Issue |
A&A
Volume 520, September-October 2010
|
|
---|---|---|
Article Number | A21 | |
Number of page(s) | 4 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/201015495 | |
Published online | 23 September 2010 |
VLT observations of the middle-aged pulsar PSR B1055-52
(Research Note)
R. P. Mignani1 - A. C. Jackson2,3 - A. Spiers2
1 - Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
2 - Ardingly College, Haywards Heath, West Sussex, RH17 6SQ, UK
3 - On leave to University College London, London, WC1E 6BT, UK
Received 29 July 2010 / Accepted 25 August 2010
Abstract
Context. The 535 kyear old radio pulsar B1055-52 is the
oldest of the ``Three Musketeers'', the group of the middle-aged
pulsars (the other two being PSR B0656+14 and Geminga) with
bright soft X-ray and high-energy -ray
emission. The identification of its optical counterpart has been only
recently confirmed through observations performed with the Hubble Space
Telescope (HST), which provided a first characterisation of the pulsar
spectrum.
Aims. We aim at measuring the pulsar flux in the B band for which only an upper limit has been obtained with the HST.
Methods. We used the deepest ground-based observations of the
PSR B1055-52 field obtained with Very Large Telescope (VLT),
available in the ESO archive.
Results. Due to the non-optimal image quality of the VLT data (
)
we could not resolve the faint pulsar emission against the higher sky
background induced by the presence of a bright star (Star A; mV=14.6)
away from the pulsar position. We determined a
upper limit of
on the pulsar brightness. This corresponds to an extinction-corrected flux
0.391
Jy which is consistent with the HST spectrum and comparable with the HST flux upper limit at a similar wavelength.
Conclusions. The presence of Star A makes it improbable to
detect PSR B1055-52 in the optical bands through ground-based
observations, unless performed under exceptional seeing conditions. The
situation might be more promising through adaptive optics high-spatial
resolution observations in the near-infrared, where the HST spectrum
hints at a steep power-law which predicts an higher flux at longer
wavelengths.
Key words: pulsars: individual: PSR B1055-52
1 Introduction
PSR B1055-52 (Vaughan & Large 1972) is a middle-aged radio pulsar with a spin-down age
kyrs, inferred from its period (P=197 ms) and period derivative (
s s-1). These also give, assuming a pure magneto-dipolar spin-down, a rotational energy loss
erg s-1 and a surface magnetic field
G. The distance to PSR B1055-52 is uncertain. The ATNF radio pulsar catalogue
reports 1.53 kpc, based on the radio dispersion measure DM = 30.1 pc cm-3 and the original galactic free electron density model of Taylor & Cordes (1993), while a distance of
726-156+154 pc is derived using the updated NE2001 model of Cordes & Lazio (2002). Recently, a distance of
350 pc has been determined by Mignani et al. (2010).
PSR B1055-52 is one of the radio pulsars detected in X-rays by Einstein (Cheng & Helfand 1983), although X-ray pulsations were discovered only later by ROSAT (Ögelman & Finley 1993). PSR B1055-52 is also a -ray pulsar, detected by the Compton Gamma-ray Observatory (CGRO) (Fierro et al. 1993).
In particular, together with the slightly younger PSR B0656+14
(111 kyrs) and Geminga (340 kyrs), PSR B1055-52 is one
of the three pulsars for which ROSAT discovered thermal X-ray emission.
Their similar age and their overall similarities in their X-ray
emission properties promptly earned them the nickname of ``The Three
Musketeers'' (Becker & Trümper 1997). PSR B1055-52 has been also studied with Chandra (Pavlov et al. 2002) and XMM-Newton (De Luca et al. 2005),
which showed that its X-ray spectrum, like that of the other two
``Musketeers'', is the combination of soft/hard blackbody (BB) and a
power-law (PL) components. Recently, PSR B1055-52 has been
observed by the Fermi Gamma-ray Space Telescope (Abdo et al. 2010).
