The Spin Period: Ups and Downs
The spin period modulation is the defining characteristic of an
intermediate polar. Therefore an attempt to measure the
spin period is a pre-requisite for accepting a CV as an IP.
Long-term tracking of the spin period is equally important,
for several reasons.
- At a pragmatic level, when we obtain, e.g., new X-ray data,
we want to be able to phase it to a known spin ephemeris.
- The relative stability of the spin period is an evidence
for the white dwarf, as opposed to neutron star, nature
of the compact object.
- One can deduce the magnetic moment of the white dwarf if
we assume that IPs are at or near their long-term spin
equilibrium. This assumption needs to be confirmed,
however. Patterson (1994)
predicts that Pdot of IPs in spin equilibrium will not be steady.
- Observational determiation of spin up/down gives a better handle
on the magnetic moment, and hence allows an observational test of spin
equilbrium (Patterson (1994)).
We might also be able to deduce something about the degree of departure
from secular mean accretion rate in these systems.
Notes on Inidividual Systems
In the descriptions below, the "shelf life" of an ephemeris
is indicated by the time it takes for the formal uncertainty to
exceed 1 complete cycle. Note that this definition of a shelf life
is rather generous. In practice, you should divide this by n
- the value of n can be debeted, but 4 might be a reasonable choice.
- Systems which have shown a complex spin history
- FO Aqr:
Patterson et al (1998)
present the most recent spin ephemeris. Both quadratic and cubic
terms are necessary (the star was spinning down in the early to mid
1980s, then swithced to spin up in more recent years). Despite the
long baseline, extrapolation of ephemeris is unlikely to be reliable,
and in fact Williams (2003)
was unable to establish a unique ephemeris through 2002.
FO Aqr is almost certainly near spin equilibrium, and "we are
fitting noise" in the period history. CBA continues to monitor
this star extensively, which is reflected in
the archive.
- EX Hya:
Hellier & Sproats
(1992) contains the most recent published spin ephemeris,
according to which this system shows a secular spin-up. However,
1998-2000 timings lag this sphemeris (by up to 0.01 day in 2000;
Walker, private communication). It is therefore highly likely that
EX Hya has a complex spin history. More data are available at
the CBA
archive
- Systems which have shown a secular spin-up
- BG CMi:
A steady spin-up is seen over 15 years; see
Hellier (1997a) for
details. Further monitoring would be desirable to see if the
Pdot is constant or not, since there is a slight difference between
the value derived by Hellier (1997a) and that in
Patterson & Thomas
(1993). This is another
regular
target of the CBA network.
- DQ Her:
Zhang et al. (1959) compiled
the spin timings over 40 years and conclude that a cubic ephemeris
is necessary. As of 1967, Pdot was -6.4x10-13 but it will
go to zero in about 70 years.
The CBA
archive mostly contains fast photometry at MDM.
- GK Per:
Secular spin-up was inferred for this source using a mixture of
X-ray and U-band data by <Patterson
(1991), but with cycle count uncertainties. Recently,
Mauche (2003)
has made a secure determination of the spin-up rate using X-ray data
alone. In the optical, U-band observations appear to be essential for
a secure detection of the spin signal. Some data (in U and other bands)
exist in the
CBA archive.
- AO Psc:
Williams (2003) has
confirmed the secular spin-up of this system, but was unable
to provide a unique ephemeris. There is also an extensive set of data
in the CBA
archive.
- Systems which have shown a secular spin-down
- AE Aqr:
Based on about 15 years of spin timing data,
de Jager et al (1994)
has derived a steady spin-down at the rate of
Pdot(33s)=5.642(20)x10-14 d d-1. This is one
important supporting evidence for the propeller model for this system.
The ephemeris is very precise, but continued monitoring is highly
desirable to confirm if the spin-down is really constant.
There are no data at the CBA, presumably because the spin period
of AE Aqr is too short for the network.
- PQ Gem:
A steady spin-down is seen over ~4 years; see
Hellier (1997a) for
details. A longer baseline would clearly be useful. Intensive
monitoring
data by the CBA network over the last couple of years, but
fewer data seem to exist before 2001.
