2001A&A...380..221V


Query : 2001A&A...380..221V

2001A&A...380..221V - Astronomy and Astrophysics, volume 380, 221-237 (2001/12-2)

An unusual Hα nebula around the nearby neutron star RX J1856.5-3754.

VAN KERKWIJK M.H. and KULKARNI S.R.

Abstract (from CDS):

We present spectroscopy and Hα imaging of a faint nebula surrounding the X-ray bright, nearby neutron star RX J1856.5-3754. The nebula shows no strong lines other than the hydrogen Balmer lines and has a cometary-like morphology, with the apex being approximately 1'' ahead of the neutron star, and the tail extending up to at least 25'' behind it. We find that the current observations can be satisfactorily accounted for by two different models. In the first, the nebula is similar to ``Balmer-dominated'' cometary nebulae seen around several radio pulsars, and is due to a bow shock in the ambient gas arising from the supersonic motion of a neutron star with a relativistic wind. In this case, the emission arises from shocked ambient gas; we find that the observations require an ambient neutral hydrogen number density nH0≃0.8cm–3 and a rotational energy loss {dot}(E)≃6x1031erg/s. In the second model, the nebula is an ionisation nebula, but of a type not observed before (though expected to exist), in which the ionisation and heating are very rapid compared to recombination and cooling. Because of the hard ionising photons, the plasma is heated up to ∼70000K and the emission is dominated by collisional excitation. The cometary morphology arises naturally as a consequence of the lack of emission from the plasma near and behind the neutron star (which is ionised completely) and of thermal expansion. We confirm this using a detailed hydrodynamical simulation. We find that to reproduce the observations for this case, the neutral hydrogen number density should be n_ H0_≃3cm–3 and the extreme ultraviolet flux of the neutron star should be slightly in excess, by a factor ∼1.7, over what is expected from a black-body fit to the optical and X-ray fluxes of the source. For this case, the rotational energy loss is less than 2x1032erg/s. Independent of the model, we find that RX J1856.5-3754 is not kept hot by accretion. If it is young and cooling, the lack of pulsations at X-ray wavelengths is puzzling. Using phenomenological arguments, we suggest that RX J1856.5-3754 may have a relatively weak, few 1011G, magnetic field. If so, it would be ironic that the two brightest nearby neutron stars, RX J1856.5-3754 and RX J0720.4-3125, may well represent the extreme ends of the neutron star magnetic field distribution, one a weak field neutron star and another a magnetar.

Abstract Copyright:

Journal keyword(s): stars: individual: RX J1856.5-3754 - stars: neutron - HII regions - hydrodynamics

Simbad objects: 13

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Number of rows : 13
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2022
#notes
1 PSR J0437-47 Psr 04 37 15.7994581776 -47 15 08.544351636           DC 829 0
2 PSR B0656+14 Psr 06 59 48.1960 +14 14 19.400   24.85 24.90     ~ 809 1
3 RX J0720.4-3125 N* 07 20 24.961 -31 25 50.21   26.1   27.0   ~ 292 0
4 PSR B1055-52 Psr 10 57 59.0450 -52 26 56.330           ~ 699 0
5 NAME Upper Sco Association As* 16 12 -23.4           ~ 1205 1
6 CD-38 10980 WD* 16 23 33.8380498896 -39 13 46.161043572 9.916 10.885 11.029 10.936 10.841 DA2 464 0
7 * zet Oph Be* 16 37 09.5390545 -10 34 01.529471 1.73 2.58 2.56 2.46 2.50 O9.2IVnn 1785 2
8 RX J1856.6-3754 Psr 18 56 35.11 -37 54 30.5     25.7     ~ 413 0
9 NAME CrA Complex reg 19 15 00.0 -37 30 00           ~ 105 1
10 PSR B1929+10 Psr 19 32 14.0570 +10 59 33.380           ~ 722 0
11 SNR G068.8+02.6 SNR 19 52 55 +32 54.2           ~ 340 1
12 PSR B1951+32 Psr 19 52 58.181 +32 52 40.98           ~ 574 2
13 PSR J1959+2048 Psr 19 59 36.7480 +20 48 14.599   20.4       ~ 751 2

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2022.08.13-01:53:46

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