A&A 479, 793-803 (2008)
DOI: 10.1051/0004-6361:20077728
S. V. Zharikov1 - Yu. A. Shibanov2 - R. E. Mennickent3 - V. N. Komarova4,5
1 - Observatorio Astronómico Nacional SPM, Instituto de Astronomía, Universidad Nacional Autónomia de México, Ensenada, BC, México
2 -
Ioffe Physical Technical Institute, Politekhnicheskaya 26,
St. Petersburg 194021, Russia
3 -
Departamento de Fisica, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile
4 -
Special Astrophysical Observatory, Russian Academy of Science, Nizhnii Arkhyz,
Russia, 369167
5 - Isaac Newton Institute of Chile, SAO Branch, Russia
Received 26 April 2007 / Accepted 1 November 2007
Abstract
Aims. We performed deep optical observations of the field of an old, fast-moving radio pulsar PSR B1133+16 in an attempt to detect its optical counterpart and a bow shock nebula.
Methods. The observations were carried out using the direct imaging mode of FORS1 at the ESO VLT/UT1 telescope in the B, R, and H bands. We also used archival images of the same field obtained with the VLT in the B band and with the Chandra/ACIS in X-rays.
Results. In the B band we detected a faint (B = 28
1
0
3) source that may be the optical counterpart of PSR B1133+16, as it is positionally consistent with the radio pulsar and with the X-ray counterpart candidate published earlier. Its upper limit in the R band implies a color index
5, which is compatible with the index values for most pulsars identified in the optical range. The derived optical luminosity and its ratio to the X-ray luminosity of the candidate are consistent with expected values derived from a sample of pulsars detected in both spectral domains. No Balmer bow shock was detected, implying a low density of ambient matter around the pulsar. However, in the X-ray and H
images we found the signature of a trail extending
behind the pulsar and coinciding with the direction of its proper motion. If confirmed by deeper studies, this is the first time such a trail has been seen in the optical and X-ray wavelengths.
Conclusions. Further observations at later epochs are necessary to confirm the identification of the pulsar by the candidate's proper motion measurements.
Key words: pulsars: general - pulsars: individual: PSR B1133+16 - stars: neutron
Table 1: Parameters of several nearby old pulsars.
Table 2: Log of the VLT/FORS1 observations of the PSR B1133+16 field during the 2003-2004 periods.
The non-thermal component, which also is frequently observed in a wider spectral range including X-rays, is believed to be powered by the NS rotational energy loss
An old, 5 Myr, nearby pulsar PSR B1133+16
has almost the same parameters as the two old objects mentioned above
(see Table 1).
It is located at a high galactic latitude,
,
implying a low interstellar extinction
E(B-V)=0.04(Schlegel et al. 1998). The direct proper
motion and annual parallax
measurements in the radio range by Brisken et al.
(2002)
yield
a high transverse velocity of
km s-1
and a short distance to the pulsar
pc.
The pulsar is younger than PSR B0950+08,
but its spin-down luminosity,
erg s-1, is about an order of magnitude
lower than those of PSR B1929+20 and PSR B0950+08.
Nevertheless, it is higher than that of
the nearby old PSR J0108-1431,
whose optical counterpart has been unsuccessfully searched
by Mignani et al. (2003).
The high transverse velocity is promising for detecting
an H
bow shock nebula expected to be produced by the supersonic
motion of the pulsar in the interstellar matter,
as has been found around several
rapidly moving pulsars
and radio-silent NSs (e.g., Gaensler & Slane 2006).
Recently, the
field of PSR B1133+16 has been observed in X-rays with
the Chandra/ACIS
and a faint
X-ray counterpart candidate was found
at a flux level of (
)
erg s-1cm-2 in 0.5-8 keV range (Kargaltsev et al. 2006).
Given
this value
and using an empirical relation between the optical
and X-ray luminosities of pulsars (Zharikov et al. 2004, 2006;
see also Zavlin & Pavlov 2004), one can expect to detect
the pulsar in the optical range at a sensitivity level of
27-29 mag.
In this paper
we present
the results of a
deep optical imaging
of the PSR B1133+16 field taken with
the ESO Very Large Telescope (VLT) in B, ,
and H
bands to search for the optical counterpart and
the
bow shock nebula of the pulsar.
