One of the biggest surprises of the INTEGRAL mission was the detection of large

numbers of magnetic cataclysmic variables – in particular the intermediate polar (IP) subclass.

Not only have many previously known systems been detected, but many new ones have also been

found and subsequently classified from optical follow-up observations, increasing the sample of IPs

by ! 15%. We have recently been using a particle hydrodynamic code to investigate the accretion

flows of IPs and determine the equilibrium spin-rates and accretion flow patterns across a wide range

of orbital periods, mass ratios and magnetic field strengths. We use the results of these accretion

flow simulations to examine whether the INTEGRAL IPs differ from the overall population and

conclude that they do not. Most IPs are likely to be INTEGRAL sources, given sufficient exposure.

Currently however, none of the 'EX Hya-like' IPs, with large spin-to-orbital period ratios and short

orbital periods, are detected by INTEGRAL. If this continues to be the case once the whole sky

has a comparable INTEGRAL exposure, it may indicate that the ring-like mode of accretion which

we demonstrate occurs in these systems is responsible for their different appearance

A. J. Norton

Christopher R. Gelino

Dawn Gelino

E. J. Barlow

G.A. Wynn

O. W. Butters

Reba M. Bandyopadhyay

Stefanie Wachter

English

Crossref

Open Research Online
The Open University’s repository of research publications
and other research outputs
Are the INTEGRAL Intermediate Polars Different?
Conference or Workshop Item
How to cite:
Norton, Andrew; Barlow, E.J.; Butters, Olly and Wynn, G.A. (2008). Are the INTEGRAL Intermediate Polars
Different? In: A Population Explosion: The Nature & Evolution of X-ray Binaries in Diverse Environments, 28 Oct -
2 Nov 2007, St Pete Beach, Florida, USA.
For guidance on citations see FAQs.
c© [not recorded]
Version: [not recorded]
Link(s) to article on publisher’s website:
http://dx.doi.org/doi:10.1063/1.2945047
http://conference.astro.ufl.edu/XRAYBIN/Proceedings.html
Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright
owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies
page.
oro.open.ac.uk
Are the INTEGRAL Intermediate Polars Di!erent?
A.J. Norton, E.J. Barlow, O.W. Butters
Dept. of Physics and Astronomy, The Open University, Milton Keynes MK7 6AA, U.K.
and G.A. Wynn
Astronomy Group, University of Leicester, Leicester LE1 7RH, U.K.
Abstract. One of the biggest surprises of the INTEGRAL mission was the detection of large
numbers of magnetic cataclysmic variables – in particular the intermediate polar (IP) subclass.
Not only have many previously known systems been detected, but many new ones have also been
found and subsequently classified from optical follow-up observations, increasing the sample of IPs
by ! 15%. We have recently been using a particle hydrodynamic code to investigate the accretion
flows of IPs and determine the equilibrium spin-rates and accretion flow patterns across a wide range
of orbital periods, mass ratios and magnetic field strengths. We use the results of these accretion
flow simulations to examine whether the INTEGRAL IPs di!er from the overall population and
conclude that they do not. Most IPs are likely to be INTEGRAL sources, given su"cient exposure.
Currently however, none of the ‘EX Hya-like’ IPs, with large spin-to-orbital period ratios and short
orbital periods, are detected by INTEGRAL. If this continues to be the case once the whole sky
has a comparable INTEGRAL exposure, it may indicate that the ring-like mode of accretion which
we demonstrate occurs in these systems is responsible for their di!erent appearance.
1. Magnetic Cataclysmic Variables
Magnetic cataclysmic variables (mCVs) consist of a magnetic white dwarf accreting material
via Roche lobe overflow from a late type, usually main sequence, donor star (for a compre-
hensive review see Warner 1995). Two subcategories are recognised. In polars, the magnetic
fields of the two stars interact to synchronize the rotation period of the white dwarf to the
orbital period of the binary. Furthermore, the accreting plasma attaches to the white dwarf’s
magnetic field immediately on leaving the inner Lagrangian point, and follows a stream-like
trajectory down to the white dwarf’s magnetic poles. Polars are characterized by polarized
optical/infrared emission and X-ray emission modulated at the orbital period of the system,
which is typically less than 4 hours.
In intermediate polars (IPs), the rotation period of the white dwarf is of order a few
hundred to a few thousand seconds, and is less than the binary orbital period of typically a
few hours. The nature of the accretion flow is uncertain, although in many cases a truncated
accretion disc probably forms. At the inner edge of the disc, plasma attaches to the field lines
and travels towards the white dwarf’s magnetic poles, forming accretion curtains which stand
above the white dwarf surface. As it accelerates towards the white dwarf surface, the material
undergoes a strong shock, below which material settles onto the surface, cooling as it does
so by the emission of optical/infrared cyclotron emission and hard X-ray bremsstrahlung.
The white dwarf surface may also be heated directly, so giving rise to a soft X-ray blackbody
