Issue |
A&A
Volume 508, Number 2, December III 2009
|
|
---|---|---|
Page(s) | 779 - 782 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200913028 | |
Published online | 27 October 2009 |
A&A 508, 779-782 (2009)
HST observations of the nebula around the central compact object in the Vela Jr. supernova remnant
(Research Note)
R. P. Mignani1 - A. De Luca2,3 - A. Pellizzoni4
1 - Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
2 -
Istituto Universitario di Studi Superiori Viale Lungo Ticino Sforza 56, Pavia 27100, Italy
3 -
INAF, Istituto di Astrofisica Spaziale, via Bassini 15, Milan 20133, Italy
4 -
INAF, Osservatorio Astronomico di Cagliari, Localitá Poggio dei Pini, Strada 54, Capoterra, Cagliari 09012, Italy
Received 30 July 2009 / Accepted 13 October 2009
Abstract
Context. A handful of young (a few thousand years) supernova
remnants (SNRs) host point-like X-ray sources, dubbed central compact
objects (CCOs), which are thought to be radio-silent isolated neutron
stars formed by the supernova explosion. So far, no CCO has been firmly
detected at other wavelengths. However, ground-based observation in the
H band
detected a nebula around CXO J085201.4-461753, the CCO in the Vela
Jr. SNR. The nebula has also been detected in deep R-band observations performed with the Very Large Telescope (VLT). Interestingly, both its extension and its flux in the R band are consistent with those measured in H
,
suggesting that the nebula spectrum is dominated by line emission,
possibly produced by a velocity-driven bow-shock in the interstellar
medium (ISM) or by its photo-ionisation from the neutron star.
Aims. The aim of this work is to resolve the morphology of the H nebula around the CCO to verify the proposed interpretations.
Methods. We performed high-resolution imaging observations of the nebula with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope (HST) through the 656N filter, almost exactly centred on the rest wavelength of the H line.
Results. Surprisingly enough, we did not detect the nebula in our WFPC2 image down to a
flux limit of
erg cm-2 s-1.
This limit is a factor of 10 fainter than the nebula flux measured in
the discovery ground-based observations which were, however, performed
with redder and broader H
filters.
Conclusions. The non-detection of the nebula in the narrower and bluer WFPC2 656N filter
suggests that the peak of the emission might actually be at longer
wavelengths. One possibility, compatible with the bow-shock scenario
only, is that the H line is red-shifted by
10-60 Å due to the neutron star motion with a radial velocity
km s-1. The other possibility is that the nebula is a knot of [NII] emission (
Å)
unrelated to CXO J085201.4-461753 and possibly associated with the
nearby star Wray 16-30. High-resolution spectroscopy observations will
help to settle the nature of the nebula.
Key words: stars: neutron - ISM: supernova remnants - ISM: clouds
1 Introduction
X-ray observations performed over the last 30 years led to the
discovery of about ten puzzling radio-silent, isolated neutron stars
(INSs) at the centre of young (0.3-20 kyears old) supernova remnants (SNRs), hence dubbed central compact objects (CCOs; Pavlov et al. 2004).
Their thermal-like X-ray spectra and lack of pulsar-wind nebulae (PWNe)
indicate a low level of magnetospheric activity, which clearly sets
CCO apart from most young INSs (see De Luca 2008 for a recent review). X-ray pulsations have been measured only for 1E 1207-5209 in PKS 1209-52 (P=424 ms; Zavlin et al. 2000), CXOU J185238.6+004020 in Kes 79 (P=105 ms; Gotthelf et al. 2005), and RX J0822.0-4300 in Puppis A (P=112 ms; Gotthelf & Halpern 2009). In all three cases, the upper limits on the period derivatives (Gotthelf & Halpern 2007; Halpern et al. 2007; Gotthelf & Halpern 2009) suggest magnetic fields
G, rotational energy losses
erg s-1, and spin-down ages a factor of
1000 larger
then the SNR age. This suggests a scenario where CCOs are born spinning
close to their present period and with very low magnetic fields,
perhaps enabling polar cap accretion from a debris disc. So far, no CCO
has been detected outside the X-ray band (see De Luca 2008, and references therein). One possible exception is the CCO in the RXJ 0852.0-4622 (G266.1-1.2) SNR.
In the optical, CXO J085201.4-461753 is embedded in a compact (
diameter) nebula detected in H
both in an archival ESO/MPG 2.2 m observation
and in a digitised UK Schmidt
Telescope (UKST) plate (Pellizzoni et al. 2002).
Recent Very Large Telescope (VLT) observations clearly detected the nebula also in the R band (Mignani et al. 2007a,b) with an extension and a flux consistent with those measured in H
,
suggesting that its spectrum is dominated by line emission. The most
likely interpretations favour either a velocity-driven bow-shock
nebula,as observed in a handful of other INSs (see, e.g. Pellizzoni
et al. 2002, and references therein), or a photo-ionisation nebula produced by the ultraviolet (UV) radiation from the neutron star.
