J/ApJ/905/87 LCO i'band LC of SAX J1808.4-3658 (Baglio+, 2020)
Probing jet launching in neutron star X-ray binaries: the variable and polarized
jet of SAX J1808.4-3658.
Baglio M.C., Russell D.M., Crespi S., Covino S., Johar A., Homan J.,
Bramich D.M., Saikia P., Campana S., D'Avanzo P., Fender R.P., Goldoni P.,
Goodwin A.J., Lewis F., Masetti N., Miraval Zanon A., Motta S.E.,
Munoz-Darias T., Shahbaz T.
<Astrophys. J., 905, 87 (2020)>
=2020ApJ...905...87B 2020ApJ...905...87B
ADC_Keywords: Pulsars; X-ray sources; Photometry, RI
Keywords: Jets ; Low-mass x-ray binary stars ; Neutron stars ;
Compact objects ; Stellar accretion disks
Abstract:
We report on an optical photometric and polarimetric campaign on the
accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658 during its
2019 outburst. The emergence of a low-frequency excess in the spectral
energy distribution in the form of a red excess above the disk
spectrum (seen most prominently in the z, i, and R bands) is observed
as the outburst evolves. This is indicative of optically thin
synchrotron emission due to a jet, as seen previously in this source
and in other AMXPs during outburst. At the end of the outburst decay,
the source enters a reflaring state. The low-frequency excess is still
observed during the reflares. Our optical (BVRI) polarimetric campaign
shows variable linear polarization (LP) throughout the outburst. We
show that this is intrinsic to the source, with low-level but
significant detections (0.2%-2%) in all bands. The LP spectrum is red
during both the main outburst and the reflaring state, favoring a jet
origin for this variable polarization over other interpretations, such
as Thomson scattering with free electrons from the disk or the
propelled matter. During the reflaring state, a few episodes with
stronger LP levels (1%-2%) are observed. The low-level, variable LP is
suggestive of strongly tangled magnetic fields near the base of the
jet. These results clearly demonstrate how polarimetry is a powerful
tool for probing the magnetic field structure in X-ray binary jets, as
for active galactic nuclei jets.
Description:
SAX J1808.4-3658 was extensively monitored during its 2019 outburst
using the Las Cumbres Observatory (LCO) 2m (Faulkes) and 1m telescopes
in the B, V, R, and i' optical bands and with the Very Large Telescope
(VLT) in Cerro Paranal, Chile, equipped with the FORS2 instrument used
in polarimetric mode (B, V, R, and I bands; program ID: 0103.D-0575;
PI: Baglio).
We observed SAX J1808.4-3658 with the LCO 2m Faulkes and 1m network
for the whole duration of the outburst (spanning 2019-08-08 to
2019-09-25), as part of a monitoring campaign of ∼50 LMXBs
(coordinated by the Faulkes Telescope Project;
Lewis+ 2008arXiv0811.2336L 2008arXiv0811.2336L). Imaging data were taken in the B, R, and
V Bessell filters and the Sloan Digital Sky Survey (SDSS) u' ,i' and
z' filters (436.1-870nm).
Objects:
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RA (ICRS) DE Designation(s)
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18 08 27.54 -36 58 44.3 SAX J1808.4-3658 = V* V4580 Sgr
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
fig2.dat 31 159 LCO i' band light-curve of the outburst of SAX J1808
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See also:
J/ApJ/716/L109 : Radio & X-ray observations of Aql X-1 (Miller-Jones+, 2010)
J/ApJ/750/99 : The Pan-STARRS1 photometric system (Tonry+, 2012)
J/MNRAS/431/L107 : Outbursts of GX339-4 at 5.5 & 9.0GHz fluxes (Corbel+, 2013)
J/A+A/572/A99 : 4U 0614+091 VLT spectrum (Baglio+, 2014)
J/A+A/591/A101 : PSR J1023+0038 & XSS J12270-4859 polarimetry (Baglio+, 2016)
J/A+A/587/A102 : 1RXSJ180408.9-342058 spectrum (Baglio+, 2016)
J/A+A/631/A104 : PSR J1023+0038 VLT, XM and Swift obs. (Baglio+, 2019)
http://www.faulkes-telescope.com/ : Faulkes Telescope Project home page
Byte-by-byte Description of file: fig2.dat
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Bytes Format Units Label Explanations
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1- 15 F15.9 d MJD [58682/58759] Modified Julian Date of observation
(JD-2400000.5)
17 A1 --- l_imag Upper limit flag on imag
18- 24 F7.4 mag imag [16.4/20.5] LCO i' band magnitude
26- 31 F6.4 mag e_imag [0.004/0.17]? Uncertainty in imag
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 27-Jul-2022