component. The plasma settling onto the white dwarf surface in IPs is expected to have a
characteristic temperature of " 108 K and so will give rise to a hard X-ray bremsstrahlung
230
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
Figure 1. The spin periods and orbital periods of the known polars and intermediate
polars. Those systems detected by INTEGRAL are shown circled.
component with a temperature of tens of keV. This has been confirmed for many years by
studies of the medium energy (2–20 keV) X-ray spectra of IPs with, for example, EXOSAT,
Ginga, RXTE and XMM-Newton (e.g. Norton & Watson 1989; Suleimanov et al. 2005; Evans
& Hellier 2007). It should therefore come as no surprise that INTEGRAL detects IPs as
hard X-ray sources in the 20–100 keV band. IPs should be significant hard X-ray sources.
2. INTEGRAL Detections of mCVs
In the third IBIS/ISGRI catalog (Bird et al. 2007), detections are reported of 10 confirmed
IPs, 3 polars, and a further 7 candidate IPs. The 7 INTEGRAL sources identified as can-
didate IPs have been associated with previously known ROSAT sources, and optical coun-
terparts to them have been identified whose spectra resemble other mCVs (Masetti et al.
2006). The IPs detected by INTEGRAL (Barlow et al. 2006) generally lie in directions for
which the INTEGRAL exposure is around 106 s or higher, and consequently many of them
lie close to the Galactic plane.
As may be seen from Figure 1, compared with polars, IPs occupy a wide range of
parameter space in the spin period versus orbital period plane. However, the IPs so far
detected by INTEGRAL lie largely amongst the ‘typical’ IPs with orbital periods greater
than 3 hours and spin-to-orbital period ratios of Pspin/Porb ! 0.01 # 0.1.
3. Accretion Flows from the Magnetic Model
We use a particle hydrodynamic code known as hydisc (see Norton, Wynn & Somerscales
(2004) for details). This code utilises a simplified treatment of magnetic fields which is valid
231
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
Figure 2. The accretion flow phase diagrams for an orbital period of 4 h and mass
ratios of 0.2, 0.5 and 0.9. The regions in which disc-like, stream-like, propeller-like and
ring-like flows occur are indicated.
throughout the flow, except for very close to the white dwarf itself. The parameterisation
of the magnetic field strength is in terms of a ‘drag parameter’ which relates the magnetic
acceleration to the relative velocity between the plasma and the field lines.
We used hydisc to generate a grid of models for various combinations of orbital period
(ranging from 80 min to 10 h), white dwarf spin period (from 100 s to the orbital period),
mass ratio (0.2, 0.5 and 0.9) and white dwarf magnetic moment (from about 1032 G cm3 to
1037 G cm3). Each model was allowed to run to steady state before the resulting accretion
flow was examined. A comprehensive analysis of the results presented in this section may
be found in Norton et al. (2008), but in summary the models reveal that broadly four
types of accretion flow are produced. These are: propellers in which most of the material
transferred from the secondary star is magnetically propelled away from the system by the
rapidly spinning magnetosphere of the white dwarf; discs in which most of the material
forms a circulating flattened structure around the white dwarf, truncated at its inner edge
by the white dwarf magnetosphere where material attaches to the magnetic field lines before
accreting onto the white dwarf surface; streams in which most of the material latches onto
the field lines immediately and follows these on a direct path down to the white dwarf; and
rings in which most of the material forms a narrow annulus circling the white dwarf at the
outer edge of its Roche lobe, with material stripped from its inner edge by the magnetic field
lines before being channeled down to the white dwarf surface.
The regions of parameter space occupied by these flow types may be seen in Figure 2,
where we show the white dwarf spin period versus magnetic moment plane corresponding to
an orbital period of 4 h for mass ratios of 0.2, 0.5 and 0.9. As can be seen, there are two
‘triple points’ at which flows may exist that are a combination of either disc-like, stream-like
and propeller-like accretion (the lower left triple point) or stream-like, ring-like and propeller-
like accretion (the upper right triple point). For a mass ratio of 0.5 for instance, these triple
points occur at Pspin/Porb ! 0.1 and ! 0.6 respectively.
The equilibrium between spin-up and spin-down of the white dwarf lies at the boundary
between disc/stream-like flows and ring/propeller-like flows. If a system finds itself in the
upper part of the plane shown in Figure 2, the disc-like or stream-like flows will spin-up
the white dwarf, so moving it vertically downwards in the plane towards the equilibrium
line. Conversely, if a system finds itself in the lower part of the plane shown in Figure 2,
232
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
the propeller-like or ring-like flows will spin-down the white dwarf, so moving it vertically
upwards in the plane towards the equilibrium line.
Other orbital periods have similar phase diagrams to Figure 2, but with the following
