Events leading up to the June 2015 outburst of V404 Cyg

0.39 MB·

by  F. Bernardini

Draftversion January20,2016
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
EVENTS LEADING UP TO THE JUNE 2015 OUTBURST OF V404 CYG
F. Bernardini 1,2, D.M. Russell1, A.W. Shaw3, F. Lewis4,5, P.A. Charles3,6, K.I.I. Koljonen1, J.P. Lasota7,8, and
J. Casares9,10,11
1NewYorkUniversityAbuDhabi,P.O.Box129188, AbuDhabi,UnitedArabEmirates;[email protected]
2INAF−OsservatorioAstronomicodiCapodimonte,SalitaMoiariello16,I-80131Napoli,Italy
3DepartmentofPhysics&Astronomy,UniversityofSouthampton, Southampton, SO171BJ,UK
4FaulkesTelescopeProject,SchoolofPhysics&Astronomy,CardiffUniversity,TheParade,CF243AA,Cardiff,Wales
6 5AstrophysicsResearchInstitute, LiverpoolJohnMooresUniversity,146BrownlowHill,LiverpoolL35RF,UK
6ACGC,UniversityofCapeTown,PrivateBagX3,Rondebosch, 7701, SouthAfrica
1 7Institutd’AstrophysiquedeParis,CNRSetSorbonneUniversit´es,UPMCParis06,UMR7095, 98bisBdArago,75014Paris,France
0 8NicolausCopernicusAstronomicalCenter,Bartycka18,00-716Warsaw,Poland
2 9InstitutodeAstrof´ısicadeCanarias,E-38205LaLaguna,SantaCruzdeTenerife,Spain
10DepartamentodeAstrof´ısica,UniversidaddeLaLaguna, E-38206LaLaguna, SantaCruzdeTenerife,Spainand
n 11DepartmentofPhysics,Astrophysics,UniversityofOxford,DenysWilkinsonBuilding,KebleRoad,OxfordOX13RH,UK
a Draft version January 20, 2016
J

9 ABSTRACT
1 On 2015 June 15 the burst alert telescope (BAT) on board Swift detected an X-ray outburst from

theblackholetransientV404Cyg. WemonitoredV404Cygforthelast10yearswiththe2-mFaulkes
] ′
E TelescopeNorthinthreeopticalbands(V,R,andi ). Wefoundthat,oneweekpriortothisoutburst,
H the optical flux was 0.1–0.3 mag brighter than the quiescent orbital modulation, implying an optical
precursor to the X-ray outburst. There is also a hint of a gradual optical decay (years) followed by
.
h a rise lasting two months prior to the outburst. We fortuitously obtained an optical spectrum of
p V404 Cyg 13 hours before the BAT trigger. This too was brighter (∼ 1mag) than quiescence, and
- showedspectrallines typicalof anaccretiondisk, with characteristicabsorptionfeatures ofthe donor
o
being much weaker. No He II emission was detected, which would have been expected had the X-ray
r
t flux been substantially brightening. This, combined with the presence of intense Hα emission, about
s 7 times the quiescent level, suggests that the disk entered the hot, outburst state before the X-ray
a
outburst began. We propose that the outburst is produced by a viscous-thermalinstability triggered
[
close to the inner edge of a truncated disk. An X-ray delay of a week is consistent with the time

2 needed to refill the inner region and hence move the inner edge of the disk inwards, allowing matter
v to reach the central BH, finally turning on the X-ray emission.
0
Subject headings: accretion, accretion disks — black hole physics — X-rays: individual (V404 Cyg,
5
GS 2023+338)
5
4
0 1. INTRODUCTION hot, optically thin, radiatively inefficient plasma
.
1 In Low Mass X-ray Binaries (LMXBs) a black hole (see e.g. Narayan, Barret & McClintock 1997;
0 Narayan& McClintock 2008) that shines in X-rays,
(BH) or a neutron star (NS) accretes matter from a low
6 mass (M ∼ M⊙) Roche lobe filling companion. Many and/or a jet (Hynes et al. 2009; Xie, Yang & Ma 2014),
1 while the outerdisk remainscool. However,fromaDIM
LMXBs are transient, alternating between long periods
: perspective the inner flow is not important, as long as it
v ofquiescence(years),wheretheX-rayluminosityisfaint
does not contribute to the dynamics of the disk (DHL).
i (≤1033erg/s) to shorter episodes of outburst where the
X V404 Cyg (=GS 2023+338), hereafter V404, is one of
X-ray luminosity strongly increases (1037−38erg/s) and
r the closest and best studied LMXBs (2.39± 0.14 kpc;
can approach the Eddington limit.
a Coriat, Fender & Dubus(2012)showedthattheglobal Miller-Jones et al. 2009). It hosts a 9 ±00..26 M⊙ BH
(Khargharia,Froning & Robinson 2010) with an orbital
behaviour of LMXB outbursts is well described by the
periodof6.4714±0.0001days(Casares & Charles1994).
thermal–viscous disk instability model (DIM; see e.g.