We also used
archival VLT images of the field obtained earlier in
the B band and the X-ray data from the Chandra archive.
The observations and data reduction are described in
Sect. 2. Astrometric and photometric referencing
are given in Sect. 3. We present our results in Sect. 4 and
discuss them in Sect. 5.
The observations
were carried out with the FOcal Reducer and low-dispersion Spectrograph
(FORS1)
at the UT1 (ANTU) unit of the ESO/VLT during several
service-mode observational runs
from the beginning of
April 2003
to the end of January 2004.
A standard resolution
mode was used with an image scale of
0
2/
and field of view
(FOV)
6
8
6
8.
The total integration time was
12 240 s in B, 8470 s in
,
and 13950 s
in H
bands.
The log of the observations is given in Table 2.
To complement our analysis, we retrieved from the ESO archive
VLT B-band images of the same field obtained under
Program 66.D-0069A of Gallant et al. (2000) in January 2001.
These unpublished data were taken with the same telescope unit and instrument
in a high resolution mode
with an image scale of
0
1/
and
3
4
3
4 FOV. The total
integration time was about 15 ks.
The observational log
is given in Table 3.
Table 3: The same as in Table 2 but for the 2001 period.
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Figure 1:
PSR B1133+16 field as seen in the B band with
the VLT/FORS1 using a standard resolution mode during the 2003-2004 period.
The position of the pulsar is marked by +.
The large box marks the FOV of
high-resolution mode observations during the 2001 period.
The ![]() |
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The data reduction
including bias subtraction, flat-fielding, removing cosmic ray traces
and alignment of each individual image to a reference frame was performed
for all the data using standard
IRAF and MIDAS tools.
Taking the data non-uniformity into account, we considered different
combinations of individual exposures in each band to get co-added images
of the best quality, deepness, and spatial resolution.
A simple sum of all available images for a given band and period
was finally chosen as optimal and subsequently analyzed.
Only very short, 30 s, exposures in the B band
(see Tables 2 and 3)
were excluded.
As a result, the FWHM of a stellar object
in the composed images was
0
75
and
1
in the B band for the
2001 and 2003-2004 periods, respectively.
The respective values in the
and H
bands
were
0
85 and
1
.
The panoramic view
of the pulsar field as seen
with the VLT in the B band is shown in
Fig. 1.
The B-band image obtained by
co-adding all B band frames taken during the 2003-2004 period was chosen as a base image
for our astrometry because its FOV is twice bigger than in the 2001 period.
To compute a precise astrometric image solution, the positions of objects
selected from the USNO-B1 astrometric catalog
were used as a reference. There are about 50 USNO-B1 objects in the FOV
in contrast to none from UCAC2. The recent release of the Guide Star Catalog
(GSC-II v2.3.2)
contains a similar number of stars in this FOV but provides no information on
proper motions, and the declared astrometric errors (0
3) are
higher than nominal 0
2 uncertainty of USNO-B1.
We discarded the reference stars with significant proper motions and catalog-positional
uncertainties
along with saturated ones. The pixel
coordinates of 30 objects
considered to be suitable astrometric reference points were computed making
use of the IRAF task imcenter.
The IRAF tasks ccmap/cctran were applied for the astrometric
transformation of the image. Formal
rms uncertainties of the initial astrometric
fit were rather large,
RA = 0
287 and
Dec = 0
251,
as compared with the 0
2 pixel
scale of the image.
Using standards with the smallest catalog
uncertainties and fit residuals significantly improved the fit.
For nine reference stars listed
in Table 4 and marked
in Fig. 1, the formal rms are
RA = 0
051 and
Dec = 0
097, which is smaller than the
pixel scale.
As can be derived
from the Table 4, the local USNO-B1 catalog rms
uncertainties of the list in RA and Dec are
0
045 and
0
039, respectively, which is a factor of four smaller
than the nominal catalog uncertainty. To refine the fit, we further
removed standards with the largest catalog uncertainties, i.e. the first and
the fourth ones from Table 4.
This provided much smaller rms,
RA = 0
0278
and
Dec = 0
068, and individual standard residuals
,
which is consistent within 2-
with the local rms of the
catalog and with the positional
uncertainties of the standards in the image (
).
We adopted this fit as a final one.