di!erences. The triple points at which disc-stream-propeller and stream-ring-propeller flows
can co-exist shift to smaller magnetic moments and shorter spin periods as the orbital period
decreases. However, the triple points exist at the same spin-to-orbital period ratio at all
orbital periods, for a given mass ratio. As can be seen in Figure 2, the triple points shift to
larger magnetic moments and larger spin-to-orbital period ratios as the mass ratio decreases.
We identify the lower left triple point with the condition that the co-rotation radius
(i.e. that radius at which material would orbit the white dwarf with the same period as the
white dwarf spins) is equal to the circularisation radius (i.e. that radius at which the specific
angular momentum equals that of matter at the inner Lagrangian point). Theory then
predicts (King & Wynn 1999) that Pspin/Porb ! (1 + q)2 (0.500 # 0.227 log q)6. Similarly we
identify the upper right triple point with the condition that the corotation radius is equal
to the distance from the white dwarf to the inner Lagrangian point. In this case, theory
predicts that Pspin/Porb ! (0.500 # 0.227 log q)3/2. The observed spin-to-orbital period ratios
of the triple points from the various phase diagrams at di!erent mass ratios are in very good
agreement with these theoretical predictions. This gives strong support to our identification
of the conditions which are satisfied at these locations in parameter space.
Many IPs cluster around Pspin/Porb ! 0.05 # 0.15 (Figure 1). This period ratio corre-
sponds to the stream-disc-propeller triple point for plausible mass ratios. In contrast, the
EX Hya-like IPs at an orbital period below 2 h with relatively large spin-to-orbital period
ratios, probably display ring-like accretion. This conclusion has been supported by recent
observational evidence from optical spectroscopy of EX Hya itself (Mhlahlo et al. 2007).
4. The Evolution of mCVs
In order to investigate how the accretion flows of IPs are likely to change as the systems
evolve, we took a standard evolutionary sequence of a CV, starting at an orbital period of
9 hr with a mass ratio of 1.2 and evolving to an orbital period of 80 min with a mass ratio
of 0.1. At hourly intervals along its evolutionary track we extracted its mass ratio and mass
accretion rate. We then ran hydisc models with these parameters at each orbital period
step for a range of three white dwarf magnetic field strengths: 60 MG, 6 MG and 0.6 MG,
corresponding to white dwarf magnetic moments of 1034 G cm3, 1033 G cm3, and 1032 G cm3
respectively. Each accretion flow simulation was allowed to spin-up or spin-down the white
dwarf until it reached a stable equilibrium period. The resulting evolutionary tracks of these
systems are plotted in Figure 3.
As IPs evolve, both their mass ratio and orbital period decrease. As noted earlier,
these trends individually cause opposite shifts in the spin-to-orbital period ratio at which
the triple points occur in the accretion flow phase diagrams. Consequently, the variation in
equilibrium spin-to-orbital period ratio cannot be easily predicted. Nonetheless, the tracks
in Figure 3 demonstrate that ‘typical’ IPs with a magnetic field strength of a few megagauss
will evolve from being disc-like accretors at long orbital period (where Pspin/Porb " 0.1), to
ring-like accretors at short orbital period (where Pspin/Porb " 0.6), providing that they don’t
synchronize along the way and become polars. This is the likely history of the IPs that
currently sit alongside EX Hya in the spin period / orbital period plane. So we conclude
that EX Hya-like IPs are systems which have avoided synchronisation as they have evolved
to short orbital periods, and now display ring-like accretion.
233
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
Figure 3. Evolutionary tracks of mCVs with three di!erent magnetic field strengths in
the spin period versus orbital period plane. For plausible mass ratios, disc-like flows will
occur at Pspin/Porb ! 0.05 # 0.15 and ring-like flows will occur at Pspin/Porb ! 0.4 # 0.7.
5. Conclusions
The short answer to the question ‘Are the INTEGRAL intermediate polars di!erent?’ is
‘No’. We expect that most IPs will be detected by INTEGRAL once the whole sky is
subject to the same INTEGRAL exposure. However, if the EX Hya-like IPs continue to
remain undetected by INTEGRAL, even when their sky locations have been subjected to
longer duration exposures, this may suggest that the ring-like accretion which occurs in
these systems produces less hard X-ray emission than in the disc-fed systems at longer
orbital periods.
References
Barlow, E.J., et al. 2006, MNRAS, 372, 224
Bird, A.J., et al. 2007, ApJS, 170, 175
Evans, P.A., Hellier, C. 2007, ApJ, 633, 1277
King, A.R., Wynn, G.A. 1999, MNRAS, 310, 203
Masetti, N., et al. 2006, A&A, 455, 11
Mhlahlo, N., et al. 2007, MNRAS, 378, 211
Norton, A.J., Watson, M.G. 1989, MNRAS, 237, 853
Norton, A.J., Wynn, G.A., Somerscales, R.V. 2004, ApJ, 614, 349
Norton, A.J., Butters, O.W., Parker, T.L., Wynn, G.A. 2008, ApJ, 672, 534
Suleimanov, V., Revnitsev, M., Ritter, H. 2005, A&A, 435, 191
Warner, B. 1995, Cataclysmic Variable Stars, C.U.P.
234
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp


