Date and Time of the Query: 2022-01-25 T23:56:23 PST
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Notes for object [HB89] 2131-021

12 note(s) found in NED.

1. 2006A&A...456..131P
Re:[HB89] 2131-021
Faint source; no significant detection in the LW7 filter. To increase the signal
to noise (S/N) we used an extraction radius of three pixels for the LW3 filter,
instead of four.

2. 2005A&A...440..831K
Re:[HB89] 2131-021
This is the highest redshift BL Lac object in the sample (z = 1.285). It has a
faint companion galaxy ~4 arcsec (47 kpc) to NE. The host galaxy remained
unresolved in the optical studies by Scarpa et al. (2000a;m(r) > -27.0_),
H04(M(I) > -28.3), and Pursimo et al. (2002) M(R) > -27.8). Despite the mediocre
seeing (FWHM ~ 1.0" ), we detected the host galaxy in the K-band with R(e) = 4.7
kpc and M(K) = -28.5).

3. 2005A&A...435..839T
4.3 Candidate inverted-spectrum sources
B2131-021: the redshift of this BL Lac object is 1.285. Cassaro et al.
(1999) made VLBI observations with the VLA of a sample of 28 BL Lacs
including this object. The source is compact but there is some extended
emission up to angular size of 9".
Lower envelope of the spectrum in Fig. 8 is flat. There is variability
at all the frequencies and the upper envelope is barely convex.

4. 2003AJ....125.2447R
Re:[HB89] 2131-021
PKS 2131-021: The 1.4 GHz map of this object in Rector & Stocke (2001)
is highly suggestive of a lensed system, with possible multiple images
of the core to the east and southeast. However, 4.86 and 8.46 GHz maps
of this object (Figs. 7 and 8) show a very interesting morphology, with
a jet that originates to the south-southeast of the core. The jet
appears to be significantly distorted and may be helical. There also
appears to be lobes roughly 5" to the east and southeast of the core. It
is not clear how these jets propagate to the lobes, although projection
effects may cause a single lobe to appear as the two seen. However,
significant bending is still required to cause both lobes to appear on
the same side of the core, even with the severe projection effects
expected in BL Lac objects. The spectral index map (Fig. 7) shows no
evidence of multiple images of the core, and the jet and lobes have
indexes ({alpha}^4.86^_8.46_ <~ -1) much steeper than the core
({alpha}^4.86^_8.46_ = +0.06). A MERLIN+EVN map of this object 8.46
shows a jet component 50 mas east-southeast from the core and multiple
components extending to the south out to 350 mas from the core,
suggesting a helical motion (Cassaro et al. 2002).

5. 2001AJ....122..565R
Re:PKS 2131-021
PKS 2131-021 (4C -02.81). - Wills & Lynds (1978) suggested a redshift
of z = 0.557; however, our spectrum shows [O II] {lambda}3727,
Mg II {lambda}2798, and C III] {lambda}1909 in emission at z = 1.285.
These emission lines are at the upper limit in W_{lambda}_ for BL Lac
objects. Drinkwater et al. (1997) independently confirm our redshift. In
addition, there are very weak absorption features at z ~ 0.36, which are
more apparent in a subset of our KPNO 2.1 m spectra. These very tentative
detections require confirmation and suggest that Mg II absorption should
be searched for at ~3800 A. Surprisingly, the extended radio structure
resembles a wide-angle tail but with the extended radio power and
leading-edge hotspots like an FR 2. This unusual radio morphology is
suggestive of a gravitationally lensed source; however, ~0.1" resolution
maps reveal a jet to the southeast of the core that bisects the lobes to
the south and east (Rector & Stocke 2001).

6. 2000MNRAS.319.1109G
Re:[HB89] 2131-021
3.7 2131-021
This source has the largest redshift among the sample objects, z = 1.28
(Drinkwater et al. 1997). The structure is very compact, with a jet
extending nearly directly east (Fig. 7); the faint jet is also visible in
the 2-cm image of Kellermann et al. (1998) and the 13- and 4-cm images of
Fey & Charlot (1997). We detected polarization only in the VLBI core, and
this polarization varied during the VLBI observations. The P map in
Fig. 7(b) was obtained for one time subinterval considered in the intraday
variability analysis (Gabuzda et al. 2000b). The degree of polarization was
only mildly variable, m ~ 1.6 per cent, while {chi}_core_ varied by about
15^deg^ over the course of the VLBI run, remaining roughly perpendicular
to the direction of the VLBI jet (to within ~30^deg^).