It is important
that by
starting with 13 standards
the image transformation became very stable
and practically
independent
of removing other standards
with the largest residuals. At these steps
coordinate reference points clustered together
only within the same image pixel,
gradually migrating to some limiting position within this pixel.
The same result was obtained using the list of stars with coordinates from GSC-II v2.3.2.
Therefore, we
are quite confident
the derived coordinate reference that is reliable
at least at the level of the nominal mean catalog
accuracy of
0
2. Combining that with the best
fit rms, we obtained secure 1-
uncertainties
of our astrometric referencing in the RA and
Dec as 0
202 and 0
211,
respectively. They are comparable with the pixel scale and
much smaller than the seeing value.
The co-added 2001 B-band image was rebinned by
pixels to get
the same pixel scale as for the 2003-2004 period.
All co-added 2001 B,
,
and H
images were
aligned to the base one using a set of suitable unsaturated stars
with an accuracy of better than 0.025 pixel size, and
the above coordinate reference of the base image was adopted for these
images.
Formal errors of this referencing are
negligible (
0
005) in comparison with the astrometric
referencing uncertainties of the base image.
The selection of another summed image,
or H
,
as a base does not change the result.
Table 4: The list of USNO-B1.0 stars used for astrometrical referencing with coordinates (epoch J2000.0) and their errors.
Table 5: PSR B1133+16 radio coordinates at the epochs of the VLT and Chandra observations.
The radio positions of PSR B1133+16 at the epochs of the VLT and Chandra observations
(Table 5) were determined using radio measurements of
the pulsar proper motion made with the VLA by Brisken et al. 2002.
The position errors in Table 5,
and
,
include the errors
of the radio position at the reference epoch
J2000 (15 mas for both coordinates) and the uncertainties in the pulsar proper motion
(0.38 mas and 0.28 mas in RA and Dec, respectively).
Combining
the errors of our astrometry
and the radio position errors,
we derived uncertainties of
the pulsar position in our images.
Observations in different bands were splinted by different sets
on a time basis from a week to 8-9 months, and
the pulsar has a large
proper motion, particularly along the declination (Table 1)
(cf. Tables 2 and 3),
therefore one has to account for a systematic uncertainty related to
a shift in the pulsar position during this
time. It is mostly significant
for the B and R band observations of the
2003-2004 period when the pulsar shifts
by 0
197 and
0
229, or by about 1 CCD pixel,
from the beginning to the end of the
observations. We added these shifts
as uncertainties squared and calculated the
expected optical counterpart coordinates as a
simple mean of those at the beginning and the end of
the observations for given resulting frames
and periods.
The results are summarized in Table 6.
As seen, despite a rather accurate astrometric
referencing of the summed
images, the resulting
position
uncertainty of the expected optical counterpart
can be as large as 1 arcsec in the Dec due to the large
proper motion of the pulsar.
For precise photometric calibration of all datasets taken in different nights and periods, a two-step approach known as ``differential photometry'' was applied. At first, absolute magnitudes of a set of relatively bright stars visible in all target frames (so called ``secondary standards'') were derived accurately. This was done using images of the primary Landolt's standards obtained during the same night chosen as a ``reference''. In the second step the co-added target images were directly calibrated using the secondary standards. An advantage of this approach is that it is not necessary to consider possible magnitude zero-points and/or extinction variations from night to night since the secondary standards and the target are in the same frames.
Table 6: Expected optical counterpart coordinates at the summed VLT frames.
The night of 02/03 April 2003
was used as a reference for photometric calibration and, for this night, the zero-points in the Johnson-Cousins B and
bands, and color terms establishing the relation between the real B and
magnitudes
and respective instrumental magnitudes
mB and mR were derived using 14
Landolt's standard stars from Rubin149, PG1047+003, and SA110 fields (Landolt 1992):
B = 27.17(3)+mB-0.04(2)(mB-mR) | (1) | ||
R = 27.38(2)+mR+0.01(2)(mB-mR). |
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(2) |
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(3) |
![]() |
(4) |
Table 7: Secondary standard stars used for photometric referencing.
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Figure 2:
Fragments of the PSR B1133+16
field images
obtained in the optical B and ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The B and
magnitudes of the stars chosen as secondary standards
were measured at the reference-night images calibrated
with the primary standards as described above.
The stars are marked by numbers in Fig. 1,
and their magnitudes with errors accounting for the zero-point errors shown in brackets of
Eq. (1) are listed in Table 7.