Are the INTEGRAL Intermediate Polars Different?

Open Research Online
The Open University’s repository of research publications
and other research outputs
Are the INTEGRAL Intermediate Polars Different?
Conference or Workshop Item
How to cite:
Norton, Andrew; Barlow, E.J.; Butters, Olly and Wynn, G.A. (2008). Are the INTEGRAL Intermediate Polars
Different? In: A Population Explosion: The Nature & Evolution of X-ray Binaries in Diverse Environments, 28 Oct -
2 Nov 2007, St Pete Beach, Florida, USA.
For guidance on citations see FAQs.
c© [not recorded]
Version: [not recorded]
Link(s) to article on publisher’s website:
http://conference.astro.ufl.edu/XRAYBIN/Proceedings.html
Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright
owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies
page.
oro.open.ac.uk
Are the INTEGRAL Intermediate Polars Different?
A.J. Norton, E.J. Barlow, O.W. Butters
Dept. of Physics and Astronomy, The Open University, Milton Keynes MK7 6AA, U.K.
and G.A. Wynn
Astronomy Group, University of Leicester, Leicester LE1 7RH, U.K.
Abstract. One of the biggest surprises of the INTEGRAL mission was the detection of large
numbers of magnetic cataclysmic variables – in particular the intermediate polar (IP) subclass.
Not only have many previously known systems been detected, but many new ones have also been
found and subsequently classified from optical follow-up observations, increasing the sample of IPs
by ∼ 15%. We have recently been using a particle hydrodynamic code to investigate the accretion
flows of IPs and determine the equilibrium spin-rates and accretion flow patterns across a wide range
of orbital periods, mass ratios and magnetic field strengths. We use the results of these accretion
flow simulations to examine whether the INTEGRAL IPs differ from the overall population and
conclude that they do not. Most IPs are likely to be INTEGRAL sources, given sufficient exposure.
Currently however, none of the ‘EX Hya-like’ IPs, with large spin-to-orbital period ratios and short
orbital periods, are detected by INTEGRAL. If this continues to be the case once the whole sky
has a comparable INTEGRAL exposure, it may indicate that the ring-like mode of accretion which
we demonstrate occurs in these systems is responsible for their different appearance.
1. Magnetic Cataclysmic Variables
Magnetic cataclysmic variables (mCVs) consist of a magnetic white dwarf accreting material
via Roche lobe overflow from a late type, usually main sequence, donor star (for a compre-
hensive review see Warner 1995). Two subcategories are recognised. In polars, the magnetic
fields of the two stars interact to synchronize the rotation period of the white dwarf to the
orbital period of the binary. Furthermore, the accreting plasma attaches to the white dwarf’s
magnetic field immediately on leaving the inner Lagrangian point, and follows a stream-like
trajectory down to the white dwarf’s magnetic poles. Polars are characterized by polarized
optical/infrared emission and X-ray emission modulated at the orbital period of the system,
which is typically less than 4 hours.
In intermediate polars (IPs), the rotation period of the white dwarf is of order a few
hundred to a few thousand seconds, and is less than the binary orbital period of typically a
few hours. The nature of the accretion flow is uncertain, although in many cases a truncated
accretion disc probably forms. At the inner edge of the disc, plasma attaches to the field lines
and travels towards the white dwarf’s magnetic poles, forming accretion curtains which stand
above the white dwarf surface. As it accelerates towards the white dwarf surface, the material
undergoes a strong shock, below which material settles onto the surface, cooling as it does
so by the emission of optical/infrared cyclotron emission and hard X-ray bremsstrahlung.
The white dwarf surface may also be heated directly, so giving rise to a soft X-ray blackbody
component. The plasma settling onto the white dwarf surface in IPs is expected to have a
characteristic temperature of ≈ 108 K and so will give rise to a hard X-ray bremsstrahlung
230
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
Figure 1. The spin periods and orbital periods of the known polars and intermediate
polars. Those systems detected by INTEGRAL are shown circled.
component with a temperature of tens of keV. This has been confirmed for many years by
studies of the medium energy (2–20 keV) X-ray spectra of IPs with, for example, EXOSAT,
Ginga, RXTE and XMM-Newton (e.g. Norton & Watson 1989; Suleimanov et al. 2005; Evans
& Hellier 2007). It should therefore come as no surprise that INTEGRAL detects IPs as
hard X-ray sources in the 20–100 keV band. IPs should be significant hard X-ray sources.
2. INTEGRAL Detections of mCVs
In the third IBIS/ISGRI catalog (Bird et al. 2007), detections are reported of 10 confirmed
IPs, 3 polars, and a further 7 candidate IPs. The 7 INTEGRAL sources identified as can-
didate IPs have been associated with previously known ROSAT sources, and optical coun-
terparts to them have been identified whose spectra resemble other mCVs (Masetti et al.