- V1223 Sgr:
A secular spin-down was inferred by
Jablonski & Steiner
(1987) based on the sideband period ephemeris. The true spin
period is reliably seen in X-rays but rarely? (ever?) seen in the
optical, and there is no long-term spin ephemeris. This star then became
almost forgotten, but there are plenty of data in the
CBA
archive since 2000.
- Systems for which spin up/down has not been measured
- HT Cam:
Based on CBA
observations during 1997-2002,
Kemp
et al (2002) have derived a very precise linear ephemeris, with
a shelf life of over 30 years. They've established an upper limit
on Pdot of 2x10-11.
- Systems for which a good ephemeris is not yet available
- XY Ari:
The spin period was determined to be 206.3 +/- 0.1 s by
Koyama et al (1991)
using Ginga X-ray data. Subsequent X-ray and IR data
only confirmed this value. Given the lack of an optical counterpart,
it would be very difficult to do much better.
- V405 Aur:
The spin ephemeris of
Skillman (1996) has a
nominal shelf life of 36 years; however, if we instead take the
difference in period between this ephemeris and that of
Allan et al (1996)
as the true uncertainty, the shelf life drops to 10 years.
In any case, CBA observers have kept
a close
eye on this source in recent years.
- V709 Cas:
Kozhevnikov (2001)
have observed this star during 1999 Oct 4-9 and detected the spin
modulation at 312.77 +/- 0.04 s; the implied ephemeris has a 28.3 day
shelf life. There are
many
datasets in the CBA archive.
- V1025 Cen:
Buckley et al (1998)
finds 2146.59 s spin period from photometry in 1995 and 1996.
There are a
couple of datasets archived at the CBA.
- TV Col:
I am not aware of any spin ephemeris for this system. The
spin is primarily seen in X-rays, mostly with insufficient duration
to establish an ephemeris. The only mention of positive detection
of this period in the optical I've seen is in the "note added in
proof" of Bonnet-Bidaud,
Motch & Mouchet (1985).
- TX Col:
Spin period inferred from X-ray and optical data (1984-1985)
is 1911 +/- 2 s (Buckley
& Tuohy (1989). Spin period is weakly seen in the optical,
and various sidebands come and makes it difficult (if not impossible)
to establish a long term ephemeris for this system.
An
extensive CBA campaign was carried out in Jan-Feb 2002.
- UU Col:
Burwitz et al (1996)
conclude that the probable spin period is 863.5 +/- 0.7 s from
4 nights of data during 1996 January 13-17. Shelf life of the
implied ephemeris is 12 days. No data have been archived at CBA
as of 2003 April 14.
- V2306 Cyg:
The Norton et al (2002)
ephemeris for half-spin period is based on 7 nights of V-band
photometry at JKT, 2000 July 9-15. Shelf life of the implied ephemeris
is 180 days. The amount of
data
at CBA is very low, probably because of the crowded field.
- YY Dra:
Table 6 of Haswell et al
(1997) contains the most up-to-date table that I know of
of the spin period, determined from X-ray and/or U band photometry
(usually via its 1st harmonic). Data spanning many years are combined
yet the uncertainty is such that the implied shelf life of the
ephemeris is only 6 months. The lack of strong signal in white light
photometry makes it difficult to establish a long-term ephemeris.
- V2400 Oph:
Buckley et al (1995) and
Buckley et al (1997)
have identified 927.66 +/- 0.41 s spin period from polarimetry and
1003.299 +/- 0.003 s synodic period from photometry during 1991-1994.
However, 1027 s period was also possible from these data. Assuming
the 1003 s period, the implied ephemeris has a shelf life of 10 years.
One campiang on this star in 2001 June and July is
archived
at CBA.
- V1062 Tau:
Remillard et al (1994)
derived a period of 1.054+/-0.005 hr = 3794 +/- 18 s from 4 nights of
I-band data, whereas Hellir,
Berdmore & Mukai (2002) derived 3704 +/- 8 s from 2 days of
(ASCA and RXTE) X-ray observations. More data are
needed to establish the spin period.
Some data
exist at CBA, particularly during 2002 December.
- NY Lup:
Haberl, Motch & Zickgraf
(2002) observed this object from ESO on three consecutive nights,
1998 May 7-9. Shelf life of the implied ephemeris is 10 days.
No data have been archived at CBA as of 2003 April 14.