The nebula was undetected in more recent VLT infrared (IR) observations which, instead, pinpointed a faint (
)
candidate counterpart to the CCO (Mignani et al. 2007b).
Here, we present high resolution H imaging observations of the nebula around CXO J085201.4-461753 taken with the Hubble Space Telescope (HST). The observations and data analysis are described in Sect. 2, while the results are discussed in Sect. 3.
2 Observations and results
As a back-up after the failure of the Advanced Camera for Survey (ACS) on January 2007, CXO J085201.4-461753 was re-scheduled for observations with the Wide Field Planetary Camera 2 (WFPC2). We obtained a set of four 500 s exposures during the allocated spacecraft orbit on January 12th 2008. In order to exploit the maximum spatial resolution of the WFPC2 we centred the target in the planetary camera (PC) chip (0






As a reference for the CXO J085201.4-461753 position we used the Chandra coordinates re-computed in Mignani et al. (2007b), i.e.
,
,
with a nominal 90% error circle of 0
6. For consistency with Mignani et al. (2007b), we checked the astrometric calibration of the WFPC2 image against the positions of stars in the 2MASS catalogue (Skrutskie et al. 2006).
We used the mosaic of the four WFPC2 chips since it provides a large enough field of view to include a sufficient number of 2MASS stars. We produced the mosaic image with the STSDAS task wmosaic
which also accounts for the correction of the geometric distortions of
the four chips. Then, we measured the pixel coordinates of the 2MASS stars through gaussian fitting
with the Graphical Astronomy and Image Analysis (GAIA) tool
and we computed the pixel-to-sky coordinates transformation using the code ASTROM
. This yielded an rms
in the astrometric fit. Finally, we added in quadrature the uncertainty in the registration of the WFPC2 image on the 2MASS reference frame (
)
.
Thus, the overall
accuracy of the WFPC2 astrometry is 0
14 (
). By adding in quadrature this value to the
accuracy of the Chandra coordinates we obtained an uncertainty of
on the CXO J085201.4-461753 position on the WFPC2 image.
No point or extended source is visible at or close to the CCO position (Fig. 1). In order to increase the signal-to-noise ratio we rebinned the image by a factor of 2 and applied a smoothing over
pixel cells. Still, our result did not change. We thus conclude that
the CXO J085201.4-461753 nebula was not detected in our images. We
evaluated the upper limit to the flux of the undetected nebula from the
number of background counts over regions with a size comparable to that
of the nebula. We thus estimated that a
detection would correspond to a count rate of 0.92 cts s-1 over such an area. According to the WFPC2 absolute flux calibration, this corresponds to a flux limit of
erg cm-2 s-1.
Assuming pure H
emission, the flux of the nebula measured by Pellizzoni et al. (2002) in the UKST image, where it is detected at
,
is a factor 10 brighter than our
upper limit.
This huge difference is far larger than any
of the different flux calibration between the UKST and of the WFPC2
images. Thus, we exclude that the non-detection of the nebula can be
attributed to an exposure shallower than expected, possibly ascribed to
a decrease in the efficiency of the WFPC2 detector.
![]() |
Figure 1:
|
Open with DEXTER |
3 Discussion
The non-detection of the nebula in the WFPC2 656N image can be explained, in principle, by invoking a decrease in brightness since the epoch of the ESO/MPG 2.2m and of the UKST observations. However, this possibility is unlikely since the brightness measured in both ground-based observations is consistent with a steady emission. Besides, it would be difficult to explain on physical grounds such a large variability of the H



![[*]](/icons/foot_motif.png)









One possibility is that the H line is red-shifted by
10 Å. However, since H
emission is indeed detected in the broader ground-based filters, the line can not be red-shifted by more than
60 Å. This corresponds to a red-shift
and it would imply that the nebula is either moving or expanding asymmetrically, with a radial velocity
km s-1.