V404 showed at least three outbursts (1938, 1956, and
Cannizzo1993;Lasota2001),whereinquiescenceacold,
non-stationarydiskfills-upwithmatteruntilatsomera- 1989; Makino 1989; Richter 1989; Z˙ycki, Done & Smith
dius its temperature havingreacheda criticalvalue trig- 1999), now followed by the burst alert telescope (BAT;
gers an outburst. Heating fronts propagate through the Barthelmy 2000) triggering on V404 on 2015 June 15
disk bringing it to a hot, quasi-stationary, bright state 18:31:38 UT (MJD 57188.772; Barthelmy et al. 2015)
at which the X-ray luminosity reaches its maximum. whenthefirstX-rayflareofanewoutburstwasdetected,
followed by multiple flares in hard X-rays with fast rise
The DIM can broadly explain the LMXB outburst cy-
cle only if the inner disk is truncated during quiescence time and durations of hours (Rodriguez et al. 2015).
(Dubus, Hameury & Lasota 2001, hereafter DHL). An optical precursor of the first X-ray flare
was detected with the 2-m Faulkes Telescope
The accretion flow structure inside the trun-
cated disk’s hole during quiescence forms a North (FTN) a week before the first BAT trigger
(Bernardini, Russell & Lewis 2015). Due to the spo-
radic nature of LMXB outburst start times, to the
[email protected]
2 Bernardini et al.
small number of transient sources currently known, and Spain. We obtainedtwo600sexposurescoveringatotal
to the lack of regular high S/N monitoring (nowadays spectral range 4173–7153 ˚A, utilising the R600B and R
difficult at X-ray wavelengths but easier at optical gratings and a 1” slit in photometric conditions of good
wavelengths), it is hard to detect a delay in the rise to (∼1”)seeing. Spectrawerereducedandextractedusing
outburst between short (X-ray) and long (IR-optical) standardIRAFprocedures. Theone-dimensionalspectra
wavelengths. werewavelengthcalibratedusingalow-orderpolynomial
Only five other sources have shown an indication of fit to the pixel-wavelength arc data. We flux-calibrated
similar behavior (Orosz et al. 1997; Shahbaz et al. 1998; thespectrumusingthenearbyfluxstandardstarBD+25
Jain et al. 2001; Wren et al. 2001; Uemura et al. 2002; 4655 (Oke 1990). The WHT observations were previ-
Buxton & Bailyn 2004; Zurita et al. 2006). This X-ray ously reported in Munoz-Darias et al. (2015).
delay of uncertain origin, is reminiscent of the well-
documented dwarf-nova outbursts UV to optical delay 3. RESULTS
(Smak1998,andreferencestherein). However,theabove
3.1. X-ray rise to outburst
LMXBsduringquiescencewerefainterthanthedetection
limitofX-rayinstruments,sotheirX-rayemissioncould We convert the BAT 15-50 keV count rate averaged
have started rising well before the first X-ray detection on each orbit (Krimm et al. 2013) at the time of trigger
(the derived X-ray delay could be much lower than re- (MJD 57188.772)and the first3σ upper limit before the
ported). trigger (MJD 57188.705) to unabsorbed 15-50 keV flux
We monitored V404 with the 2-m FTN since 2006 using WebPIMMS.Since the spectrum ofthe firstflare
and report here on the detailed analysis of the optical is highly absorbed and the spectral shape not well con-
lightcurvesfrom2006uptothe time ofthe BAT trigger, strained, we used a power law with slope Γ = 0.3−1.2
and on the optical spectrum of V404 fortuitously col- (Kuulkers et al. 2015). We measure F= 4.0 ± 0.5 ×
lectedwiththeWilliamHerschelTelescope(WHT)∼13 10−8ergcm−2s−1 and F< 4.2× 10−9ergcm−2s−1, re-
hours before the trigger. This is the closest in time an spectively. V404 displays X-ray variability a factor of
opticalspectrumofanLMXBhasbeenacquiredpriorto a few in quiescence (Bernardini & Cackett 2014). We
its firstX-rayoutburst detection. We discuss the results convertthe lowestliterature X-ray quiescent 0.3–10keV
in the framework of the DIM (DHL). flux (2006 XMM-Newton pointing; Bradley et al. 2007)
to 15-50 keV flux using WebPIMMS, Γ = 1.85 and
2. OBSERVATIONS AND DATA REDUCTION N = 1 × 1022cm−2 (Rana et al. 2015), and obtain
H
2.1. Optical photometry F∼ 1×10−12 ergcm−2s−1. Assuming that the rise to
outburstofthe firstX-rayflarewasmonotonicandslow,
ObservationsofV404weretakenwiththe 2-mFaulkes
e.g. a single exponential rise starting from the quies-
TelescopeNorth(FTN,Haleakala,Maui,USA).Imaging
cent level or above it, it must have begun after MJD
was obtained in Bessell V, Bessell R and Sloan Digital
57188.530.