7. 1999ApJS..121..131F
Re:[HB89] 2131-021
There are only 2 nights of infrared data, showing a variation of 0.8
mag (J=16.06-16.88, H=15.05-15.97, K=14.23-14.88), which is comparable
with the optical variation of {DELTA}m=1.2 mag (Stickel et al. 1993).
Mean infrared color indices of (J-H)=0.96+/-0.07, (J-K)=1.91+/-0.12, and
(H-K)=0.96+/-0.16 can be obtained. P_opt_=16.9% is reported by Kuhr &
Schmidt (1990).

8. 1999A&AS..139..601C
Re:[HB89] 2131-021
2131-021: the unresolved core is located at the NW edge of the radio
emission (Fig. 31). Two jet like structures are oriented in PA ~ -90 deg
and PA ~ -170 deg; all this is reminiscent of NAT/WAT morphology. The
B array image at 1.36 GHz does not reveal any further extended emission
and we can therefore consider that the total angular size of the
extended radio emission is about 9". Observations of this source were
made by Perley (1982) and he found an extended flux of about 50 mJy.
Recently a VLA image in B configuration at 1.49 GHz was published by
Hutchings et al. (1998). The lower resolution of their observations did
not allow to properly separate the core flux density from the
tail-shaped extended emission yielding to underestimate the total
extended flux density.

9. 1998AJ....115.1295K
Re:[HB89] 2131-021
2131-021.--This object contains a closely spaced double.

10. 1997MNRAS.284...85D
Re:[HB89] 2131-021
(ix) PKS 2131-021: also observed with the ANU 2.3-m telescope on 1995 September
28; combined spectrum used. The redshift is based on O II and Mg II in our
spectra and a reported `definite' line at 3541 A (Baldwin, Wampler & Gaskell
1989) which we identify with C IV.

11. 1993A&AS..100..521V
Re:PKS 2131-021
PKS 2131-021 (Wall et al. 1971; Wright et al. 1982), 4C-02.81 (Gower et
al. 1967) OX-053 (Ehman et al. 1970) has been identified by Bolton & Wall
(1970) with a 19 mag. UVX object with a high degree of optical
polarization (Kuhr & Schmidt 1990; Impey & Tapia 1990). No definite lines
have been seen in the spectrum (Browne et al. 1975), while Wills & Lynds
(1978) have suggested a very uncertain redshift, z = 0.557. The radio
(Perley, 1982) and optical (Torres & Wroblewski 1987) positions are in
good agreement. The object is an X-ray source (Schwarz & Ku 1983).
The radiosource is compact (Wills 1979) with a VLBI core (Booth et al.
1979; Preston et al. 1985) and variable (Brandie & Bridle 1974; O'Dea et
al. 1986; Aller et al. 1985). Its spectrum is flat (Wall 1972; Brandie &
Bridle 1974; Edelson 1987).
Our spectrum is continuous and does not confirm the redshift
z = 0.557. However, emission lines with equivalent width of 5 A would not
have been detected on our spectrum which has a relatively low signal to
noise ratio; lines twice as strong could not have escaped detection.
PKS 2131-021 has therefore an inconclusive spectrum.

12. 1993A&AS...98..393S
Re:PKS 2131-021
The direct image shows this object to be unresolved with a faint
companion object ~4" to the northeast. From a single spectrogram Wills &
Lynds (1978) suggested an uncertain redshift of z = 0.557 for 2131-021.
Baldwin et al. (1989) detected definitely one and possibly two other
narrow emission lines which could, however, not be identified with any of
the typical emission lines seen in these objects. Our spectrum is
featureless showing no obvious, broad emission line and appears to be too
noisy to confirm the presence of the weak, narrow emission lines observed
by Baldwin et al. (1989).

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