PSR B1055-52 is the last of the ``Musketeers'' to have been firmly
identified at optical/near-ultraviolet (nUV) wavelengths, and one of
the very few pulsars identified optically (see Mignani 2010a,b
for recent reviews). Indeed, optical observations of PSR B1055-52
had been tried since its X-ray detection by Einstein, but they had been
frustrated by the presence of a bright (
)
F-type field star (Star A; Manchester et al. 1978), positioned
away from the pulsar, which made all ground-based observations unsuccessful (Cheng & Helfand 1983; Bignami et al. 1988; Mignani 1994). Indeed, Mignani et al. (1997)
could identify the candidate pulsar counterpart only thanks to the
sharp resolution and high nUV sensitivity of the Faint Object Camera (
FOC) aboard the Hubble Space Telescope (HST). The identification has
been confirmed through new HST observations (Mignani et al. 2010)
which also yielded the first measurement of the pulsar proper motion.
Like for the other ``Musketeers'', the PSR B1055-52 spectrum in
the optical/nUV is the combination of a PL, with spectral index
,
and a Rayleigh-Jeans (R-J), with temperature
K (Mignani et al. 2010), where d350 and
are the pulsar distance in units of 350 pc and the emitting radius
in units of 13 km. No detection of PSR B1055-52 was obtained
in the B band, which is important to constrain the relative slopes of the PL and the R-J.
Here we present the results of optical observations of PSR B1055-52 performed with the Very Large Telescope (VLT). Observations and data analysis are described in Sect. 2 while results are presented in Sect. 3.
2 Observations and data analysis
VLT observations of the PSR B1055-52 field, obtained on 2002 March 21, are available in the public ESO archive
(Programme 68.D-0407A, PI. P. Lundqvist). The observations were
performed with FOcal Reducer/low dispersion Spectrograph (FORS1), a
multi-mode camera for imaging and long-slit/multi-object spectroscopy.
At the time of the observations, FORS1 was still equipped with the
original four-port
CCD detector. The camera was operated in high resolution (HR) mode (0
1 pixel size) but with a windowing of the CCD, which decreased the instrument field of view to
.
A sequence of 18 short (140 s) exposures to avoid the saturation of Star A were obtained through the Bessel B filter (
Å;
Å)
for a total integration time of 2520 s. Another sequence of 12
exposures of the field was obtained but, strangely enough, not centred
on the PSR B1055-52 position. Single exposures were dithered by
steps of
50
along the X and Y axis. The pulsar was observed close to its maximum altitude, with an airmass of
.
The average image quality during the exposures was
,
as measured directly on the frames.
Observations were obtained with the Moon at
illumination, at a distance of
from our target and at
on the horizon at the time of the observations.
We retrieved the data from the ESO archive together with the associated
calibration files (bias and flat fields) and we reduced them using
standard programs in MIDAS
for bias subtraction, flat-fielding, cosmic-ray removal, image
alignment and stacking. Unfortunately, no standard star observations in
HR mode were obtained for the night of 2002 March 21. For this
reason, we computed the FORS1 photometry calibration using archival
images of the standard star fields PG 1323 and SA 101
(Landolt 1992), taken on
2002 March 11 and 12. Since these observations were obtained a few days
before those of PSR B1055-52, we verified that in both cases the
night conditions were of comparable photometric quality according to
the ESO ambient conditions database. We then fitted Landolt to instrumental magnitudes computed through aperture photometry using a fixed radius of 10
,
for comparison with values measured by the FORS1 photometric
calibration pipeline. For the airmass correction we applied the average
B-band atmospheric extinction coefficient at the Paranal Observatory measured with FORS1
and closest in time to our observations (
).
The overall uncertainty of our photometric calibration corresponds to
an error of less than 0.1 mag on the value of the zero point (ZP=26.98).
![]() |
Figure 1:
(Left) VLT/ FORS1
|
Open with DEXTER |
Given the presence of Star A, a very accurate astrometry of the
FORS1 image is crucial to precisely locate the pulsar position and to
properly account for the effects of the local background enhancement
produced by the wings of the star point spread function (PSF). We
performed the astrometric calibration of the FORS1 image using the
GSC-2.3 (Lasker et al. 2008)
as a reference catalogue. We measured the pixel coordinates of the
GSC-2.3 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
.
