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 later.

Isolated Nearby Neutron Stars: An Overview

Contents

What is an Isolated Neutron Star?

There are about 109 neutron stars in the galaxy

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

What else do you want to know?

The Lifecycle of a Neutron Star

The canonical picture (e.g., Treves et al. 2000):

Alternatives:

Heating Mechanisms

Neutron stars that are bright enough to be seen are either coolers or accretors.

Observational consequences:

The Surveys

Meet the Candidates

The Candidates II. What are they?

RX J185635-3754

Kinematics
Spectral Energy Distribution
Implications for the Small Radius

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 nebulosity. 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 v100-1 AU.
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 D-2 photons/cm2/s
where X is the neutral fraction of the ISM, L33 is the pulsar spindown luminosity, and D is the distance in kiloparsecs.

If there is no relativistic wind, pressure balance between the neutron star magnetic field and the ISM ram pressure leads to a standoff distance

r = 0.02 B121/3 n-1/6 v100-1/3 AU

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 1031 erg/s, 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.

Conclusions

If RX J185635-3754 is a misdirected pulsar:

Outstanding Questions:

References

Arnett, Schramm, & Truran 1989 ApJ 339, L25.
Belloni, Zampieri, & Campana 1996 A&A 127, 145.
Blaauw 1991 in "Physics of Star Formation and Early Spectral Evolution", p 125.
Blaes & Madau 1993 ApJ 403, 690.
Brazier & Johnston 1998 MNRAS 305, 671.
Caraveo et al. 1996 ApJL 461, L91.
Chakrabarty et al. 2000 astro-ph/0001026.
Colpi, Campana, & Treves 1993 A&A 278, 161.
Colpi, Turolla, Zane, & Treves 1998 ApJ 501, 150.
Cordes, Romani, & Lundgren 1993 Nature 362, 133.
Gaensler, Bock, & Stappers 2000 ApJ 537, L35.
Geppert, Page, & Zannias 1999 A&A 345, 847.
Greiner 1996 in Proc. of MPE X-Ray Conference, p 350.
Haberl et al. 1997 A&A 326, 662.
Haberl et al. 1998 AN 319, 97.
Haberl et al. 1999 A&A 351, L53.
Helfand, Chanan, & Novick 1980 Nature 283, 337.
Heyl & Hernquist 1998 MNRAS 297, L69.
Kulkarni & van Kerkwijk 1998 ApJL 507, L4
9. Lorimer, Bailes, & Harrison 1997 MNRAS 289, 592.
Lyne & Graham-Smith 1990 "Pulsar Astronomy".
Madau & Blaes 1994 ApJ 423, 748.
Manning, Jeffries, & Willmore 1996 MNRAS 278, 577.
Motch et al. 1997 A&A 318, 111.
Motch & Haberl 1998 A&A 333, L59.
Motch et al. 1999 A&A 351, 177.
Motch 2000, astro-ph/0008485.
Narayan & Ostriker 1990 ApJ 352, 222.
Neuhäuser & Trüuemper 1999 A&A, 343, 151.
Ostriker, Rees, & Silk 1970 Ap Lett. 6, 179.
Pavlov et al. 1996 ApJ 472, L33.
Popov & Prokhorov 2000 A&A 357, 164.
Rajagopal & Romani 1996 ApJ 461, 327.
Schwope et al. 1999 A&A, 341, L51.
Thomas et al. 1998 A&A 335, 467.
Treves & Colpi 1991 A&A 241, 107.
Treves et al. 2000 PASP, 112, 297.
Tsuruta 1998 Phys Rep 292, 1.
van den Bergh & Tammann 1991 ARAA 29, 363.
Vasisht et al. 1997 ApJ 476, L43.
Walter, 2000, ApJ, in press; astro-ph/0009031.
Walter & Matthews, 1997 Nature 389, 358.
Walter, Wolk, & Neuhäauser 1996, Nature 379, 233.
Zane et al. 1995 ApJ 451, 739.
Zane et al. 1996 A&A 309, 469.