2006). The IPs detected by INTEGRAL (Barlow et al. 2006) generally lie in directions for
which the INTEGRAL exposure is around 106 s or higher, and consequently many of them
lie close to the Galactic plane.
As may be seen from Figure 1, compared with polars, IPs occupy a wide range of
parameter space in the spin period versus orbital period plane. However, the IPs so far
detected by INTEGRAL lie largely amongst the ‘typical’ IPs with orbital periods greater
than 3 hours and spin-to-orbital period ratios of Pspin/Porb ∼ 0.01− 0.1.
3. Accretion Flows from the Magnetic Model
We use a particle hydrodynamic code known as hydisc (see Norton, Wynn & Somerscales
(2004) for details). This code utilises a simplified treatment of magnetic fields which is valid
231
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
Figure 2. The accretion flow phase diagrams for an orbital period of 4 h and mass
ratios of 0.2, 0.5 and 0.9. The regions in which disc-like, stream-like, propeller-like and
ring-like flows occur are indicated.
throughout the flow, except for very close to the white dwarf itself. The parameterisation
of the magnetic field strength is in terms of a ‘drag parameter’ which relates the magnetic
acceleration to the relative velocity between the plasma and the field lines.
We used hydisc to generate a grid of models for various combinations of orbital period
(ranging from 80 min to 10 h), white dwarf spin period (from 100 s to the orbital period),
mass ratio (0.2, 0.5 and 0.9) and white dwarf magnetic moment (from about 1032 G cm3 to
1037 G cm3). Each model was allowed to run to steady state before the resulting accretion
flow was examined. A comprehensive analysis of the results presented in this section may
be found in Norton et al. (2008), but in summary the models reveal that broadly four
types of accretion flow are produced. These are: propellers in which most of the material
transferred from the secondary star is magnetically propelled away from the system by the
rapidly spinning magnetosphere of the white dwarf; discs in which most of the material
forms a circulating flattened structure around the white dwarf, truncated at its inner edge
by the white dwarf magnetosphere where material attaches to the magnetic field lines before
accreting onto the white dwarf surface; streams in which most of the material latches onto
the field lines immediately and follows these on a direct path down to the white dwarf; and
rings in which most of the material forms a narrow annulus circling the white dwarf at the
outer edge of its Roche lobe, with material stripped from its inner edge by the magnetic field
lines before being channeled down to the white dwarf surface.
The regions of parameter space occupied by these flow types may be seen in Figure 2,
where we show the white dwarf spin period versus magnetic moment plane corresponding to
an orbital period of 4 h for mass ratios of 0.2, 0.5 and 0.9. As can be seen, there are two
‘triple points’ at which flows may exist that are a combination of either disc-like, stream-like
and propeller-like accretion (the lower left triple point) or stream-like, ring-like and propeller-
like accretion (the upper right triple point). For a mass ratio of 0.5 for instance, these triple
points occur at Pspin/Porb ∼ 0.1 and ∼ 0.6 respectively.
The equilibrium between spin-up and spin-down of the white dwarf lies at the boundary
between disc/stream-like flows and ring/propeller-like flows. If a system finds itself in the
upper part of the plane shown in Figure 2, the disc-like or stream-like flows will spin-up
the white dwarf, so moving it vertically downwards in the plane towards the equilibrium
line. Conversely, if a system finds itself in the lower part of the plane shown in Figure 2,
232
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
the propeller-like or ring-like flows will spin-down the white dwarf, so moving it vertically
upwards in the plane towards the equilibrium line.
Other orbital periods have similar phase diagrams to Figure 2, but with the following
differences. The triple points at which disc-stream-propeller and stream-ring-propeller flows
can co-exist shift to smaller magnetic moments and shorter spin periods as the orbital period
decreases. However, the triple points exist at the same spin-to-orbital period ratio at all
orbital periods, for a given mass ratio. As can be seen in Figure 2, the triple points shift to
larger magnetic moments and larger spin-to-orbital period ratios as the mass ratio decreases.
We identify the lower left triple point with the condition that the co-rotation radius
(i.e. that radius at which material would orbit the white dwarf with the same period as the
white dwarf spins) is equal to the circularisation radius (i.e. that radius at which the specific
angular momentum equals that of matter at the inner Lagrangian point). Theory then
predicts (King & Wynn 1999) that Pspin/Porb ∼ (1 + q)2 (0.500− 0.227 log q)6. Similarly we
identify the upper right triple point with the condition that the corotation radius is equal
to the distance from the white dwarf to the inner Lagrangian point. In this case, theory