Sky Survey i′ filters since April 2006, as part of a moni-
The flares detected by INTEGRAL (Winkler et al.
toringcampaignof∼40LMXBs(Lewis et al.2008). We
2003) at 25–100 keV after the BAT trigger show a fast
present more than nine yearsof data (from 2006 April 8
rise (. 1 h) and duration of hours (Rodriguez et al.
to 2015 June 9). Observations were typically made once
2015). Moreover, during its 1989 outburst, the 1.2–
per weekwhenV404wasvisible,andexposuretimes are
37 keV lightcurve of V404 showed flares with very fast
200 seconds in each filter. Automatic pipelines de-bias
rises (minutes; Kitamoto et al. 1989; Terada et al. 1994;
and flat-field the FTN science images.
Z˙ycki, Done & Smith1999). Acomplexbehaviorforthe
There is a field star of magnitude V = 18.90±0.02,
R=17.52±0.01, i′ =16.92±0.01 just 1.4 arcsec north X-ray rise to outburst, e.g. a slow rise followed by a
sudden increase, cannot be excluded since the source in
of V404 (Udalski & Kaluzny 1991; Casares et al. 1993;
quiescence is below the BAT detection limit. We con-
Barentsen et al. 2014). The two stars are blended in
clude that MJD 57188.53 can be safely considered as a
all images, so we performed aperture photometry (us-
lowerlimit to the rise to outburst of the first X-ray flare
ing PHOT in IRAF) adopting an optimum fixed aper-
detected by BAT.
ture radius of 12 pixels (3.6”) to encompass the flux of
both stars. The same aperture was used for photome-
3.2. Optical photometry
try on four comparison stars 13–34 arcsec from V404.
These were used for flux calibration, and themselves In Fig. 1 we show the optical lightcurve in V, R and
calibrated using field stars of known magnitudes listed i′ bands, from2006up to the 2015BAT trigger. A week
in Udalski & Kaluzny (1991) for V-band, Casares et al. ′
beforeit(thelasttwopoints),theV,Randi magnitudes
(1993) for R-band and the second data release of the
are the highest recorded throughout this time.
IPHAS (INT Photometric Hα Survey of the Northern Using P = 6.4714 ± 0.0001 day and
Galactic Plane catalogue; Barentsen et al. 2014) for i′. T = 24488o1r3b.873± 0.004HJD as orbital ephemerides
0
Magnitudes of V404 were obtained in total from 392 us-
(Casares & Charles 1994), we generate the orbital
able images.
lightcurve (Fig. 2). The relative uncertainty on the
phase determined with these ephemerides is ∼ 0.05,
2.2. Optical spectroscopy
across the entire duration of our observations, but
V404 was observed on 2015 June 15 at UTC 0500 only ∼ 0.005 from one year to the next. The typical
(MJD57188.208)withtheIntermediatedispersionSpec- variation due to the tidally distorted donor’s ellipsoidal
troscopic and Imaging System (ISIS) on the 4.2m WHT modulation is present. Low amplitude flaring,likely due
atObservatoriodelRoquedeLosMuchachos,LaPalma, toresidualaccretionactivity,isalsopresent,ashasbeen
Events leading up to the June 2015 outburst of V404 Cyg 3
8&&( 8&&) 8&&9 8&%& 8&%% 8&%8 8&%: 8&%# 8&%!
