Recent outburst activity
of the supersoft X-ray binary AG Draconis
IBWS 2017 - Karlovy Vary - April 6, 2017
Jaroslav Merc1 , Rudolf Gális1 , Laurits Leedjärv2
1
Institute of Physics, Faculty of Science, Pavol Jozef Šafárik University
Park Angelinum 9, 040 01 Košice, Slovakia
2
Tartu Observatory
Observatooriumi 1, 616 02 Tõravere, Estonia
Content
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
Observational data
Photometric behaviour
Spectral characteristics
Photometric behaviour
Temperature of the white dwarf
Methods
Spectral characteristics
Our previous results
Recent activity stage (2015 - now)
Temperature of the white dwarf
Methods
Our previous results
Type of the outbursts
Comparison with previous outbursts
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Symbiotic system AG Draconis
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
2
AG Dra as supersoft X-ray source
Outburst types
AG Dra is one of the best studied symbiotic systems.
I
the cool component is a red giant of early spectral type (K0 – K4) and
higher luminosity than that of standard class III (giant)
I
the hot component is considered to be a white dwarf sustaining a high
luminosity ( ∼ 103 L ) and temperature (∼ 105 K) due to the TN burning
of accreted matter
I
system is in a circumbinary nebula (thanks to the stellar wind of the cool
giant), partially ionized by the white dwarf
I
the system undergoes characteristic symbiotic activity with alternating
quiescent and active stages; the active ones consist of several
outbursts repeating at about a one-year interval
Observational data
Photometric behaviour
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Symbiotic system AG Draconis
Supersoft X-ray source
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
3
Outburst types
Observational data
Photometric behaviour
Moreover, AG Dra is the most intense X-ray source among symbiotic stars,
and one of the best representatives of the supersoft X-ray objects.
I
X-ray flux remains constant during the quiescent phase but during optical maxima there is usually remarkable decrease in X-ray flux (Greiner
et al., 1996)
I
the behaviour during the major outbursts is explained by expansion and
cooling of the white dwarf atmosphere (which explains also the anticorrelation between optical/ UV and X–ray fluxes)
I
the minimum X–ray flux observed also during the minor outbursts might
be attributed to the increased opacity of the WD envelope
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Symbiotic system AG Draconis
Supersoft X-ray source
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
4
Outburst types
Observational data
Photometric behaviour
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Figure: Comparison of the AAVSO optical light curve of AG Dra (dots) with the
X-ray light curves (top of the figure). The solid points superposed on the light
curve mark the visual magnitude at the times of the X-ray observations. The
plots in the upper part of the figure show the EXOSAT, ROSAT and XMM light
curves (units are counts per sec). Source: González-Riestra et al. (2008).
Symbiotic system AG Draconis
Outburst types
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
5
Observational data
Photometric behaviour
Spectral characteristics
Major outbursts occur in intervals of 12–15 yr (in 1936, 1951, 1966, 1980,
1994 and 2006), and are usually followed by minor-scale outbursts in intervals
of about 1 yr.
I
Temperature of the white dwarf
Methods
Our previous results
I
Recent activity stage (2015 - now)
Type of the outbursts
I
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
using UV and X-ray observations, González-Riestra et al. (1999) showed
that there are two types of outbursts: cool and hot ones
major outbursts at the beginning of active phases are usually cool ones
in our recent paper (Leedjärv et al., 2016) we demonstrated that the
outburst types can be clearly distinguished also according to behavior
of the prominent optical emission lines
Observational data
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Photometry
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
I
all available photometric observation of AG Dra have been used
I
new photometric data covering recent outbursts (2015 - 2017) were obtained from AAVSO and from Martin Vrašt’ák
6
Photometric behaviour
Spectral characteristics
Spectroscopy
Temperature of the white dwarf
Methods
Our previous results
I
intermediate-dispersion spectroscopy of AG Dra was carried out at the
Tartu Observatory in Estonia (more than 500 spectra obtained over about
14 years (1997–2011) on the 1.5-metre telescope, R = 6 000, 7 000 and
20 000).
I
new spectra were obtained from ARAS (Astronomical Ring for Access to
Spectroscopy, 25 – 35 cm telescopes, R = 1 800 – 11 000)
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Photometric behaviour
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra regularly undergoes quiescent and active stages which consist of
the series of individual outbursts repeating at about a one-year interval.