predicts that Pspin/Porb ∼ (0.500− 0.227 log q)3/2. The observed spin-to-orbital period ratios
of the triple points from the various phase diagrams at different mass ratios are in very good
agreement with these theoretical predictions. This gives strong support to our identification
of the conditions which are satisfied at these locations in parameter space.
Many IPs cluster around Pspin/Porb ∼ 0.05 − 0.15 (Figure 1). This period ratio corre-
sponds to the stream-disc-propeller triple point for plausible mass ratios. In contrast, the
EX Hya-like IPs at an orbital period below 2 h with relatively large spin-to-orbital period
ratios, probably display ring-like accretion. This conclusion has been supported by recent
observational evidence from optical spectroscopy of EX Hya itself (Mhlahlo et al. 2007).
4. The Evolution of mCVs
In order to investigate how the accretion flows of IPs are likely to change as the systems
evolve, we took a standard evolutionary sequence of a CV, starting at an orbital period of
9 hr with a mass ratio of 1.2 and evolving to an orbital period of 80 min with a mass ratio
of 0.1. At hourly intervals along its evolutionary track we extracted its mass ratio and mass
accretion rate. We then ran hydisc models with these parameters at each orbital period
step for a range of three white dwarf magnetic field strengths: 60 MG, 6 MG and 0.6 MG,
corresponding to white dwarf magnetic moments of 1034 G cm3, 1033 G cm3, and 1032 G cm3
respectively. Each accretion flow simulation was allowed to spin-up or spin-down the white
dwarf until it reached a stable equilibrium period. The resulting evolutionary tracks of these
systems are plotted in Figure 3.
As IPs evolve, both their mass ratio and orbital period decrease. As noted earlier,
these trends individually cause opposite shifts in the spin-to-orbital period ratio at which
the triple points occur in the accretion flow phase diagrams. Consequently, the variation in
equilibrium spin-to-orbital period ratio cannot be easily predicted. Nonetheless, the tracks
in Figure 3 demonstrate that ‘typical’ IPs with a magnetic field strength of a few megagauss
will evolve from being disc-like accretors at long orbital period (where Pspin/Porb ≈ 0.1), to
ring-like accretors at short orbital period (where Pspin/Porb ≈ 0.6), providing that they don’t
synchronize along the way and become polars. This is the likely history of the IPs that
currently sit alongside EX Hya in the spin period / orbital period plane. So we conclude
that EX Hya-like IPs are systems which have avoided synchronisation as they have evolved
to short orbital periods, and now display ring-like accretion.
233
Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp
Figure 3. Evolutionary tracks of mCVs with three different magnetic field strengths in
the spin period versus orbital period plane. For plausible mass ratios, disc-like flows will
occur at Pspin/Porb ∼ 0.05− 0.15 and ring-like flows will occur at Pspin/Porb ∼ 0.4− 0.7.
5. Conclusions
The short answer to the question ‘Are the INTEGRAL intermediate polars different?’ is
‘No’. We expect that most IPs will be detected by INTEGRAL once the whole sky is
subject to the same INTEGRAL exposure. However, if the EX Hya-like IPs continue to
remain undetected by INTEGRAL, even when their sky locations have been subjected to
longer duration exposures, this may suggest that the ring-like accretion which occurs in
these systems produces less hard X-ray emission than in the disc-fed systems at longer
orbital periods.
References
Barlow, E.J., et al. 2006, MNRAS, 372, 224
Bird, A.J., et al. 2007, ApJS, 170, 175
Evans, P.A., Hellier, C. 2007, ApJ, 633, 1277
King, A.R., Wynn, G.A. 1999, MNRAS, 310, 203
Masetti, N., et al. 2006, A&A, 455, 11
Mhlahlo, N., et al. 2007, MNRAS, 378, 211
Norton, A.J., Watson, M.G. 1989, MNRAS, 237, 853
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One of the biggest surprises of the INTEGRAL mission was the detection of large numbers of magnetic cataclysmic variables – in particular the intermediate polar (IP) subclass. Not only have many previously known systems been detected, but many new ones have also been found and subsequently classified from optical follow-up observations, increasing the sample of IPs by ~15%. We have recently been using a particle hydrodynamic code to investigate the accretion flows of IPs and determine the equilibrium spin-rates and accretion flow patterns across a wide range of orbital periods, mass ratios and magnetic field strengths. We use the results of these accretion flow simulations to examine whether the INTEGRAL IPs differ from the overall population and conclude that they do not. Most IPs are likely to be INTEGRAL sources, given sufficient exposure. Currently however, none of the 'EX Hya-like' IPs, with large spin-to-orbital period ratios and short orbital periods, are detected by INTEGRAL. If this continues to be the case once the whole sky has a comparable INTEGRAL exposure, it may indicate that the ring-like mode of accretion which we demonstrate occurs in these systems is responsible for their different appearance