In the bow-shock scenario, the first case would be compatible with both
the CCO and the shock front moving along the radial direction. This
would be consistent with the shape of the nebula which features a
somewhat spherical symmetry (Pellizzoni et al. 2002; Mignani et al. 2007b)
instead of the more typical arc-like one. On the other hand, a nebula
expanding asymmetrically would be incompatible with the fact that the
shock front is expected to be almost stationary with respect to the
neutron star. An asymmetric nebula expansion would also be incompatible
with the photo-ionisation scenario, unless the radiation pressure from
the neutron star improbably sweeps the ISM in the radial direction
only. This means that a red-shifted H line
would be compatible with the bow-shock scenario only. In this case, we
could constrain the CXO J085201.4-461753 rotational energy loss
from its spatial velocity and from the H
luminosity of the nebula (Chatterjee & Cordes 2002). We inferred the radial velocity
from the assumed red-shift of the H
line, while we derived the transverse velocity
from the CCO displacement from the SNR centre (
)
which, for a distance of
1 kpc (Slane et al. 2001) and for an age of
1000-3000 years (Katsuda et al. 2009), implies a value of
500-1500 km s-1. This corresponds to a spatial velocity
V=700-3000 km s-1, large but not unheard of
for a neutron star, as it is shown by the Guitar Nebula pulsar
PSR B2224+65 (see, e.g. Chatterjee & Cordes 2002, 2004). For an H
luminosity of
erg s-1, computed for a SNR distance of 1 kpc, and for a fraction of neutral hydrogen in the ISM
,
the bow-shock formation requires that
erg km s-2 (see Fig. 3 of Pellizzoni et al. 2002).
Thus, our limits on the CXO J085201.4-461753 spatial velocity
translates into lower limits on its rotational energy loss of
erg s-1 and
erg s-1, respectively.
We note that the lower bound on the
of CXO J085201.4-461753 would be still compatible with the
upper limits of both CXOU J185238.6+004020 in Kes 79 (Halpern et al. 2007) and of RX J0822.0-4300 in Puppis A (Gotthelf & Halpern 2009), and so be consistent with the emerging picture of CCOs as young, low-energetic INSs.
A second possibility is that the emission line is
indeed [NII] (
Å).
In this case, the nebula would neither be produced by a velocity-driven
bow-shock nor by photo-ionisation from the neutron star. Most likely,
it would be a fore/background emission knot in the SNR or in the
diffuse emission structures observed close to the
CXO J085201.4-461753 position (see, e.g. Fig. 1 of Pellizzoni
et al. 2002
and Fig. 2). At this stage, we can only speculate on the nebula
origin. Because of its proximity, the nebula might be associated with
Star Z. This possibility is also suggested by the isophotal
contours of the UKST H
image (Fig. 1) which, within their
angular resolution, do not allow to exclude a physical connection
between the two. The classification of Star Z is, however,
uncertain. For instance, based on its B and R-band magnitudes Pavlov et al. (2001) concluded that it might be an early G-type background star at a distance of 2.3 kpc, for an
.
Diffuse emission around main sequence stars could, in principle, be
produced by gas outflows from the outer shells. However, if associated
with Star Z, the nebula structure would require an unipolar outflow,
difficult to explain for a main sequence star. Recently, Reynoso
et al. (2006),
based on 13 and 20 cm radio observations of the Vela Jr.
SNR, suggested that star Wray 16-30 is actually a planetary nebula
(PN). One could then argue whether the putative [NII] nebula seen
around CXO J085201.4-461753 might be an emission knot of the
external rim of the PN. However, a physical association between the two
is not so evident since neither the ground-based images of Pellizzoni
et al. (2002) nor our WFPC2 image (Fig. 2)
show evidence of extended structures connecting Wray 16-30 to the
CXO J085201.4-461753 position. We investigated the morphology of
Wray 16-30 in our WFPC2 image.
Thanks to the WFPC2 spatial resolution (0
1 for the wide field camera
chips), we could resolve for the first time the structure of Wray
16-30. We found that part of the object structure is indeed consistent
with that of a bright, saturated, star. However, we also resolved an
extended, clumpy emission structure south of it. It is not clear
whether this is associated with Wray 16-30 or is part of the much more
extended diffuse emission structures west/north-west of it, observed in
the ground-based images and also partially visible in our WFPC2 image. Thus, from the object morphology we can not confirm the identification of Wray 16-30 as a PN.
![]() |
Figure 2:
|
Open with DEXTER |
4 Summary
Our HST/WFPC2 observations of the CXO J085201.4-461753 field with the 656N filter, almost exactly centred on the rest wavelength of the H










R.P.M. thanks STFC for support through a Rolling Grant and K. Wu (UCL-MSSL) for discussions.
References
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Footnotes
- ... remnant
- Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc. under contract No NAS 5-26555.
- ... tool
- star-www.dur.ac.uk/ pdraper/gaia/gaia.html
- ... ASTROM
- http://star-www.rl.ac.uk/Software/software.htm
- ...
)
- As in Lattanzi et al. (1997),
, where
is the number of 2MASS stars and
is their conservative mean positional error (Skrutskie et al. 2006).
- ...Å
- www.eso.org/sci/facilities/lasilla/instruments/wfi/inst/filters/
All Figures
![]() |
Figure 1:
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
|
Open with DEXTER | |
In the text |
Copyright ESO 2009
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