Table 8: The parameters of the optical objects detected around the expected position of PSR B1133+16.
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Figure 3:
The observed E-W spatial profiles of the regions
containing objects A and Bextracted from the 2001 (top) and 2003 (bottom)
B-band images of Fig. 2 along the slit
with PA of 90![]() ![]() ![]() |
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Using the secondary standard magnitudes, the resulting zero-points obtained
for the images summed over the whole 2003-2004 period are
and
.
For the 2001 period
the resulting B band zero-point
is
.
We also estimated
detection limits of a point-like object in the
co-added images. In the B band it
is
28
2 for the both 2001
and 2003-2004 periods, and
28
6 for the sum
of the two periods.
It is
27
9 in the
band.
There are no significant difference between the B band formal
detection limits for both periods. However,
an ``eye visibility'' of a
object
is better for the 2001 period.
This is due to better seeing conditions and
a higher FORS1 resolution setup during this period (see Sect. 2).
Stellar magnitudes Mj
can be transformed into absolute fluxes Fj
(in erg s-1 cm-2 Hz-1), whenever necessary,
using standard equations
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(6) |
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(7) |
The calibration was performed using the night of 19 January 2004
as a reference. From count-rate variations of stars in the pulsar field
with the airmass, we derived a mean atmospheric extinction factor
in the H
band
.
Then the calibration constant for this night
was derived to be
.
The H
magnitudes of the secondary standards
measured using the reference night image
are listed in Table 7. Based on
these magnitudes the derived
flux detection limit for a stellar object
in the summed H
image is
,
and it is
for a surface brightness of an extended object.
There are no significant objects within the pulsar position error ellipse in
the 2001 B-band image.
The nearest detected object, marked A, is outside the ellipse and lies
1
1 away from the pulsar 2001 position in a direction not coinciding
with the pulsar proper motion. Its B magnitude is
or only
about
of the formal detection limit. The outer contour of this object,
where it merges with backgrounds, is overlaid on the 2003 B-band
image. As seen, it only partially overlaps
the pulsar error ellipse at the epoch of 2003,
although its center is very near to the ellipse border.
At the same time, within the pulsar position uncertainty ellipse in the 2003 B-band image,
we find another faint object (B)
with a similar magnitude
.
By a good positional coincidence with the
expected pulsar coordinates, the object Bcan be considered as a candidate pulsar
optical counterpart. The object A also can be resolved
in this image within its contour of the 2001 epoch (see below).
However, it sits in
the east wing of the candidate
and is not visible in the 2003 B-band
image of Fig. 2, where
the count range is chosen in a such way as to underline the
presence of object B. The positions and magnitudes
of objects A and B are listed in Table 8.
A thorough inspection of the region containing objects A and B in the 2001 and 2003 B-band images was made with using individual exposures. It confirmed that both objects are real but not artifacts caused by, e.g., a poor cosmic ray removing or flat-fielding. Owing to their closeness and faintness, these objects may represent bright parts of the same unresolved extended background feature. To verify this possibility we considered the changes in spatial profiles of the region containing both objects from one observing epoch to another. Within positional uncertainties, the objects almost have the same declinations, while their right ascensions differ by about one arcsecond. This allows us to compare only 1D E-W spatial profiles at a fixed declination.
In Fig. 3 we present
the 1D profiles extracted from the 2001 and
2003 B-band images, shown in Fig. 2,
along a horizontal slit
with PA = 90
and Dec = +15:51:11.49.
The slit length and width
are 4
6 and 0
6, respectively, and the centers of
both objects A and B are within this slit.
The coordinate origin of the horizontal axis in Fig. 3 corresponds
to the eastern edge of the slit with RA = 11:36:03.317.
Only object A is seen in
the 2001 profile (top panel).
The 2003 profile is significantly wider, about twice, and has
an asymmetric shape
suggesting two sources of
different intensities with overlapping
profiles (bottom panel). Its peak is shifted towards west
and corresponds to the position of object B, while
object A is probably responsible for
the ``shoulder'' in the east profile wing.
The objects are too faint to be resolved reliably as two point-like
sources with standard PSF modeling and subtraction
tools. To analyze the situation,
we applied a simplified approach based on the fact that
only a single object A is resolved in the 2001 B-band image and profile.