Norton, A.J.

Barlow, E.J.

Butters, O.W.

Wynn, G.A.

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsAre the INTEGRAL Intermediate Polars Different?Conference or Workshop ItemHow to cite:Norton, Andrew; Barlow, E.J.; Butters, Olly and Wynn, G.A. (2008). Are the INTEGRAL Intermediate PolarsDifferent? In: A Population Explosion: The Nature & Evolution of X-ray Binaries in Diverse Environments, 28 Oct -2 Nov 2007, St Pete Beach, Florida, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://conference.astro.ufl.edu/XRAYBIN/Proceedings.htmlCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukAre the INTEGRAL Intermediate Polars Different?A.J. Norton, E.J. Barlow, O.W. ButtersDept. of Physics and Astronomy, The Open University, Milton Keynes MK7 6AA, U.K.and G.A. WynnAstronomy Group, University of Leicester, Leicester LE1 7RH, U.K.Abstract. One of the biggest surprises of the INTEGRAL mission was the detection of largenumbers of magnetic cataclysmic variables – in particular the intermediate polar (IP) subclass.Not only have many previously known systems been detected, but many new ones have also beenfound and subsequently classified from optical follow-up observations, increasing the sample of IPsby ∼ 15%. We have recently been using a particle hydrodynamic code to investigate the accretionflows of IPs and determine the equilibrium spin-rates and accretion flow patterns across a wide rangeof orbital periods, mass ratios and magnetic field strengths. We use the results of these accretionflow simulations to examine whether the INTEGRAL IPs differ from the overall population andconclude that they do not. Most IPs are likely to be INTEGRAL sources, given sufficient exposure.Currently however, none of the ‘EX Hya-like’ IPs, with large spin-to-orbital period ratios and shortorbital periods, are detected by INTEGRAL. If this continues to be the case once the whole skyhas a comparable INTEGRAL exposure, it may indicate that the ring-like mode of accretion whichwe demonstrate occurs in these systems is responsible for their different appearance.1. Magnetic Cataclysmic VariablesMagnetic cataclysmic variables (mCVs) consist of a magnetic white dwarf accreting materialvia Roche lobe overflow from a late type, usually main sequence, donor star (for a compre-hensive review see Warner 1995). Two subcategories are recognised. In polars, the magneticfields of the two stars interact to synchronize the rotation period of the white dwarf to theorbital period of the binary. Furthermore, the accreting plasma attaches to the white dwarf’smagnetic field immediately on leaving the inner Lagrangian point, and follows a stream-liketrajectory down to the white dwarf’s magnetic poles. Polars are characterized by polarizedoptical/infrared emission and X-ray emission modulated at the orbital period of the system,which is typically less than 4 hours.In intermediate polars (IPs), the rotation period of the white dwarf is of order a fewhundred to a few thousand seconds, and is less than the binary orbital period of typically afew hours. The nature of the accretion flow is uncertain, although in many cases a truncatedaccretion disc probably forms. At the inner edge of the disc, plasma attaches to the field linesand travels towards the white dwarf’s magnetic poles, forming accretion curtains which standabove the white dwarf surface. As it accelerates towards the white dwarf surface, the materialundergoes a strong shock, below which material settles onto the surface, cooling as it doesso by the emission of optical/infrared cyclotron emission and hard X-ray bremsstrahlung.The white dwarf surface may also be heated directly, so giving rise to a soft X-ray blackbodycomponent. The plasma settling onto the white dwarf surface in IPs is expected to have acharacteristic temperature of ≈ 108 K and so will give rise to a hard X-ray bremsstrahlung230Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jspFigure 1. The spin periods and orbital periods of the known polars and intermediatepolars. Those systems detected by INTEGRAL are shown circled.component with a temperature of tens of keV. This has been confirmed for many years bystudies of the medium energy (2–20 keV) X-ray spectra of IPs with, for example, EXOSAT,Ginga, RXTE and XMM-Newton (e.g. Norton & Watson 1989; Suleimanov et al. 2005; Evans& Hellier 2007). It should therefore come as no surprise that INTEGRAL detects IPs ashard X-ray sources in the 20–100 keV band. IPs should be significant hard X-ray sources.2. INTEGRAL Detections of mCVsIn the third IBIS/ISGRI catalog (Bird et al. 2007), detections are reported of 10 confirmedIPs, 3 polars, and a further 7 candidate IPs. The 7 INTEGRAL sources identified as can-didate IPs have been associated with previously known ROSAT sources, and optical coun-terparts to them have been identified whose spectra resemble other mCVs (Masetti et al.2006). The IPs detected by INTEGRAL (Barlow et al. 2006) generally lie in directions forwhich the INTEGRAL exposure is around 106 s or higher, and consequently many of themlie close to the Galactic plane.As may be seen from Figure 1, compared with polars, IPs occupy a wide range ofparameter space in the spin period versus orbital period plane. However, the IPs so fardetected by INTEGRAL lie largely amongst the ‘typical’ IPs with orbital periods greaterthan 3 hours and spin-to-orbital period ratios of Pspin/Porb ∼ 0.01− 0.1.3. Accretion Flows from the Magnetic ModelWe use a particle hydrodynamic code known as hydisc (see Norton, Wynn & Somerscales(2004) for details). This code utilises a simplified treatment of magnetic fields which is valid231Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jspFigure 2. The accretion flow phase diagrams for an orbital period of 4 h and massratios of 0.2, 0.5 and 0.9. The regions in which disc-like, stream-like, propeller-like andring-like flows occur are indicated.throughout the flow, except for very close to the white dwarf itself. The parameterisationof the magnetic field strength is in terms of a ‘drag parameter’ which relates the magneticacceleration to the relative velocity between the plasma and the field lines.We used hydisc to generate a grid of models for various combinations of orbital period(ranging from 80 min to 10 h), white dwarf spin period (from 100 s to the orbital period),mass ratio (0.2, 0.5 and 0.9) and white dwarf magnetic moment (from about 1032 G cm3 to1037 G cm3). Each model was allowed to run to steady state before the resulting accretionflow was examined. A comprehensive analysis of the results presented in this section maybe found in Norton et al. (2008), but in summary the models reveal that broadly fourtypes of accretion flow are produced. These are: propellers in which most of the materialtransferred from the secondary star is magnetically propelled away from the system by therapidly spinning magnetosphere of the white dwarf; discs in which most of the materialforms a circulating flattened structure around the white dwarf, truncated at its inner edgeby the white dwarf magnetosphere where material attaches to the magnetic field lines beforeaccreting