RX J185635-3754 - November 2000 Update

Contents

• What is an Isolated Neutron Star?
• Why Look for Isolated Neutron Stars?
• The Lifecycle of a Neutron Star
• Heating Mechanisms
• The Surveys
• Meet the Candidates
• The Candidates II. What are they?
• RX J185635-3754
• Implications for the RX J1856-37 Bowshock
• Conclusions
• References

• 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

• over 1000 radio pulsars (Princeton Pulsar catalog).
• a few hundred accreting neutron stars in X-ray binaries.
• < 20 isolated neutron star candidates.

These other candidates include

• Geminga.
• Soft X-ray point sources in young supernova remnants (Brazier & Johnston 1998). An example is the point source discovered in the CHANDRA image of Cas A.
• Soft X-ray point sources with f_X / f_V >> 1000.

Isolated neutron stars are radio-quiet.

• Radio pulsars tell us about magnetospheric physics (image from Astronomy Picture of the Day, 27 May 1998).
  

• Accreting neutron stars in X-ray binaries tell us about accretion and disk physics(image from Astronomy Picture of the Day, 19 December 1999).
  

• Isolated neutron stars allow a glimpse of the surface of a neutron star.

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 T_eff⁴).
• 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?

• 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).

Alternatives:

• 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).

• 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 n_H / (v²+c²)^1.5 or approximately 3 x 10⁹ gm/sec for n_H=1 cm^-3, M=1.4 solar masses, and V=100 km/s (c is the sound speed). The accretion will maintain a temperature T_BB = 15 (n_H / (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.

Observational consequences:

• 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 not 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)

• 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 f_X / f_V >> 1000. I know of 6.
  • 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)
  kT for blackbody fits range from about 50 to 120 eV. (100~eV = 1.16 x 10⁶K)

• 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 f_X / f_V >> 1000.
  • RX J0420.0-5022 and RX J0720.4-3125 are X-ray pulsars, with periods of 22 and 8 seconds, respectively. These may be magnetars, or may be related to the Anomalous X-ray Pulsars (Heyl & Hernquist 1998).

    RX J0720.4-3125 has an optical counterpart with V>26. (Kulkarni & van Kerkwijk 1998; Motch & Haberl 1998)

  • RX J0806.4-4123, RX J130848+2127, and RX J1605.3-3249 are faint and without bright optical counterparts. Not much else is known.
  • RX J185635-3754 is an isolated neutron star detected at X-ray, EUV, UV, and optical wavelengths.

• Proper motion: 332 +/-1 milli-arcsec/yr at position angle 100.3^o (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)

• 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)?

The spindown luminosity of a 10 km, 1.4 solar mass pulsar is

The H-alpha flux F_H-alpha from the bow shock is (from Cordes et al. 1993)

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

The ionization front will have a radius

Any emission on scales of about 1 arcsec must be from a bow shock.

With our data, we estimate:

• 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 compositions. (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.

Outstanding Questions:

• 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?