J/A+A/416/1037 BVRIK Photometry for the field of 4U 0142+61 (Hulleman+, 2004)
The anomalous X-ray pulsar 4U 0142+61: variability in the infrared and a
spectral break in the optical.
Hulleman F., van Kerkwijk M.H., Kulkarni S.R.
<Astron. Astrophys. 416, 1037 (2004)>
=2004A&A...416.1037H 2004A&A...416.1037H
ADC_Keywords: Pulsars ; X-ray sources ; Photometry, UBVRI ;
Photometry, infrared ; Positional data
Keywords: pulsars: individual: 4U 0142+61 - stars: neutron -
stars: magnetic fields - X-rays: general
Abstract:
We present new optical and infrared observations of the counterpart
to the Anomalous X-ray Pulsar (AXP) 4U 0142+61 taken with the Keck I
telescope. The counterpart is found to be variable in the infrared.
This contrasts with our optical observations, which do not show any
evidence for variability. Apart from the variability the AXP shows a
remarkable spectral energy distribution. In particular, we find a
sudden drop in flux going from V to B, presumably due to a spectral
feature. We compare our results to those obtained for the two other
securely identified AXP counterparts, to 1E 2259+586 and 1E
1048.1-5937. 4U 0142+61 is very similar to the former source in its
X-ray timing and spectral properties, and we find that this similarity
extends to the quiescent infrared to X-ray flux ratio. For 1E
1048.1-5937, which has different X-ray properties, the situation is
less clear: in one observation, the infrared to X-ray flux ratio was
much larger, but another observation gave an upper limit which is
consistent with that observed for 4U 0142+61. Assuming the quiescent
ratios are all similar, we estimate the optical and infrared
brightnesses for the three AXPs that remain to be identified as well
as for the four Soft Gamma-ray Repeaters. We also discuss briefly how
the observed optical and infrared emission might arise, in particular
in the context of the magnetar model.
Description:
Infrared images in the K band were obtained on the nights of 1999
February 7 and 8 with the Near Infrared Camera, mounted on the Keck I
telescope.
Further images were obtained on 2001 October 31 in the Ks band.
Finally, an additional set of 18 frames, each consisting of five
co-added exposures of 10s, was obtained on 2002 December 18.
Optical observations were obtained with the Low Resolution Imaging
Spectrograph at either of the two Keck telescopes.
For photometric calibration, we also used observations taken with the
60-inch telescope on Palomar mountain, using the direct imaging camera
at the Cassegrain focus.
Observations in the B band were made for us on 2000 September 3.
On 2000 December 29, further LRIS observations were taken, in Band R.
On 2000 July 23, V, R and I-band images were taken under photometric
conditions.
In order to verify the calibration, as well as to obtain a calibration
for the B band, additional images were taken on 2000 November 18 (B,
V, R and I) and 19 (B and V).
Table3 contains photometry for 4U 0142+61 and objects in the field.
The errors listed are measurement uncertainties, and do not include
the zero-point uncertainty of 0.03mag in all bands. The astrometry is
based on the K-band image. Measurement errors range from 0.02arcsec
for objects as faint as 4U 0142+61, to 0.005arcsec for the brighter
ones. The astrometry is on the USNO-A2.0 system to within 0.1arcsec;
it should be on the International Celestial Reference Frame to about
0.2arcsec.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 100 43 BVRI photometry for objects in the field of
AXP 4U 0142+61
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Right ascension (J2000.0)
4- 5 I2 min RAm Right ascension (J2000.0)
7- 12 F6.3 s RAs Right ascension (J2000.0)
14 A1 --- DE- Declination sign (J2000.0)
15- 16 I2 deg DEd Declination (J2000.0)
18- 19 I2 arcmin DEm Declination (J2000.0)
21- 25 F5.2 arcsec DEs Declination (J2000.0)
27- 32 F6.3 mag Bmag ? B band magnitude (1)
34- 38 F5.3 mag e_Bmag ? Error in B band magnitude
40- 45 F6.3 mag Vmag ? V band magnitude
47- 51 F5.3 mag e_Vmag ? Error in V band magnitude
53- 58 F6.3 mag Rmag ? R band magnitude
60- 64 F5.3 mag e_Rmag ? Error in R band magnitude
66- 71 F6.3 mag Imag ? I band magnitude
73- 77 F5.3 mag e_Imag ? Error in I band magnitude
79- 84 F6.3 mag Kmag ? K band magnitude (1), (2)
86- 90 F5.3 mag e_Kmag ? Error in K band magnitude
92- 94 A3 --- Ident Identification (3)
96- 97 A2 --- Other Previous identification (4)
99 A1 --- Flag [F] bad photometry flag (5)
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Note (1): The B band magnitude for the AXP (A) is marginal detection,
the 3σ limit is B>27.4, its K band magnitude varied between
K=19.7 and K=20.2.
Note (2): Average of K and Ks band magnitudes
Note (3): Identificiations:
A for 4U 0142+61 ([HVK2000] A, Hulleman et al., 2000Natur.408..689H 2000Natur.408..689H)
Numbers 102 to 143 for the field stars
Note (4): Numbering scheme introduced by Reid et al. (1980AJ.....85.1062R 1980AJ.....85.1062R)
and subsequently followed and extended by White et al.
(1987MNRAS.226..645W 1987MNRAS.226..645W) and Coe & Pightling (1998MNRAS.299..223C 1998MNRAS.299..223C).
= [DAB78] 2S 0142+614 NNa in Simbad
Note (5): Indicates that magnitudes are uncertain, because
the object was saturated in (part of) the images.
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Acknowledgements: Ferdi Hulleman
(End) Ferdi Hulleman [UU, Netherlands], Patricia Bauer [CDS] 12-Feb-2004