Unfortunately, due to the windowing of the CCD, only 9 suitable
stars (after sigma clipping) were available for the astrometry
calibration. From the astrometric fit we then obtained an rms of
on the position residuals. To this, we added in quadrature the
uncertainty in the registration of the FORS1 image on the GSC-2.3
reference frame (0
17). As in Lattanzi et al. (1997), we estimated it as
,
where
accounts for the free parameters in the astrometric fit,
is the mean positional error of the GSC-2.3 coordinates (Lasker et al. 2008), and
is the number of GSC-2.3 stars used to compute the astrometric solution. Accounting for the 0
15 uncertainty (Lasker et al. 2008)
on the link of the GSC-2.3 to the International Celestial Reference
Frame (ICRF), the overall accuracy of the FORS1 astrometry is, thus,
(1
). The uncertainty on the star centroids is
0.1 pixel (
)
and is negligible with respect to the overall uncertainty of our absolute astrometry calibration.
3 Results
As a reference for the PSR B1055-52 position we used the coordinates derived from HST astrometry (Mignani et al. 2010):
(J2000) with a position error
.
Since these coordinates refer to epoch 2008.18 while the epoch of the
VLT observations is 2002.21, we applied the correction for the pulsar
proper motion (Mignani et al. 2010):
yr
yr-1.The computed PSR B1055-52 position overlaid on the FORS1 image is shown in Fig. 1 (left), where the size of the error circle (0
4)
accounts for both the uncertainty of the PSR B1055-52 coordinates
at the reference epoch (2008.18), of the proper motion extrapolation,
and of our astrometric calibration. Unfortunately, since the FORS1
image is affected by a non-optimal image quality (
), the wings of the Star A PSF extend up to a radial distance of
(Fig. 1, right). This produces a
enhancement
of the sky background at the pulsar position, which obviously makes it
more difficult to detect emission from a faint point source, even if
the FORS1 image was taken in high-resolution mode. Using the tools
available in the IRAF package daophot,
we tried to subtract Star A after fitting the image PSF computed from a
number of reference stars selected in the image field of view. However,
probably due to its much higher brightness, the subtraction of Star A
produced quite large residuals and, thus, it did not help to detect any
excess of signal at the pulsar position. Thus, we concluded that
PSR B1055-52 is not detected in the VLT images.
We used the VLT/FORS1 observations of the PSR B1055-52 field to derive an upper limit on its spectral flux in the B band, to be compared with that derived in the HST F450W filter by Mignani et al. (2010), which has a similar wavelength and band pass (
Å;
Å). Following, e.g., Newberry (1991), we determined the number of counts corresponding to a 3
detection limit in a photometry aperture with diameter equal to the full-width at half maximum (FWHM) of the image PSF (1
2) assuming the standard deviation of the background (
)
sampled within a
radius around the computed pulsar position. We note that, thanks to the small uncertainty of our astrometry, the value of
does not vary more than
in the region of interest. To the number of counts thus determined and
normalised to the average frame exposure time of 140 s, we then
applied the aperture correction computed using the growth curve from a
number of unsaturated field stars. After applying the photometric zero
point and correcting for the air mass we thus derived a 3
upper limit of
on the flux of PSR B1055-52, which conservatively accounts for
both the <0.1 mag uncertainty on our photometry calibration
(see Sect. 2) and for the
0.3 magnitude upper limit uncertainty due the fluctuations of
around the pulsar position (see Fig. 1-right). Our value is slightly above the 3
upper limit of
obtained by Mignani et al. (2010),
once the difference between the Johnson and the STMAG photometric
systems is taken into account. According to the simulations performed
with the synphot tool
using a set of suitable templates for the pulsar spectrum we estimated that this difference corresponds to a
.
By using the magnitude-to-flux conversion tool available on the STScI web site
, we obtained an observed spectral flux upper limit
Jy from our FORS1 B-band
measurement. In order to compare the observed flux with the pulsar
optical/nUV spectrum, we corrected this value for the estimated
interstellar reddening towards the pulsar. As done in Mignani
et al. (2010), we assumed as a reference
E(B-V)=0.07. We derived the extinction coefficient in the B band from the extinction curves of Fitzpatrick (1999), which yield
AB = 0.29. We finally derived an extinction-corrected B-band flux upper limit
Jy.