onto the white dwarf surface; streams in which most of the material latches ontothe field lines immediately and follows these on a direct path down to the white dwarf; andrings in which most of the material forms a narrow annulus circling the white dwarf at theouter edge of its Roche lobe, with material stripped from its inner edge by the magnetic fieldlines before being channeled down to the white dwarf surface.The regions of parameter space occupied by these flow types may be seen in Figure 2,where we show the white dwarf spin period versus magnetic moment plane corresponding toan orbital period of 4 h for mass ratios of 0.2, 0.5 and 0.9. As can be seen, there are two‘triple points’ at which flows may exist that are a combination of either disc-like, stream-likeand propeller-like accretion (the lower left triple point) or stream-like, ring-like and propeller-like accretion (the upper right triple point). For a mass ratio of 0.5 for instance, these triplepoints occur at Pspin/Porb ∼ 0.1 and ∼ 0.6 respectively.The equilibrium between spin-up and spin-down of the white dwarf lies at the boundarybetween disc/stream-like flows and ring/propeller-like flows. If a system finds itself in theupper part of the plane shown in Figure 2, the disc-like or stream-like flows will spin-upthe white dwarf, so moving it vertically downwards in the plane towards the equilibriumline. Conversely, if a system finds itself in the lower part of the plane shown in Figure 2,232Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jspthe propeller-like or ring-like flows will spin-down the white dwarf, so moving it verticallyupwards in the plane towards the equilibrium line.Other orbital periods have similar phase diagrams to Figure 2, but with the followingdifferences. The triple points at which disc-stream-propeller and stream-ring-propeller flowscan co-exist shift to smaller magnetic moments and shorter spin periods as the orbital perioddecreases. However, the triple points exist at the same spin-to-orbital period ratio at allorbital periods, for a given mass ratio. As can be seen in Figure 2, the triple points shift tolarger magnetic moments and larger spin-to-orbital period ratios as the mass ratio decreases.We identify the lower left triple point with the condition that the co-rotation radius(i.e. that radius at which material would orbit the white dwarf with the same period as thewhite dwarf spins) is equal to the circularisation radius (i.e. that radius at which the specificangular momentum equals that of matter at the inner Lagrangian point). Theory thenpredicts (King & Wynn 1999) that Pspin/Porb ∼ (1 + q)2 (0.500− 0.227 log q)6. Similarly weidentify the upper right triple point with the condition that the corotation radius is equalto the distance from the white dwarf to the inner Lagrangian point. In this case, theorypredicts that Pspin/Porb ∼ (0.500− 0.227 log q)3/2. The observed spin-to-orbital period ratiosof the triple points from the various phase diagrams at different mass ratios are in very goodagreement with these theoretical predictions. This gives strong support to our identificationof the conditions which are satisfied at these locations in parameter space.Many IPs cluster around Pspin/Porb ∼ 0.05 − 0.15 (Figure 1). This period ratio corre-sponds to the stream-disc-propeller triple point for plausible mass ratios. In contrast, theEX Hya-like IPs at an orbital period below 2 h with relatively large spin-to-orbital periodratios, probably display ring-like accretion. This conclusion has been supported by recentobservational evidence from optical spectroscopy of EX Hya itself (Mhlahlo et al. 2007).4. The Evolution of mCVsIn order to investigate how the accretion flows of IPs are likely to change as the systemsevolve, we took a standard evolutionary sequence of a CV, starting at an orbital period of9 hr with a mass ratio of 1.2 and evolving to an orbital period of 80 min with a mass ratioof 0.1. At hourly intervals along its evolutionary track we extracted its mass ratio and massaccretion rate. We then ran hydisc models with these parameters at each orbital periodstep for a range of three white dwarf magnetic field strengths: 60 MG, 6 MG and 0.6 MG,corresponding to white dwarf magnetic moments of 1034 G cm3, 1033 G cm3, and 1032 G cm3respectively. Each accretion flow simulation was allowed to spin-up or spin-down the whitedwarf until it reached a stable equilibrium period. The resulting evolutionary tracks of thesesystems are plotted in Figure 3.As IPs evolve, both their mass ratio and orbital period decrease. As noted earlier,these trends individually cause opposite shifts in the spin-to-orbital period ratio at whichthe triple points occur in the accretion flow phase diagrams. Consequently, the variation inequilibrium spin-to-orbital period ratio cannot be easily predicted. Nonetheless, the tracksin Figure 3 demonstrate that ‘typical’ IPs with a magnetic field strength of a few megagausswill evolve from being disc-like accretors at long orbital period (where Pspin/Porb ≈ 0.1), toring-like accretors at short orbital period (where Pspin/Porb ≈ 0.6), providing that they don’tsynchronize along the way and become polars. This is the likely history of the IPs thatcurrently sit alongside EX Hya in the spin period / orbital period plane. So we concludethat EX Hya-like IPs are systems which have avoided synchronisation as they have evolvedto short orbital periods, and now display ring-like accretion.233Downloaded 18 Jun 2008 to 81.129.214.58. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jspFigure 3. Evolutionary tracks of mCVs with three different magnetic field strengths inthe spin period versus orbital period plane. For plausible mass ratios, disc-like flows willoccur at Pspin/Porb ∼ 0.05− 0.15 and ring-like flows will occur at Pspin/Porb ∼ 0.4− 0.7.5. ConclusionsThe short answer to the question ‘Are the INTEGRAL intermediate polars different?’ is‘No’. We expect that most IPs will be detected by INTEGRAL once the whole sky issubject to the same INTEGRAL exposure. However, if the EX Hya-like IPs continue toremain undetected by INTEGRAL, even when their sky locations have been subjected tolonger duration exposures, this may suggest that the ring-like accretion which occurs inthese systems produces less hard X-ray emission than in the disc-fed systems at longerorbital periods.ReferencesBarlow, E.J., et al. 2006, MNRAS, 372, 224Bird, A.J., et al. 2007, ApJS, 170, 175Evans, P.A., Hellier, C. 2007, ApJ, 633, 1277King, A.R., Wynn, G.A. 1999, MNRAS, 310, 203Masetti, N., et al. 2006, A&A, 455, 11Mhlahlo, N., et al. 2007, MNRAS, 378, 211Norton, A.J., Watson, M.G. 1989, MNRAS, 237, 853Norton, A.J., Wynn, G.A., Somerscales, R.V. 2004, ApJ, 614, 349Norton, A.J., Butters, O.W., Parker, T.L., Wynn, G.A. 2008, ApJ, 672, 534Suleimanov, V., Revnitsev, M., Ritter, H. 2005, A&A, 435, 191Warner, B. 1995, Cataclysmic Variable Stars, C.U.P.234Downloaded 18 Jun 2008 to 81.129.214.58. 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Are the INTEGRAL Intermediate Polars Different?

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