7 7 7 7 7 7 7 7 7
01 2+34
✆
%’ 5 2+34
✆
,
+
* %( **+,-./0,1*+,-./0,1+,-./0,1(cid:0)✂☎ $)"$$)$)"$$)$)"$$) &"$0.1$!$!!&)*+,)-./0123!&)*+,)-./0123✁✄ &"$&"$
&"&$0.01 &"&$&"&$
6 2+34 !5!"".##7$$1!!5!%%$$1&&0’’4 !!!5""".###7(((2%!%%$$$1&&&0’’4’
✆ 2M233J44D
%)
!"#$%&# !"!$%&# !"’$%&# !"($%&#
-./
′
Fig.1.— Optical lightcurve in i (triangles), R (squares), and V (circles) band from 2006 to 2015. Magnitudes include flux from the
nearbycontaminatingstar. Thedot-dashedlineatMJD57188.772showstheX-raytrigger. Theredpointsare2015June8(MJD57181.5)
′
and9(MJD57182.5). Noobservationini bandwasperformedon2015June8. Intheinsert,azoom-inofthe2015Rbanddatacompared
withthe3σ BATdetections (bluecrosses).
′
documented before (Shahbaz et al. 2003; Zurita et al. bottom panel of Fig. 3 we show the average of R and i
2004; Hynes et al. 2004, 2009; Bernardini & Cackett bands,<∆i′, ∆R>=0.5(<∆i′ >+<∆R>),where
2014). we use three points per bin. The decreasing-increasing
We fit the orbital lightcurves (except the last two trendisnowclearer. Wefitthefirstpartofthelightcurve
points) with a constant plus double sinusoid function, (up to MJD 55834.5)with a constant plus a linear func-
whose phases are free to vary so as to account for un- tion. AnF-testgivesa4.4σ significancefortheinclusion
equal minima and maxima, as frequently seen in qui- of the latter. Between MJD 53860 and 55834.5 we mea-
escent LMXBs including V404 (Zurita et al. 2004). On sure a decrease of ∼0.02mag/year.
2015 June 8 (MJD∼ 57181.5, φ ∼ 0.11), and 9 (MJD∼ We detect an optical precursor to the first X-ray flare
57182.5, φ∼ 0.25) the optical magnitude in all bands is registeredbyBAT.Assumingthatthetwoeventsaredi-
significantly brighter than the average quiescent modu- rectlylinkedandthattheriseoftheX-rayflareismono-
lationlevelby0.1–0.3mag,andabovethelowamplitude tonic,weestimateadelayintheriseoftheX-ray(MJD∼
flaring behavior (where ∆mag . 0.1). We subtract the 57188.5)comparedto the optical(MJD∼57181.5)emis-
bestfittingmodelfromthe orbitallightcurves,andshow sion of at least 7 days. It could be as long as 13 days,
in Fig. 3 the residual lightcurves. We note that the last considering that the optical flux may have started ris-
two points of the lightcurves have the highest residuals, ing immediately after the pointing before the precursor
that the residuals in the different bands look correlated (MJD∼ 57175.6). The delay’s length is similar to those
on short timescales (days) and that a long-term trend seen in the other four LMXBs that showed indication of
(years) seems present (first a decay and then a rise). this behavior. We have also found evidence of a long-
We measure the statistical significance of the correla- term trend. The last 2 points of the residual lightcurve
′
tion using Spearman’s rank test on the R and i bands, beforetheoutburstaresignificantlyabovethisquiescent
wheretheS/NishighercomparedtoV.TheSpearman’s downward trend. Consequently, we can constrain the
coefficientisρ=0.78,andthenullhypothesisprobability long-termrisetohavestartedbeforeMJD57129.5(2015
is P =3.5×10−27, so the residuals are positively corre- April 17).
lated. This and the small error bars on each data point,
suggest that the observed variability is intrinsic to the 3.3. Spectral Energy Distribution
sourceandislikelyaccretionactivityontimescaleslonger
To construct the Spectral Energy Distribution (SED)
than minutes (the time between two consecutive expo-
′ of the optical precursor, we first measure the lower
sures). We combine the i and R band residuals. In the
envelope of the modulation (see Zurita et al. 2004),
4 Bernardini et al.
0011 22**3344
(cid:0)✁
’’%% 55 22**3344
(cid:0)✁
++
**
)) ’’((
66 22**3344
(cid:0)✁
’’&&
!! !!""## !!""$$ !!""%% !!""&& ’’
,,--**..//
Fig.2.—Orbitallightcurves. Thedashedlineisaconstantplustwosinesbestfittingmodel. Thedot-dashedlineisthelowerenvelope.
