J/ApJ/660/1428 Astrometry around RX J0720.4-3125 (Kaplan+, 2007)
The distance to the isolated neutron star RX J0720.4-3125.
Kaplan D.L., Van Kerkwijk M.H., Anderson J.
<Astrophys. J., 660, 1428-1443 (2007)>
=2007ApJ...660.1428K 2007ApJ...660.1428K
ADC_Keywords: X-ray sources ; Positional data
Keywords: astrometry - stars: individual (RX J0720.4-3125) - stars: neutron -
X-rays: individual (RX J0720.4-3125)
Abstract:
We have used a set of dedicated astrometric data from the Hubble Space
Telescope to measure the parallax and proper motion of the nearby
neutron star RX J0720.4-3125. At each of eight epochs over 2 years, we
used the High Resolution Camera of the Advanced Camera for Surveys to
measure the position of the B=26.6 target to a precision of ∼2mas
(∼0.07pix) relative to 22 other stars. From these data we measure a
parallax of plx=2.8±0.9mas (for a distance of 360+170-90pc) and
a proper motion of mu=107.8±1.2mas/yr. Exhaustive testing of every
stage of our analysis suggests that it is robust, with a maximum
systematic uncertainty on the parallax of 0.4mas. The distance is
compatible with earlier estimates made from scaling the optical
emission of RX J0720.4-3125 relative to the even closer neutron star
RX J1856.5-3754. The distance and proper motion imply a transverse
velocity of 180+90-40km/s, comparable to velocities observed for
radio pulsars. The speed and direction suggest an origin for RX
J0720.4-3125 in the Trumpler 10 OB association ∼0.7Myr ago, with a
possible range of 0.5-1.0Myr given by the uncertainty in the distance.
Description:
Our main data set consists of 64 exposures taken during eight visits
of the field of RX J0720.4-3125 with HST, between 04 Jul 2002 and 19
Mar 2004. All data were taken with the High Resolution Camera (HRC;
with a plate scale of 28.27mas/pix) and the F475W (SDSS g') filter,
which has good sensitivity to the blue colors of RX J0720.4-3125.
Objects:
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RA (2000) DE Designation(s)
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07 20 24.961 -31 25 50.21 X = RX J0720.4-3125
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 70 23 *Astrometry of reference sources
table7.dat 130 1172 ePSF position measurements for all exposures
table9.dat 56 156 Merged positions for each epoch
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Note on table4.dat: Object No. 117 not in table4.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- ID Object identifier (G1)
5 I1 --- o_ID [1/8] Number of epochs for which a source
could be measured
7- 11 F5.1 mag mag Instrumental magnitude (2)
13- 21 F9.5 arcsec DRA Offset in right ascension (3)
23- 24 I2 10mas e_DRA rms uncertainty on DRA
27- 35 F9.5 arcsec DDE Offset in declination (3)
37- 39 I3 10mas e_DDE rms uncertainty on DDE
42- 47 F6.2 mas/yr pmRA Proper motion along RA (4)
49- 52 F4.2 mas/yr e_pmRA rms uncertainty on pmRA
55- 59 F5.2 mas/yr pmDE Proper motion along DE (4)
61- 64 F4.2 mas/yr e_pmDE rms uncertainty on pmDE
67- 70 F4.2 --- Rchi2 Reduced χ2 (5)
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Note (2): Instrumental magnitudes defined as -2.5log10a, where a is the
average amplitude of the effective PSF (ePSF) required to fit a given
star in a single exposure.
Note (3): Position offsets at MJD 52645.1, relative to star A, excluding
parallactic offsets. Reference position: 07:20:24.4837-31:25:51.786
(J2000.0)
Note (4): Proper motion relative to that of star A. The systematic
uncertainty on the proper motion, from assuming that star A has mu=0,
is ∼1mas/yr.
Note (5): Reduced χ2 values for fitting, with degrees of freedom
2Nepoch-Npar, where the number of parameters Npar=5 for RX
J0720.4-3125, 0 for A, and 4 for all other stars (note that these
χ2red values for individual sources are not robust estimates.
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Byte-by-byte Description of file: table7.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- ID Object identifier (G1)
5- 9 A5 --- Exp Exposure identifier (hrcNN)
11- 20 F10.5 pix DXpix Distortion-corrected X position
22- 28 F7.5 pix e_DXpix Uncertainty in Xpix
30- 39 F10.5 pix DYpix Distortion-corrected Y position
41- 47 F7.5 pix e_DYpix Uncertainty in Ypix
49- 53 F5.3 --- Num Number of bad pixel and distance to
nearest bad one (1)
55- 62 F8.3 --- Rchi2 Reduced χ2 of the ePSF fit
64- 73 F10.5 pix Xpix Measured X position on chip
75- 81 F7.5 pix e_Xpix Uncertainty in Xpix
83- 92 F10.5 pix Ypix Measured Y position on chip
94-100 F7.5 pix e_Ypix Uncertainty in Ypix
102-110 F9.2 --- Amp Amplitude of effective PSF (ePSF)
112-117 F6.2 --- e_Amp Uncertainty in Amp
119-124 F6.3 --- Sky Sky level
126-130 F5.3 --- e_Sky Uncertainty in Sky (0 = fixed sky value)
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Note (1): This is the sum of the number of bad pixels and the distance from
the initial guess of the centroid to the nearest bad pixel, divided by
1000. Only measurements with values of 0 (no bad pixels) were used in
our analysis.
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Byte-by-byte Description of file: table9.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- ID Object identifier (G1)
5- 8 A4 --- Exp Exposure identifier (hrcN)
10- 19 F10.5 pix DXpix Distortion-corrected X position
21- 27 F7.5 pix e_DXpix Uncertainty in D-Xpix
29- 38 F10.5 pix DYpix Distortion-corrected Y position
40- 46 F7.5 pix e_DYpix Uncertainty in D-Ypix
48- 49 I2 --- Num Number of measurements (2x number of exposures)
51- 56 F6.3 --- Rchi2 Reduced χ2 (=χ2/(Num-6))
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Global notes:
Note (G1): Star number, with letters following Haberl et al.
(1997A&A...326..662H 1997A&A...326..662H), i.e.
A = [HMB97] A = [KVA2007] 102
B = [HMB97] B = [KVA2007] 103
X = RX J0720.4-3125 = [KVA2007] 101
For other sources, [KVA2007] NNN in Simbad.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Patricia Vannier [CDS] 13-Mar-2009