We used its profile shape as a template of a single source profile
to fit the suggested A+B blend at the 2003 period
by a sum of the emissions from two sources, assuming
that B has the similar profile shape.
Within uncertainties, this model is in a good agreement with the
2003 observations
if the peak intensity of object A is by a factor of 1.5 lower than that of B. This corresponds to about a 0.4 mag
difference, which is within the uncertainties of the objects'
magnitude measurements.
The distance between A and B is about
0
8-1
0. This simplified approach supports the
interpretation that A does not change its position and
brightness significantly from the 2001 epoch to 2003, while B appears
in the 2003 image as an additional source roughly of the
same brightness, and by positional coincidence it can be associated with the
pulsar. We discuss this and other possibilities of the A+Binterpretation in Sect. 5.
Neither of the objects A and B are resolved
in the R-band image, which is obviously shallower due to a shorter integration
time (cf. Sect 2).
Nevertheless, the absence of a ``red'' source at the expected pulsar positions
allows us to constrain the color index for the candidate detected in the B band as
.
To better compare the 2005 X-ray and the 2003 B-band images we overlaid the contours
of the optical image onto the X-ray image. The pixel scale of the ACIS-S image is
about 0
5, which is comparable to the proper motion shift of the pulsar between
the 2003 and 2005 epochs, but about
three times smaller than the FWHM of the ACIS PSF. Within these uncertainties and a nominal
1
Chandra/ACIS pointing accuracy,
the contour of the possible optical counterpart,
object B, fits the position
of the candidate
X-ray counterpart nicely, which is the brightest object in the X-ray image.
This implies, at least, that we probably see the same object in the optical and X-rays.
The position of the X-ray candidate with 0
2 accuracy coincides with the
expected radio position of the pulsar at the epoch of the Chandra observations
(Kargaltsev et al. 2006).
We also note that counterparts of some other optical objects in the field may
be found in the X-ray image and vice versa.
The 2004 H-image of the same field and the difference in the H
and normalized R-band
images, where most of the continuum emission is subtracted, are shown
in Fig. 4. The contours of the
X-ray image, which is also shown here but
with a slightly different gray scale
than in Fig. 2,
are overlaid on the (H
)-image and vice versa. As in Fig. 2, all images
are smoothed with the Gaussian kernel of about 1
,
which roughly corresponds
to the seeing of the resulting optical images.
In the H
image we find a faint point-like source (C), which lies within the pulsar
position error ellipse. It is near the detection limit and its star-like H
magnitude
is
(
formal detection limit), which corresponds to the flux
ergs cm-2 s-1 Hz-1 or
photon cm-2 s-1.
The object is not detected in the B and
bands, while
its possible artifact origin
in the narrow band was ruled out
after a careful inspection of individual H
frames.
We do not see any characteristic extended bow shock
structure around the pulsar, as is produced, for instance, around another high-velocity pulsar
PSR B2224+65, known as the Guitar nebula (Cordes et al. 1993). Instead of that, we see a faint and rather
clumpy H
emission most likely associated with the recombination of a heated ambient matter around
background objects of the field. Some of them may have X-ray counterparts.
However, there is one faint structure (about 4.5
length) south of
object C.
This apparent structure is roughly aligned with
the pulsar proper motion direction and located behind its 2003 position.
It is better seen in the (H
-R) image where it merges
with
object C. There are no signs of the structure in the broad band optical images. At the same time,
in the X-ray image, we also see a marginal tail-like structure behind the pulsar counterpart
candidate, though of smaller spatial extent. It was not mentioned by Kargaltsev et al. (2006), who only reported
on the detection of the X-ray pulsar candidate.
If both
structures are not background fluctuations, they can be considered as
cometary-like pulsar tail candidates.
![]() |
Figure 4:
The same as in Fig. 2 but for the H![]() ![]() ![]() |
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One of the reasons is the faintness of the object, which is resolved at a level very
close to the estimated 3-
detection limits (see Sect. 3.2).
To estimate the probability of detecting an object with
in our data,
we selected almost a homogeneous subset of 2001 B-band images, B12-B17, which provides
a maximal number of frames with the same seeing, background, and atmospheric extinction
(see Table 3). Then, we selected about 100 faint objects chosen
randomly in the sum of these 6 frames and checked that these objects are
detected in the each of the individual frames. That was done in two ways.
