RX J185635-3754 - November 2000 Update
This update incorporates material reported at the UCSB ITP in October 2000 and
at the November 2000 HEAD meeting, with links. Some figures will be supplied
Isolated Nearby Neutron Stars: An Overview
What is an Isolated Neutron Star?
There are about 109 neutron
stars in the galaxy
- from estimates of supernova rates.
(van den Bergh & Tammann 1991)
- from estimates of galactic nucleosynthesis.
(Arnett, Schramm, & Truran 1989)
- from pulsar birthrates. (Lyne & Graham-Smith 1990;
Narayan & Ostriker 1990)
Only about 1500 neutron stars in the galaxy are identified, including
These other candidates include
Isolated neutron stars are radio-quiet.
Why Look for Isolated Neutron Stars?
Are they not likely to be boring when compared to pulsars and accreting neutron
stars in X-ray binaries?
Neutron stars are stars, with gaseous atmospheres.
Observations of the thermal spectrum can yield the
- surface composition, from the spectral energy distribution.
- angular diameter = (f / sigma Teff4).
- mass and surface gravity, because spectral lines can give
- the gravitational redshift z (proportional to M/R)
- the surface gravity g (proportional to M/R2)
What else do you want to know?
The Lifecycle of a Neutron Star
The canonical picture
(e.g., Treves et al. 2000):
- NS born hot, with B approximately 1012G,
P approximately 0.01 seconds, and
T approximately 1011K.
- NS cool to approximately 106K within 105 yrs.
(e.g., Tsuruta 1998)
- Standard cooling curves suggest that NS will cool rapidly at
about 106 years.
- Pulsars are ejectors.
After decay of the relativistic wind they become propellors.
After the magnetic field decays they become accretors from
the interstellar medium (ISM).
- Neutron stars may be born highly magnetized but slowly rotating
(Vasisht et al. 1997).
- Neutron stars may not be highly magnetized due to fallback
(Geppert, Page, & Zannias 1999).
Neutron stars that are bright enough to be seen are either
coolers or accretors.
- Internal heat. Isolated neutron stars are just cooling off.
- Internal reheating. Many mechanisms have been suggested.
- Accretion from the ISM. Generally considered to be Bondi-Hoyle
accretion, with mass accretion rate =
4 pi (G M)2 nH /
or approximately 3 x 109 gm/sec for
M=1.4 solar masses, and V=100 km/s (c is the sound speed).
The accretion will maintain a temperature
15 (nH / (f V1003)0.25 eV
where f is the fraction of the surface accreting and V100 is
the velocity in units of 100 km/s.
- Young NS will be bright thermal emitters,
L>1032 erg/s, peaking in the soft X-ray/EUV.
- Old NS (>106 years) will just fade away ...
- ... unless they reheat.
- The Predictions:
Isolated neutron stars should be detectable to a few hundred parsecs (pc).
There will be few coolers around, since there have been
few supernovae nearby the Sun in the past 106years.
(Note: within the
last 107 years, there have been supernovae
in - and runaway O stars from
- the Sco-Cen-Lup and Orion associations. (cf. Blaauw 1991).
But with 109 accretors plowing though the ISM,
thousands of NS could be
detected in the ROSAT and EUVE all sky surveys.
(e.g., Ostriker, Rees, & Silk 1970; Helfand, Chanan, & Novick 1980;
Treves & Colpi 1991, Blaes & Madau 1993, Zane et al. 1995,1996).
Isolated NS should be
most common in molecular clouds, where the interstellar density is highest.
(e.g., Colpi, Campana, & Treves 1993; Madau & Blaes 1994)
- The Reality:
- Thousands of accreting NS were
observed in the all sky surveys.
(e.g., Manning et al. 1996; Greiner 1996; Thomas et al. 1998)
- The LogN-LogS curves lie well below predictions for accretors.
(Neuhäuser & Trümper 1999)
- The Excuses:
- Column densities in molecular clouds are too high, and absorb all the
flux (e.g., Belloni, Zampieri, & Campana 1996; Motch et al. 1997)
- NS do not accrete efficiently because
- the mean velocity is high. (e.g., Lorimer, Bailes, & Harrison 1997)
- their magnetic fields do not decay. (Popov & Prokhorov 2000)
- most are in the propellor phase. (Colpi et al. 1998)
Meet the Candidates
- Geminga is a gamma- and X-ray pulsar. It may be a low frequency
radio pulsar. The age is 350,000 years; the distance is
160 pc. (Caraveo et al. 1996)
- Soft X-ray point sources in young supernova remnants are one of the
interesting early results from CHANDRA. The temperatures and
locations near the supernovae
centers are consistent with their being hot
young neutron stars.
