J/A+A/706/A41 Unveiling the white dwarf in HU Aqr (Schwope+, 2026)
Unveiling the white dwarf in the eclipsing polar HU Aquarii.
Schwope A.D., Marsh T.R., Parsons S.G., Vogel J., Dhillon V.S.
<Astron. Astrophys. 706, A41 (2026)>
=2026A&A...706A..41S 2026A&A...706A..41S (SIMBAD/NED BibCode)
ADC_Keywords: Binaries, eclipsing ; Binaries, cataclysmic ; Optical ;
Ultraviolet ; Photometry
Keywords: binaries: eclipsing - stars: magnetic field - novae -
cataclysmic variables - stars: individual: HU Aqr
Abstract:
We present an analysis of high-speed u- and r-band photometry of the
eclipsing polar HU Aqr that was obtained with ULTRACAM mounted on the
VLT. The observations were performed during a low state, permitting us
for the first time to determine the contact points of the white dwarf.
Using LCURVE we could determine its size, and hence mass, with a
direct method and with unprecedented accuracy. We determined the mass
of the white dwarf as 0.78±0.02M☉, the mass ratio
Q=MWD/Msec=4.59, and the orbital inclination i=87.4±0.9°. An
extended warm region with a central temperature of ∼33000K was
observed in the u-band at the location of the previous high-state
accretion spot. Weak accretion was ongoing in the low state that led
to cyclotron emission that could best be studied with the r-band data.
It has a diameter of only 3° to 4° and is located much closer
to the binary meridian than the accretion-heated region studied in the
u-band. The longitudinal shift of the two accretion regions is of
order 30°, due to early and late coupling of accreted matter onto
the magnetic field lines in low and high accretion states,
respectively. The low-state cyclotron-emitting region has a vertical
extent of 0.005-0.016 RWD, a value that seems to be correlated
to the instantaneous accretion rate.
Description:
Provided are 9 ULTRACAM log files from runs at VLT/UT3 in May 2005.
Log files are the final products of the data reduction chain.
ULTRACAM is operated with three CCDs. For each CCD of each frame
reduced, information is provided as one record.
Objects:
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RA (2000) DE Designation(s)
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21 07 58.19 -05 17 40.5 HU Aquarii = V* HU Aqr
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
run013.dat 251 26190 Reduced data 14/05/2005, 08:32:34.00,
ULTRACAM/VLT
run014.dat 251 32853 Reduced data 14/05/2005, 09:59:02.00,
ULTRACAM/VLT
run020.dat 251 36297 Reduced data 16/05/2005, 06:33:09.00,
ULTRACAM/VLT
run021.dat 251 36537 Reduced data 16/05/2005, 07:49:14.00,
ULTRACAM/VLT
run022.dat 251 9912 Reduced data 16/05/2005, 09:28:42.00,
ULTRACAM/VLT
run033.dat 251 12249 Reduced data 17/05/2005, 09:55:48.00,
ULTRACAM/VLT
run039.dat 251 34806 Reduced data 15/05/2005, 08:20:30.00,
ULTRACAM/VLT
run040.dat 251 34773 Reduced data 15/05/2005, 08:29:39.00,
ULTRACAM/VLT
run041.dat 251 6123 Reduced data 15/05/2005, 10:03:30.00,
ULTRACAM/VLT
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See also:
J/MNRAS/448/1118 : HU Aqr planetary system mid-egress moments
(Gozdziewski+, 2015)
Byte-by-byte Description of file (# headlines): run*.dat
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Bytes Format Units Label Explanations
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1- 8 I8 --- Number Frame number
10- 25 F16.10 d MJD Modified Julian Date (UTC) centre of exposure
27 I1 --- Flag [0/1] Time flag (1=OK,0=Not OK)
29 I1 --- Nsat Number satellites
32- 39 F8.6 s ExpTime Exposure time
41 I1 --- CCD [1/3] CCD number (1=red,2=green,3=uv).
43- 49 F7.3 --- FWHM Fitted FWHM, =0 if no fit made.
51- 57 F7.3 --- beta ?=0 Fitted Moffat exponent
60 I1 --- Naper1 Aperture number
63- 69 F7.3 pix X1 X-position of aperture 1
72- 78 F7.3 pix Y1 Y-position of aperture 1
81- 87 F7.3 pix Xm1 Measured aperture position (0,0) for aperture 1
90- 96 F7.3 pix Ym1 Invalid and/or linked apertures for aperture 1
98-103 F6.3 pix e_Xm1 ?=-1 1-sigma uncertainty in measured aperture
position for aperture 1
105-110 F6.3 pix e_Ym1 ?=-1 (-1,-1) for invalid and/or linked
apertures for aperture 1
115-121 F7.1 ct Counts1 Estimated target counts for aperture 1
123-129 F7.1 ct e_Counts1 ?=-1 1-sigma uncertainty for aperture 1
133-139 F7.2 ct Sky1 Sky background in counts for aperture 1
141-143 I3 --- NSky1 Number of sky pixels available for aperture 1
146-148 I3 --- NRej1 Number of sky pixels rejected for aperture 1
150-151 I2 --- Worst1 Value of worst bad pixel in aperture (0 OK)
for aperture 1
153-154 I2 --- errorfl1 Code for aperture 1 (1)
157 I1 --- Naper2 Aperture number
160-166 F7.3 pix X2 X-position of aperture 2
169-175 F7.3 pix Y2 Y-position of aperture 2
178-184 F7.3 pix Xm2 Measured aperture position (0,0) for aperture 2
187-193 F7.3 pix Ym2 Invalid and/or linked apertures for aperture 2
195-200 F6.3 pix e_Xm2 ?=-1 1-sigma uncertainty in measured aperture
position for aperture 2
202-207 F6.3 pix e_Ym2 ?=-1 (-1,-1) for invalid and/or linked
apertures for aperture 2
212-218 F7.1 ct Counts2 Estimated target counts for aperture 2
220-226 F7.1 ct e_Counts2 ?=-1 1-sigma uncertainty for aperture 2
230-236 F7.2 ct Sky2 Sky background in counts for aperture 2
238-240 I3 --- NSky2 Number of sky pixels available for aperture 2
243-245 I3 --- NRej2 ? Number of sky pixels rejected for aperture 2
247-248 I2 --- Worst2 Value of worst bad pixel in aperture (0 OK)
for aperture 2
250-251 I2 --- errorfl2 Code for aperture 2 (1)
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Note (1): Codes as follows:
0 = All OK
1 = At least one attempt was made to remove a cosmic ray from the star
aperture (non-fatal)
2 = Sky annulus overlaps edge of data window (non-fatal)
3 = Sky annulus overlaps edge of data window and at least one cosmic ray
was zapped (non-fatal)
4 = Negative sky values may indicate a bad bias frame which leads to
underestimated errors in the photon case (non-fatal)
5 = No sky pixels (non-fatal, although could be bad!)
6 = Aperture has extra apertures but these are ignored for optimal
extraction (non-fatal)
7 = Aperture lies outside all data windows (fatal)
8 = Target aperture overlaps edge of data window (fatal)
9 = Aperture has been invalidated (fatal)
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Acknowledgements:
Axel Schwope, aschwope(at)aip.de
(End) Patricia Vannier [CDS] 04-Dec-2025