In the first, we used the optimal aperture, with 3 pixel radius, and an object
was accepted as detected at an individual image if its magnitude error was
less that 0.36, which corresponded to
.
In the second way we estimated
the object by eye in the images smoothed with the Gaussian kernel.
In the first case the probability
of detecting an object with
was 60%-70%. In the second, less formal case,
it was about 80%-90%. This means that, if we have two sets of VLT observations in the same band
with approximately equal conditions and exposure lengths and detect
a faint object with
in one of them, the probability of losing this
object in backgrounds at the second set of observations by pure count statistic
is about 10%-40%. Accounting for not quite homogeneous conditions for
the 2001 and 2003 periods, a conservative estimate for losing the suggested
pulsar optical counterpart at the expected
position at the 2001 epoch is about 50%.
In addition, a slightly higher background, e.g.,
due to the closeness of a nearby bright object east of the pulsar path (cf. Fig. 2)
or to a small increase in the interstellar extinction (e.g., by
)
for the line of sight towards the 2001 pulsar
position can drown the candidate in backgrounds in the 2001 B image.
The pulsar optical brightness variation cannot be excluded also, since it is known to show a pulse nulling phenomenon in the radio range (e.g., Bhat et al. 2007 and references therein). It spends up to 15% of its time in the null state, and shows epochs when its radio flux changes by a factor of two. There are still no direct simultaneous radio and optical observations of pulsars. However, one cannot rule out a link between pulsar activities in these domains since both emissions are governed by complicated NS magnetosperic processes. The 2001 observing period of one week duration is much shorter than the 2003 one spanning 8 months and a chance to meet a comparable flux depression phase that might completely hide the counterpart in the 2001 epoch is higher.
We also cannot exclude the possibility that B is simply a time variable background object or a part of an extended unresolved variable feature that includes A and B as its relatively brighter regions. Another possibility is that A and B represent the same object displaced between 2001 and 2004 epochs by its own proper motion. However, our analysis has shown (Sect. 4.1) that A and B are likely to be two single objects and A does not show any significant variability or proper motion towards the position of Bto explain the variability of the latter. We have also found no other variable objects in the pulsar neighborhood at the time base of three years. Possible variability of B can be checked only by further observations. If they will show that B survives and moves consistently with the pulsar this will be a strong proof that we see the real optical couterpart.
A good positional coincidence of B with the
candidate X-ray pulsar counterpart suggests that we see
the same object in the optical and X-rays.
Its color index,
,
is compatible with the indices of Vela pulsar, PSR B0656+14, Geminga, and
PSR B0950+08,
which all have B-R in a narrow range of 0.46-0.6
(Mignani & Caraveo 2001; Shibanov et al. 2006; Zharikov et al. 2004).
Let us assume that the optical object B and
the respective X-ray source are indeed the pulsar
PSR B1133+16. The spectral fit of the Chandra/ACIS-S
data of the X-ray counterpart candidate
by an absorbed power law (Kargaltsev et al. 2006)
yields the non-thermal X-ray luminosity of the pulsar
in the 2-10 keV range at the distance of 357 pc, or
.
The corresponding X-ray efficiency is
,
which is not exceptional and lies near the efficiency
range of the two other old pulsars, PSR B1929+10 and PSR B0950+08, detected in the
optical and X-rays
(Zharikov et al. 2004,2006).
![]() |
Figure 5:
The evolution of the pulsar radio, optical and X-ray luminosities,
and respective efficiencies with the characteristic age ![]() |
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Using
and the empirical relation between the non-thermal optical and X-ray efficiencies
of ordinary pulsars reported by Zharikov et al. (2004,2006),
we would expect to find the PSR B1133+16 optical counterpart with a magnitude in a range
of about 29-30 and with the optical efficiency
.
The proposed
candidate is a magnitude brighter and the respective
optical luminosity and efficiency,
and
,
are also about one order of magnitude higher than
is expected from the reported
relation. However, this may simply reflect
inaccuracies in the empirical relation, which is based on a very limited
sample of the optically detected pulsars.
To study how this discrepancy can modify the relation and the evolution tendencies
considered by Zharikov et al. (2004,2006),
we included
the proposed candidate in the sample. The results are shown in Figs. 5 and 6.