(e.g., Chakrabarty et al. 2000; Gaensler, Bock, & Stappers 2000)
- Soft X-ray point sources with fX / fV
>> 1000. I know of 6.
kT for blackbody fits range from about 50 to 120 eV.
(100~eV = 1.16 x 106K)
- RX J0420.0-5022 (Haberl et al. 1999)
- RX J0720.4-3125 (Haberl et al. 1997)
- RX J0806.4-4123 (Haberl et al. 1998)
- RX J130848+2127 (Schwope et al. 1999)
- RX J1605.3-3249 (Motch et al. 1999)
- RX J185635-3754 (Walter, Wolk, & Neuhäuser 1996)
The Candidates II. What are they?
- Geminga may be a misdirected pulsar (its radio pulse cone is not
directed at Earth).
- Soft X-ray point sources in young supernova remnants are coolers,
if indeed they are neutron stars. Are they misdirected pulsars?
- Soft X-ray point sources with fX / fV
Spectral Energy Distribution
- Proper motion: 332 +/-1 milli-arcsec/yr at position angle
(moving just south of east).
- Distance: 61 (+9,-8) pc.
- Likely origin: Upper Scorpius association.
- Age: 0.9 Myr (assuming origin)
- Space velocity: 108 (+13,-7) km/s (heliocentric); 104 km/s
relative to Upper Sco.
- Kick velocity: 200 km/s (assuming origin)
Implications for the Small Radius
- Thermal spectrum: no evidence of non-thermal heating.
- Surface composition: heavy elements.
- Heat source: accretion unlikely.
- Observed radius (at infinity) apporiximately 6km.
- Lies on standard cooling curve for 1 Myr.
- We see only a hot polar cap?
- A very soft equation of state?
- A strange star (with quarks in the interior)?
Implications for the RX J1856-37 Bowshock
M. van Kerkwijk & S. Kulkarni (ESO PR 19/00)
show a deep H-alpha image of the RX J1856-37 region (5 hours on one
of the 8m VLT telescopes). The region is dominated by a
They suggest that the H-alpha emission
nebulosity is an ionization front. We disagree. The shape suggests
a bowshock, as seen around a few fast moving pulsars.
The spindown luminosity of a 10 km, 1.4 solar mass pulsar is
L = 9.6 x 1030 B122 P-4 erg/s
where B12 is the magnetic field strength in units of
1012 Gauss and P is the rotation priod in seconds.
Pressure balance between the relativistic wind and the ISM
gives a standoff distance
r = 26 B120.5 P-2 n0.5
where n is the density of the ISM in particles per cm3, and
v100 is the velocity of the neutron star in units of 100 km/s.
The H-alpha flux FH-alpha
from the bow shock is (from Cordes et al. 1993)
FH-alpha = 2.5 x 10-4 v100 X L33
where X is the neutral fraction of the ISM,
L33 is the pulsar spindown luminosity, and D is the distance in
If there is no relativistic wind, pressure balance between
the neutron star magnetic field and the ISM ram pressure leads to a
r = 0.02 B121/3 n-1/6
The ionization front will have a radius
rI = 4000 [(L31)/(v7 n)]0.5 AU,
where L31 is the luminosity of ionizing photons in units of
or about 70 arcsec for n=1/cm3.
The partially ionized zone extends out about twice this distance.
Any emission on scales of about 1 arcsec must be from a bow shock.
With our data, we estimate:
We conclude that RX J185635-3754 is a misdirected or dead pulsar.
- the standoff shock radius is approximately 60 AU.
- B > 0.4 x 1012 G
- P > 0.5 second.
- Isolated neutron stars do exist, and have been detected.
- The ROSAT logN-logS curve is consistent with that expected from coolers
alone. (Motch 2000)
- The spectral energy distribution can be used to distinguish surface
(Pavlov et al. 1996; Rajagopal & Romani 1996)
- Good quality X-ray spectra are needed to confirm surface compositions.
If RX J185635-3754 is a misdirected pulsar:
- its properties appear normal for an age of 106 years.
- the atmosphere may be magnetic, complicating the modelling.
- Are all NS born with 1012G fields?
- Are all NS born as rapid rotators?
- Do magnetic fields of isolated NS decay?
- Do NS reheat from internal means?
- Do NS accrete from the ISM?
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