J/ApJ/930/20 Monitoring of the neutron star XRB Centaurus X-4 (Baglio+, 2022)
A misfired outburst in the neutron star X-ray binary Centaurus X-4.
Baglio M.C., Saikia P., Russell D.M., Homan J., Waterval S., Bramich D.M.,
Campana S., Lewis F., Van den Eijnden J., Alabarta K., Covino S.,
D'Avanzo P., Goldoni P., Masetti N., Munoz-Darias T.
<Astrophys. J., 930, 20 (2022)>
=2022ApJ...930...20B 2022ApJ...930...20B
ADC_Keywords: Stars, neutron ; Binaries, X-ray ; Photometry, ugriz; Ultraviolet;
X-ray sources
Keywords: Neutron stars ; Low-mass x-ray binary stars ; Stellar accretion disks
Abstract:
We report on a long-term optical monitoring of the neutron star X-ray
binary Centaurus X-4 performed during the last 13.5yr. This source has
been in quiescence since its outburst in 1979. Our monitoring reveals
the overall evolution of the accretion disk; we detect short-duration
flares, likely originating also in the disk, superimposed with a
small-amplitude (<0.1 mag) ellipsoidal modulation from the companion
star due to geometrical effects. A long-term (∼2300 days) downward
trend, followed by a shorter (∼1000 days) upward one, is observed in
the disk light curve. Such a rise in the optical has been observed for
other X-ray binaries preceding outbursts, as predicted by the disk
instability model. For Cen X-4, the rise of the optical flux proceeded
for ∼3yr, and culminated in a flux increase at all wavelengths
(optical-UV-X-rays) at the end of 2020. This increase faded after
∼2 weeks, without giving rise to a full outburst. We suggest that the
propagation of an inside-out heating front was ignited due to a
partial ionization of hydrogen in the inner disk. The propagation
might have stalled soon after the ignition due to the increasing
surface density in the disk that the front encountered while
propagating outward. The stall was likely eased by the low-level
irradiation of the outer regions of the large accretion disk, as shown
by the slope of the optical/X-ray correlation, suggesting that
irradiation does not play a strong role in the optical, compared to
other sources of emission.
Description:
Cen X-4 has been regularly monitored in the optical during the last
∼13.5yr with the LCO 2m and 1m robotic telescopes, from 2008-Feb-14
(MJD54510) to 2021-Jun-30 (MJD59395), mostly using V, R, and filters.
See Section 3.1.
Cen X-4 was observed on January 5, 2021 (MJD59219) with the 60cm Rapid
Eye Mount (REM) telescope (La Silla, Chile). Strictly simultaneous,
300s integration time observations have been obtained using the
optical SDSS filters, for a total of nine observations per filter.
The system was then observed again on 2021 May 22 (MJD59356) with REM.
See Section 3.2.
The Neil Gehrels Swift Observatory observed Cen X-4 16 times between
2020 December 28 and 2021 January 23 with the X-Ray Telescope (XRT)
and Ultraviolet and Optical Telescope (UVOT) instruments.
See Section 3.3.
The Neutron-star Interior Composition Explorer (NICER) observed Cen
X-4 extensively in early 2021. We analyzed all observations made
between January 1 and February 19. See Section 3.4.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
----------------------------------------------------------
14 58 21.93 -31 40 07.5 Centaurus X-4 = X Cen X-4
----------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
fig1.dat 144 294 13.5 years of optical monitoring of Cen X-4
performed with LCO
fig2.dat 203 104 Swift/UVOT, Swift/XRT and NICER observations
of the 2021 flare
fig6.dat 131 9 g', r', i', z' light curves obtained with REM on
2021 Jan 05 (MJD59219), showing minute-timescale
variability
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See also:
J/AJ/143/130 : VIJH light curves of the X-Ray binary GX 339-4 (Buxton+, 2012)
J/A+A/616/A37 : Close encounters to the Sun in Gaia DR2 (Bailer-Jones+, 2018)
J/ApJ/867/105 : ATLAS all-sky stellar ref. cat., ATLAS-REFCAT2 (Tonry+, 2018)
J/ApJ/905/87 : LCO i'band LC of SAX J1808.4-3658 (Baglio+, 2020)
Byte-by-byte Description of file: fig1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 F11.5 d MJD-ip Modified Julian Date of i' band
observation
13- 19 F7.4 mag ipmag [16.58/17.63] LCO i' band magnitude
21- 26 F6.4 mag e_ipmag [0.0006/0.3] Error in ipmag
28- 38 F11.5 d MJD-rp ? Modified Julian Date of r' band
observation
40- 46 F7.4 mag rpmag [17/18]? LCO r' band magnitude
48- 53 F6.4 mag e_rpmag [0.01/0.2]? Error in rpmag
55- 65 F11.5 d MJD-gp ? Modified Julian Date of g' band
observation
67- 74 F8.5 mag gpmag [17.65/19.01]? LCO g' band magnitude
76- 86 F11.9 mag e_gpmag [0.02/0.2]? Error in gpmag
88- 102 F15.8 d MJD-R ? Modified Julian Date of R band
observation
104- 110 F7.4 mag Rmag [17.1/18.2]? LCO R band magnitude
112- 117 F6.4 mag e_Rmag [0.003/0.3]? Error in Rmag
119- 129 F11.5 d MJD-V ? Modified Julian Date of V band
observation
131- 137 F7.4 mag Vmag [17.4/18.8]? LCO V band magnitude
139- 144 F6.4 mag e_Vmag [0.001/0.3]? Error in Vmag
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Byte-by-byte Description of file: fig2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 F11.5 d MJD-uvw1 ? Modified Julian Date, Swift/UVOT uvw1
13- 23 F11.8 d MJD-uvw1o ? MJD-uvw1 - 59200
25- 29 F5.2 mag uvw1mag [19.6/21]? Swift/UVOT UVW1 AB magnitude
31- 34 F4.2 mag e_uvw1mag [0.09/0.3]? Error in uvw1mag
36- 46 F11.5 d MJD-uvw2 ? Modified Julian Date, Swift/UVOT uvw2
48- 58 F11.8 d MJD-uvw2o [21/30]? MJD-uvw2 - 59200
60- 64 F5.2 mag uvw2mag [20/21]? Swift/UVOT UVW2 AB magnitude
66- 69 F4.2 mag e_uvw2mag [0.1/0.3]? Error in uvw2mag
71- 80 F10.4 d MJD-uvm2 ? Modified Julian Date, Swift/UVOT uvm2
82- 92 F11.8 d MJD-uvm2o [14.2/19]? MJD-uvm2 - 59200
94- 98 F5.2 mag uvm2mag [19.2/19.6]? Swift/UVOT UVM2 AB magnitude
100- 103 F4.2 mag e_uvm2mag [0.08/0.1]? Error in uvm2mag
105- 114 F10.4 d MJD-u ? Modified Julian Date, Swift/UVOT u band
116- 126 F11.8 d MJD-uo [32.9777]? MJD-u - 59200
128- 132 F5.2 mag umag [20.26]? Swift/UVOT U band AB magnitude
134- 137 F4.2 mag e_umag [0.12]? Error in umag
139- 146 F8.2 d MJD-XRT ? Modified Julian Date, Swift XRT
148- 152 F5.2 d MJD-XRTo [11.46/37.62]? MJD-XRT -59200
154- 161 E8.2 ct/s/cm2 XRTct [5.5e-05/0.004]? Swift XRT Count rate,
counts/s/cm/cm
163- 170 E8.2 ct/s/cm2 e_XRTct [3.3e-05/0.0002]? Error in XRTct
172- 179 E8.2 d MJD-NICER Modified Julian Date, NICER
181- 185 F5.2 d MJD-NICERo [15.93/64.28] MJD-NICER -59200
187- 194 E8.2 ct/s/cm2 NICERct [0.00018/0.006] NICER count rate,
counts/s/cm/cm
196- 203 E8.2 ct/s/cm2 e_NICERct [1.7e-05/0.0002] Error in NICERct
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Byte-by-byte Description of file: fig6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 F11.5 d MJD-gp ? Modified Julian Date of REM g' band
observation
13- 19 F7.4 mag gpmag ? REM g' band magnitude
21- 31 F11.9 mag e_gpmag ? Error in gpmag
33- 43 F11.5 d MJD-rp Modified Julian Date of REM r' band
observation
45- 51 F7.4 mag rpmag REM r' band magnitude
53- 63 F11.9 mag e_rpmag Error in rpmag
65- 75 F11.5 d MJD-ip Modified Julian Date of REM i' band
observation
77- 83 F7.4 mag ipmag REM i' band magnitude
85- 95 F11.9 mag e_ipmag Error in ipmag
97- 107 F11.5 d MJD-zp Modified Julian Date of REM z' band
observation
109- 119 F11.8 mag zpmag REM z' band magnitude
121- 131 F11.9 mag e_zpmag Error in zpmag
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 14-Mar-2024