AG Dra as supersoft X-ray source
Outburst types
Observational data
Photometric behaviour
7
I
complex period analysis revealed two real periods present in this
symbiotic system:
Porb = 551.d 0 ± 1.d 5 (the orbital motion)
Ppul = 355.d 7 ± 1.d 9 (the cool-component pulsations)
I
the period analysis of active stages confirmed the presence of a period of
around 365 d, which is the median of the time interval between outbursts
I
it is worth noting that these time intervals vary from 300 - 400 d without
an apparent long-term trend
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Photometric behaviour
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
Photometric behaviour
8
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Figure: Light curves of AG Dra in UBV over the period 1963 - 2017. The LC is divided
into active (C - G) and quiescence (Q4 - Q6) stages. Particular outbursts are assigned as
C1 - C2, D1 - D5, E0 - E10, F1, F2 and G0, G1.
Pavol Jozef Šafárik University
Košice, Slovakia
17
Spectral characteristics
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
We analyzed the equivalent widths, absolute fluxes and radial velocities
of prominent spectral emission lines Hβ , He II 4686 Å, Hα , He I 6678 Å, and
Raman-scattered O VI 6825 Å1 .
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
Photometric behaviour
Spectral characteristics
9
Characteristics of emission lines clearly confirmed the presence of the two
types of outbursts in AG Dra.
Temperature of the white dwarf
Methods
Our previous results
I
major outbursts in the beginning of active phases are usually cool ones,
during which the expanding white dwarf cools down and the He II Zanstra
temperature drops
I
in smaller scale hot outbursts, the He II Zanstra temperature increases
or remains unchanged
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
1 Broad emission lines in the spectra of symbiotic stars at 6825 and 7082 Å are a product of
Raman scattering of the photons of the O VI resonance lines at 1032 and 1038 Å off the atoms of
neutral hydrogen.
Spectral characteristics
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
Photometric behaviour
Spectral characteristics
10
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Figure: The curves of equivalent widths for particular emission spectral lines over the
period 1997 - 2017. Active and quiescence stages are marked.
Pavol Jozef Šafárik University
Košice, Slovakia
17
Temperature of the white dwarf
Methods
Recent outburst activity of AG Dra
The He II/H β ratio can be considered as a proxy to the temperature of the
hot component (Iijima 1981):
s
2.22F4686
+ 5.13
(1)
Thot (in 104 K ) = 19.38
(4.16FHβ + 9.94F4471 )
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
Photometric behaviour
Spectral characteristics
Temperature of the white dwarf
Methods
11
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Sokoloski et al. (2006) have neglected the flux of the He I 4471 Å line in the
case of Z And, because F4471 ≤ 0.1FHβ . Approximately the same relation is
valid for AG Dra (Leedjärv et al., 2016). Making a similar simplification allows
to use ratio of the EWs instead of the flux ratio:
s
EW4686
4
+ 5.13
(2)
Thot (in 10 K ) ≈ 14.16
EWHβ
Greiner et al. (1996) used for example the ratio of He II 1640 Å to the far-UV
continuum at 1340 Å as an indicator of Zanstra temperature.
Temperature of the white dwarf
Our previous results
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
The time dependence of He II /Hβ can be
considered as a plot of the hot component’s
temperature variations in time.
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
Photometric behaviour
I
the long-term average of the ratio
over the period 1997–2011 was 0.70
± 0.18 which correspond the temperature of the hot component about 170
000 K
I
values for the maximum (1.26
for hot outburst E8) and minimum
(0.16 for cold outburst F1) are
210 000 and 108 000 K, respectively
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
12
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Figure: Ratio of the EWs of the two strong
emission lines, Hβ and He II 4686 Å, in time.
Recent activity stage (2015 - now)
Recent outburst activity of AG Dra
After seven years of flat quiescence following the 2006-08 major outbursts, AG
Dra begun rising again in brightness.
I in the late spring of 2015 there was a new minor outburst
I outburst activity of AG Dra was definitely confirmed by more prominent
outburst in April 2016
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
Photometric behaviour
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
13
Type of the outbursts
Comparison with previous outbursts
Figure: Light curves of AG Dra in UBV over the period 2004 - 2017.
Pavol Jozef Šafárik University
Košice, Slovakia
17
Recent activity stage
Temperature of the system
Recent outburst activity of AG Dra
The recent active stage of AG Dra manifest the spectroscopic behaviour of
both "hot" and "cool" outburst.
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
Photometric behaviour
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
14
Comparison with previous outbursts
Pavol Jozef Šafárik University
Košice, Slovakia
17
Figure: The light curve in B filter (top), temperature (middle) and the curves of EWs of Raman-scattered
O VI 6825 Å and 7082 Å (two bottom curves) during the recent G0 and G1 outburst of AG Dra.
