the site of scattering
and absorption of X-rays from the neutron star.
References
1. U. Kraus, S. Blum, J. Schulte,
H. Ruder and P. Meszaros 1996, ApJ, 467, 794
2. T. Takeshima, T. Dotani,
K. Mitsuda and Fumiaki Nagase 1991, PASJ, 43, L43
3. R. L. Kelley, S. Rappaport,
G. W. Clark a,d L. D. Petro 1983, ApJ, 268, 790
4. Brian P. Flannery and
Roger K. Ulrich 1977, ApJ, 212, 533
5. T. D. C. Ash, A. P.
Reynolds, P. ROche, A. J. Norton, M. D. Still et al. 1999, MNRAS, 307, 357
Her X-1
Hercules X-1 was discovered by the X-ray satelite Uhuru in 1971. It was the
first X-ray source discovered in the constellation of Hercules, hence the
name. Hercules X-1 is a binary star system, comprising of a neutron star
(pulsar) and a "normal" companion called HZ Herculis, which is about twice
as massive as our Sun with surface temperature about 8000 degrees Kelvin.
The pulsation period is of 1.24 seconds and the binary orbital period is
of 1.7 days.Of all the X-ray binaries, a very few other systems have a normal
star with the mass near that of Hercules X-1. Most X-ray binaries have normal
stars with either masses much more than that of the Sun, or several times
less.
References
1. J. Trumper, P. Kahabka, H. Ogelman,
W. Pietsch and W. Voges 1986, ApJ, 300, L63
2. V. Urpin and U. Gepart
1996, MNRAS, 278, 471
3. D. A. Leahy 1995, A&A
Suppl., 113, 21
4. Ian D. Howarth and Bob
Wilson 1983, MNRAS, 204, 1091
5. James Chiang 2001, ApJ,
549, 537
LMC X-4
LMC X-4 is one of the bright X-ray sources in the Large Magellanic Cloud
that were discovered in the early days of Uhuru satellite. It is an eclipsing
high-mass disk-fed accretion-powered binary X-ray pulsar. The optical counterpart
of the pulsar is found to be a 14th magnitude OB star which was found to
exhibit ellipsoidal light variations with a 1.4 day period. Subsequently,
eclipses occurring with the 1.4 day orbital period were discovered in X-ray.
The X-ray properties of LMC X-4 include occasional episodes of X-ray flaring,
X-ray eclipses and a pronounced modulation in the hard X-ray with a period
of 30.5 day. The X-ray intensity varies by a factor of ~ 60 between high
and low states with a periodic cycle time of 30.5 day. This flux modulation
at super-orbital period (30.5 day) is believed to be due to blockage of the
direct X-ray beam by its precessing tilted accretion disk, similar to the
archetypal system Her X-1. Flaring events of duration ranging from ~ 20 s
to 45 minutes are seen about once in a day during which the source intensity
increases by factors up to ~ 20. A spin period of 13.5 s was discovered which
was later detected in the EXOSAT observations of the source during the non-flaring
and out-of-eclipses state. The approximate values for the projected orbital
radius and component masses are found to be
a sin(i) = 26.0 +/- 0.6 lts
M (Compact object) = 1.34 +/- 0.5 Solar mass
M (binary companion) = 14 Solar mass.
References
1. Claude Chevalier and Sergio
A. Iloviasky 1977, A&A, 59, L9
2. F. L. Lang, A. M. Levine,
M.Bautz, S. Hauskins, S. Howe et al. 1981, ApJ, 246, L21
3. Jonathan W. Woo, George
W. Clark, A. M. Levine, R. H. D. Corbett, F. Nagase 1996, ApJ, 467, 811
4. Samar Safi Harb, H.
Ogelman and K Dennerl 1996, ApJ, 456, L37
5. S. D. Vrtilek, B. Boroson,
F. H. Cheng, R. McCray and F. Nagase 1997, ApJ, 490, 377
NOTE: The papers are in the 'pdf' format which can be read by "Adobe
Acrobat Reader" included with this CD.
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