−−00..11
i’ 00
∆
00..11
−−00..22
R 00
∆
00..22
−−00..22
V 00
∆
00..22
−−00..11
>
R
∆ 00
,
i’
∆
< 00..11
5.4×104 5.5×104 5.6×104 5.7×104
MJD
′
Fig.3.—Residuallightcurves. Thebottom panel showstheaverageoftheresidualinthei andRband. Thedashedlineisafitmade
withaconstant plusalinearcomponent.
Events leading up to the June 2015 outburst of V404 Cyg 5
which is the donor contribution at each phase (Fig.
2). We combine the 2015 June 8 and 9 images (in
′ ✎✏(cid:0)✑✒✓
the i band only June 9 is available), and we subtract
the lower envelope from the derived magnitudes. We (cid:0)✳✽
de-redden the fluxes of the residuals using A = 4
V
(Casares et al. 1993; Hynes et al. 2009) and the extinc-
tsiuornelaawproefcuCrasrodreslpli,ecCtrlaayltionnde&x αMa=th−is0.(31598±9)0,.3a2n,dwmheeare- ✦✶✌✍✮ (cid:0)✳✻ ✏✳(cid:0)(cid:0)✎✏(cid:0)✑✒✔ ✷✳(cid:0)✎✏(cid:0)✑✒✔
Fν ∝ να. This is consistentwith a 7500±1500K black- ☞✦✶s (cid:0)✳✼✁ ✏✳✁
body, which peaks in the visible. These errors do not ✦❝☛ (cid:0)✳✹ (cid:0)✳✁(cid:0) ✏✳(cid:0)
take into accountany uncertainties in the extinctionAV ✞✟✠✡ (cid:0)✳✷✁
(Hynes et al. 2009). ✝ (cid:0)✳✁
☎✆
The SED could be consistent, within 1σ confidence ❋ ✹✻(cid:0)(cid:0) ✹✼✁(cid:0) ✹✕(cid:0)(cid:0)
✻✹(cid:0)(cid:0) ✻✁✁(cid:0) ✻✼(cid:0)(cid:0)
(cid:0)✳✷
level,withbothoptically-thinsynchrotronemissionwith
α ∼ −0.7, or optically-thick (flat) synchrotron emission
with α ∼ 0. However, Bernardini et al. (2016, sub-
mitted) showed that during the 1989 outburst the jet (cid:0)✳(cid:0)
dominates the optical flux in the hard state, but makes ✹(cid:0)(cid:0)(cid:0) ✹✁(cid:0)(cid:0) ✁(cid:0)(cid:0)(cid:0) ✁✁(cid:0)(cid:0) ✻(cid:0)(cid:0)(cid:0) ✻✁(cid:0)(cid:0) ✼(cid:0)(cid:0)(cid:0) ✼✁(cid:0)(cid:0)
marginal contribution in quiescence. ❲✂✈❡❧❡♥❣t❤✭❆û✄
We measured a de-reddened spectral index from the Fig.4.—Averaged optical spectrum (smoothed usinga5-point
WHT spectrum of α= −1.85±0.08 for the continuum, boxcaralgorithm)obtainedwithWHT/ISISon2015June15. The
spectrum has been flux calibrated using the flux standard star
by removing the hydrogen lines from the red arm, simi-
BD+25 4655. The left and right insets show the zoomed in re-
lar to the spectral index of some flares seen during qui- gionaroundHβ andthebaseofHα,respectively.
escence (Shahbaz et al. 2003); this suggests a variable
spectrum during the initial rise into outburst.
3.4. Optical spectroscopy
✄✳✽
The optical spectrum is dominated by a strong Hα
emission line (Fig. 4). Hβ is also present, along with
multiple He I emission lines. However, He II (4686 ˚A), ✄✳✻
typical of X-ray illuminated disks in outburst, is absent.