Recent activity stage (2015 - now)
Comparison with previous outbursts
Recent outburst activity of AG Dra
1
J. Merc, R. Gális, L. Leedjärv
B
1
0.78
0.33
0.51
0.63
-0.09
0.45
He II
0.78
1
0.4
0.6
0.65
0.38
0.46
Symbiotic system AG Draconis
0.8
0.6
1
B
1
0.45
0.67
0.07
-0.06
-0.79
-0.53
He II
0.45
1
0.74
-0.26
-0.14
0.32
-0.08
H beta
0.67
0.74
1
0.26
0.33
-0.18
-0.73
H alpha
0.07
-0.26
0.26
1
0.84
-0.37
-0.67
He I
-0.06
-0.14
0.33
0.84
1
-0.49
-0.64
O VI
-0.79
0.32
-0.18
-0.37
-0.49
1
0.69
T
-0.53
-0.08
-0.73
-0.67
-0.64
0.69
1
B
He II
H beta
H alpha
He I
O VI
T
0.4
H beta
AG Dra as supersoft X-ray source
0.33
0.4
1
0.78
0.75
0.23
-0.34
H alpha
0.51
0.6
0.78
1
0.58
0.34
0.15
0
0.2
-0.2
He I
Observational data
0.63
0.65
0.75
0.58
1
0.22
0.46
-0.09
0.38
0.23
0.34
0.22
1
-0.07
T
0.45
0.46
-0.34
0.15
0.46
-0.07
1
Photometric behaviour
Spectral characteristics
-0.6
-0.8
-0.4
-1
B
He II
H beta
H alpha
He I
O VI
T
B
1
0.85
0.81
0.63
0.85
0.62
0.56
He II
0.85
1
0.91
0.98
0.99
0.97
0.55
0
-0.2
-0.4
O VI
0.6
0.4
0.2
Outburst types
0.8
-0.6
-0.8
-1
Temperature of the white dwarf
1
Methods
Our previous results
0.8
0.6
1
B
1
0.69
0.74
0.51
0.85
-0.5
0.23
He II
0.69
1
0.27
0.5
0.68
0.06
0.26
H beta
0.74
0.27
1
0.83
0.6
-0.49
-0.37
H alpha
0.51
0.5
0.83
1
0.77
-0.64
-0.4
He I
0.85
0.68
0.6
0.77
1
-0.41
-0.1
O VI
-0.5
0.06
-0.49
-0.64
-0.41
1
0.36
T
0.23
0.26
-0.37
-0.4
-0.1
0.36
1
B
He II
H beta
H alpha
He I
O VI
T
0.4
Recent activity stage (2015 - now)
H beta
0.81
0.91
1
0.98
0.92
0.85
0.26
H alpha
Comparison with previous outbursts
0.63
0.98
0.98
1
0.98
0.94
-0.01
15
0
0.2
-0.2
He I
0.85
0.99
0.92
0.98
1
0.95
0.6
T
0.62
0.97
0.85
0.94
0.95
1
0.15
0.56
0.55
0.26
-0.01
0.6
0.15
1
B
He II
H beta
H alpha
He I
O VI
T
-0.6
-0.8
0
-0.2
-0.4
O VI
0.6
0.4
0.2
Type of the outbursts
0.8
-0.4
-1
-0.6
-0.8
-1
Figure: Corelation between B light curves, the curves of EWs of prominent emission lines and temperature
during outbursts (upper left stage G, upper right centre of outburst G1, bottom left E4-E5 and bottom right
F).
Pavol Jozef Šafárik University
Košice, Slovakia
17
Recent activity stage (2015 - now)
Recent outburst activity of AG Dra
J. Merc, R. Gális, L. Leedjärv
Symbiotic system AG Draconis
AG Dra as supersoft X-ray source
Outburst types
Observational data
The photometric and spectroscopic observations suggest that recent outburst
activity of AG Dra has behavior of both "hot" and "cool" outburst.
Photometric behaviour
Spectral characteristics
Temperature of the white dwarf
Methods
Our previous results
Recent activity stage (2015 - now)
Type of the outbursts
Comparison with previous outbursts
16
Pavol Jozef Šafárik University
Košice, Slovakia
17
I
Is it a new type of activity? Or we have already seen it (activity stage C)?
I
Can we expect the major "cool" or minor "hot" outburst during the spring
of 2017?
Thank you for your attention.