J/ApJ/794/54 Polarization and photometry for blazar W Comae (Sorcia+, 2014)
Rotation of the optical polarization angle associated with
the 2008 γ-ray flare of blazar W Comae.
Sorcia M., Benitez E., Hiriart D., Lopez J.M., Cabrera J.I., Mujica R.
<Astrophys. J., 794, 54 (2014)>
=2014ApJ...794...54S 2014ApJ...794...54S (SIMBAD/NED BibCode)
ADC_Keywords: BL Lac objects ; Accretion ; Polarization ; Photometry
Keywords: BL Lacertae objects: individual: ON231, W Comae - galaxies: jets -
galaxies: photometry - polarization
Abstract:
An R-band photopolarimetric variability analysis of the TeV bright
blazar W Comae between 2008 February 28 and 2013 May 17 is presented.
The source showed a gradual tendency to decrease its mean flux level
with a total change of 3 mJy. A maximum and minimum brightness states
in the R band of 14.25±0.04 and 16.52±0.1 mag, respectively, were
observed, corresponding to a maximum variation of ΔF=5.40 mJy.
We estimated a minimum variability timescale of Δt=3.3 days.
A maximum polarization degree P=33.8%±1.6%, with a maximum
variation of ΔP=33.2%, was found. One of our main results is
the detection of a large rotation of the polarization angle from
78° to 315° (Δθ∼237°) that coincides in time
with the γ-ray flare observed in 2008 June. This result
indicates that both optical and γ-ray emission regions could be
co-spatial. During this flare, a correlation between the R-band flux
and polarization degree was found with a correlation coefficient of
rF-p =0.93±0.11. From the Stokes parameters, we infer the
existence of two optically thin synchrotron components that contribute
to the polarized flux. One of them is stable with a constant
polarization degree of 11%. Assuming a shock-in jet model during the
2008 flare, we estimated a maximum Doppler factor δD∼27 and a
minimum of δD∼16; a minimum viewing angle of the jet
∼2.0°; and a magnetic field B∼0.12 G.
Description:
The observations were carried out with the 0.84 m f/15
Ritchey-Chretien telescope at the Observatorio Astronomico Nacional
of San Pedro Martir (OAN-SPM) in Baja California, Mexico and the
instrument POLIMA. The differential R-band magnitudes of W Comae were
calculated using the standard star A distant about ∼1.2 arcminutes to
the southeast from the studied object. The magnitude of the comparison
star A in the R band is (11.72±0.04) mag (Fiorucci & Tosti 1996,
J/A+AS/116/403). Because of the narrow field of view of the
instrument, ∼4 arcminutes, this was the only standard star available
for calibration with a reasonable flux level. The exposure time was 80 s
per image for W Comae. Polarimetric calibrations were made using the
polarized standard stars ViCyg12 and HD 155197 and the unpolarized
standard stars GD319 and BD+332642 (Schmidt et al. 1992AJ....104.1563S 1992AJ....104.1563S).
W Comae was observed between 2008 February 28 and 2013 May 17. During
this period, 32 observing runs of 7 nights per run were carried out
around the new moon phase; in total, we collected 141 data points.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
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12 21 31.69 +28 13 58.5 W Comae = QSO B1219+28
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 62 141 Polarization and Photometry in the R-Band
for W Comae
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See also:
J/A+AS/116/403 : VRI photometry of stars in 12 BL Lac objects (Fiorucci+, 1996)
J/A+AS/128/315 : BVRI photometry of blazars (Jia Guibin+ 1998)
J/ApJ/833/77 : gamma-ray bright blazars optical polarization (Itoh+, 2016)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- Cycle Cycle number
5- 15 A11 "Y/M/D" Date Date of the observation
17- 25 F9.4 d JD Julian Date of the observation (JD -2450000.00)
27- 30 F4.1 % p Polarization degree
32- 34 F3.1 % e_p Uncertainty in p
36- 38 I3 deg theta Orientation of electric vector position angle
40- 41 I2 deg e_theta Uncertainty in theta
43- 47 F5.2 mag Rmag R band magnitude
49- 52 F4.2 mag e_Rmag Uncertainty in Rmag
54- 57 F4.2 mJy RFlux R band flux density
59- 62 F4.2 mJy e_RFlux Uncertainty in RFlux
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 26-Apr-2017