As seen (Fig. 5), our candidate does not invalidate
the previous conclusion regarding the high optical and X-ray efficiencies of old pulsars,
which become comparable to those of the young Crab-like pulsars.
At the same time, its position on the
-
plane significantly deviates
from the previous relation (Fig. 6). Two of its neighbors,
PSR B1929+10 and PSR B0950+08, also show apparent deviations.
However, if we exclude
from the sample
the two youngest pulsars, the Crab pulsar and PSR B0540-69, we obtain a new relation
that fits the rest pulsars nicely, including the PSR B1133+16 candidate.
This is not a surprise, since noticed Zharikov et al. (2004) that
the young pulsars are distinct from the older ones by their
X-ray and optical efficiencies and displacement on the
-
and
-
planes. Therefore, in both respects the proposed candidate is not outstanding.
That can be considered as additional evidence that it is the real counterpart of PSR B1133+16.
![]() |
Figure 6:
Relations between the optical
![]() ![]() |
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Despite the high transverse velocity of PSR B1133+16,
we did not detect any developed Balmer bow shock nebula of the pulsar.
This suggests low ISM density in the pulsar
neighborhood that is compatible with
its high galactic latitude 69
,
low E(B-V) = 0.04 towards the pulsar, and a small hydrogen column
density
= (1-2)
1020 cm-2 (Kargaltsev et al. 2006) estimated from the measured pulsar dispersion measure
of 4.86 cm-3 pc and the spectral fit of the candidate X-ray
counterpart.
Nevertheless, the emission detected at
about the
detection limit in the pulsar position error ellipse
in the H
image
(object C) may come out from the brightest part
of the bow shock head and/or from a clump of the ISM with a higher density.
The low S/N ratio does not allow us to conclude this confidently.
The absence of object C in
our B and R broad band images prabably excludes its
background
origin. Accounting for deep B and R magnitude upper
limits and a high galactic latitude with low E(B-V),
object C certainly cannot
be a star from our Galaxy. Any background extragalactic object is also very unlikely since
it would be detected first of all in our deep broad band images.
Instead of the bow shock,
we note two marginal but likely spatially correlated extended features in the H
emission and X-rays,
which are reminiscent of a ``tail'' behind the pulsar.
If the X-ray and the H
tails are indeed
associated with the pulsar, this would present the first example of when the pulsar tail is detected simultaneously
in the H
and X-rays. The combinations of the X-ray tails and H
bow shocks for a few pulsars
have been reported in the literature (see, e.g., Zavlin
2006, for a short review).
Most
recently such a tail has been reliably detected in X-rays
with the XMM behind PSR B1929+10 (Becker et al. 2006),
which is also a member of the sample considered above.
However, despite these efforts, no
counterparts of the tails have been detected so far in the
optical range to our knowledge.
The formation of the H
tails
does not look unreasonable when a compressed pulsar wind
behind a supersonically moving NS cools via X-ray
emission that ionizes the ambient matter on larger spatial
scales, which then recombines producing H
photons. An apparent offset of the X-ray tail
candidate from the H
elongated structure in our case
(cf. the images with contours in the H
and X-ray panels in Fig. 4)
is consistent with such a scenario.
The suggested candidate
optical and X-ray counterparts of PSR B1133+16 and its tail can be easily verified by
a followup imaging of the field in the optical and X-rays on the basis of a few years.
Given the pulsar proper motion, the candidates, if they are the real counterparts,
will be shifted from the detected positions roughly to the north by
about 1
4-1
8 by the end of 2008.
Such a shift can be reliably measured using the subarcsecond spatial resolutions
of 8 m-class optical telescopes
and the Chandra X-ray observatory.
Acknowledgements
We are grateful to the anonymous referee for many useful comments and suggestion that allowed us to improve this paper considerably. This work has been partially supported by CONACYT 48493 and PAPIIT IN101506 projects, RFBR (grants 05-02-16245 and 05-02-22003), FASI (grant NSh-9879.2006.2), and Fondecyt 1070705. We used the USNOFS Image and Catalogue Archive operated by the United States Naval Observatory, Flagstaff Station (http://www.nofs.navy.mil/data/fchpix/). This work is based on observations made with the European Southern Observatory telescopes obtained from the ESO/ST-ECF Science Archive Facility. The Munich Image Data Analysis System is developed and maintained by the European Southern Observatory.