![]() |
Figure 2:
Optical/nUV spectrum of PSR B1055-52 (updated from Mignani
et al. 2010). HST flux measurements are marked with filter
numbers. The solid and dashed lines are the best fit to the data points
and their Rayleigh-Jeans (RJ; red) and power-law (PL |
Open with DEXTER |
The FORS1 B-band flux upper limit is shown in Fig. 2, together with the HST flux measurements and their computed best fit (Mignani et al. 2010). We note that our B-band upper limit is comparable with the value obtained in the F450W filter, which is 0.340 Jy.
This is consistent with the estimated
0.4 mag difference between the Johnson and the STMAG photometric systems. In particular, the FORS1 B-band upper limit is still above the best fit to the HST fluxes (solid line in Fig. 2).
Thus, it can not be used to constrain the spectrum of PSR B1055-52
in the optical band and, hence, to weight the contribution of the
thermal and non-thermal components. Observations deeper by at least
0.7 mag would be needed to detect PSR B1055-52 in the B band,
which are possible with 8 m-class telescopes, although dependent
on the seeing conditions, and with the refurbished HST.
4 Summary
We used archival FORS1 observations at the VLT to measure the PSR B1055-52 B-band flux for which only an upper limit is available from HST observations (Mignani et al. 2010). Unfortunately, due to the non-optimal image quality (
), the pulsar is not detected in the FORS1 data down to a 3
limit of
.
This limit, the deepest so far reported from ground-based observations, is comparable to that obtained with the HST in the F450W filter
and, thus, it is still above the optical/nUV best-fit spectrum. Due to
the presence of the bright and nearby Star A, ground-based optical
imaging of PSR B1055-52 is obviously seeing-limited and, perhaps,
challenging even under sub-arcsecond seeing conditions. While the study
of the pulsar in the optical/
band can be better pursued with the HST, more detections chances with
8 m-class telescopes, however, might come from high spatial
resolution observations in the near-infrared (NIR) with adaptive
optics devices like NAos-COnica (NACO) at the VLT. Indeed, the
contribution of star A PSF (spectral type F) should be lower
in the nIR, while the PSR B1055-52 flux should be higher, as
suggested by its power-law tail hinted in the HST spectrum (Fig. 2).
These observations would be critical to constrain the slope of the
power-law continuum and, thus, to characterise the pulsar multi-band
spectrum.
R.P.M. thanks Oleg Kargaltsev for reproducing Fig. 2 from Mignani et al. (2010). A.C.J. thanks MSSL for hospitality during her visit as A-level work experience student, when this work was carried out. We thank the anonymous referee for her/his positive comments to the manuscript.
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Footnotes
- ... PSR B1055-52
- Based on observations collected at the European Southern Observatory (ESO), La Silla and Paranal, Chile under programme ID 68.D-0407(A).
- ... catalogue
- http://www.atnf.csiro.au/research/pulsar/psrcat/
- ... archive
- www.eso.org/archive
- ... database
- http://archive.eso.org/asm/ambient-server
- ... FORS1
- http://www.eso.org/observing/dfo/quality/FORS1/qc/qc1.html
- ... tool
- star-www.dur.ac.uk/ pdraper/gaia/gaia.html
- ... ASTROM
- http://www.starlink.rl.ac.uk/star/docs/sun5.htx/sun5.html
- ... tool
- stsdas.stsci.edu
- ... site
- http://www.stsci.edu/hst/nicmos/tools/conversion_form.html
All Figures
![]() |
Figure 1:
(Left) VLT/ FORS1
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Optical/nUV spectrum of PSR B1055-52 (updated from Mignani
et al. 2010). HST flux measurements are marked with filter
numbers. The solid and dashed lines are the best fit to the data points
and their Rayleigh-Jeans (RJ; red) and power-law (PL |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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