At first sight the absorption features of the companion
✄✳✹
seem also absent, but a closer look reveals clear iden- ✞✟✠
tifications hidden within the noise (Fig. 5). A cross- ❋✆✝☎
correlation of the Hα pre-outburst spectrum and the ✄✳✷
average of 220 quiescent spectra of V404 obtained be-
tween 1990 and 2009 (see Casares 2015) with the spec-
✄✳(cid:0)
trum of the radial velocity template HR 8857 yields
clearpeaksatheliocentricvelocitiesconsistentwiththose
of the donor star at the orbital phase of our spec- (cid:0)✳✽
tra. The ∼ 7× increase in Hα flux compared with the ✦✻(cid:0)(cid:0)(cid:0) ✦✹(cid:0)(cid:0)(cid:0) ✦✷(cid:0)(cid:0)(cid:0)❱✁❧♦❝✐t②(cid:0)✭❦♠s✂✶✮ ✷(cid:0)(cid:0)(cid:0) ✹(cid:0)(cid:0)(cid:0) ✻(cid:0)(cid:0)(cid:0)
quiescent level (Casares & Charles 1992) suggests that
Fig.5.—June15pre-outburstspectrum (offsetby0.3mJyand
the accretion disk is much brighter than in quiescence. smoothed with a Gaussian kernel of FWHM=2 pixels) compared
We estimate the inner radius of the truncated disk, withthe20yearaveragequiescentspectrum(bottom), zoomedin
R = 0.5(csin(i)/v )2, by studying Hα in emission. ontheregionsurroundingHα. Spectrahavebeenaveraged inthe
in in restframeofthecompanion.
By measuring its half-width at zero intensity (HWZI)
weestimatethe velocityatthe inneredgeofthe diskv
in
(Narayan, McClintock & Yi 1996). We use a non-linear
least squares algorithm to fit a low-order polynomial to 2010) we find that Rin < 6200 Schwarzschild radii
thecontinuum(maskingHα)andsubtractingit. We ap- (Rs =2GMBH/c2) at MJD 57188.208.
ply the same approach to fit a double Gaussian to the For comparison, the HWZI of the average quies-
Hα profile, finding it more accurate than a single Gaus- cence Hα line has HWZI. 1500km/s, implying Rin &
sian due to a broader component being present at the 17000Rs. Therefore, the inner disk radius may have de-
base of the line (Fig. 4). We utilize the lower amplitude creased by a factor ∼ 3 in our pre-outburst spectrum
broader Gaussian to estimate the HWZI and hence v with respect to quiescence.
in
by measuring 5σ.
From fits to the lower amplitude broad Gaussian we 4. DISCUSSION
find a slightly redshifted Hα peak at 6564.3 ± 0.1 ˚A The DIM predicts (see Eq. 51 in Lasota 2001) that
with HWZI= 54.0 ± 0.5 ˚A, which translates to v = inLMXBs, only inside-outoutbursts canoccur (e.g. the
in
2468±23kms−1. This is actually a lower limit on v , instability is triggered well inside the accretion disk and
in
ashighervelocitystructureispresentaroundthe baseof will propagate outwards). Inside-out outbursts do not
the line profile, though difficult to fit. Using the derived start exactly at the inner disk edge and fronts propa-
inclination of i = 67◦ (Khargharia,Froning & Robinson gate both ways. To explain the long recurrence time
6 Bernardini et al.
and the intensity of the outbursts of LMXBs, and their this upper limit is consistent with R(V) derived from
quiescent X-ray luminosities, the DIM requires that the DIM equations.
inner quiescent disk is truncated far from the compact We can use Eq. A.1 in Lasota, Dubus & Kruk (2008)
object(DHL).Theheatingfrontspropagaterapidlywith to estimate the criticaleffective disk temperature (T+ )
a speed ∼αc , where α is the the hot disk viscosity pa- eff
s neededto ionize hydrogenand startthe outburst. Using
rameter and cs the speed of sound. The in-going front the derived R , T+ ≈ 7000K (the dependency on R,
dies out quickly reaching the truncation radius without in eff
andMintheequationisweak). Wenotethatatthetime
affecting much of the disk structure and then the hole is
of the optical precursor the disk temperature (7500±
refilled in a viscous time. Since the disk X-ray emission
1500K)is consistent with T+ .
is mainly emitted in the disk inner regions, a delay of eff
severaldays in the rise to outburst of the X-ray with re- All versions of the DIM predict a constantly increas-
spect to the optical emission is expected. In the case of ing optical flux during quiescence (Lasota 2001), while
a non-truncated disk the delay would be at most 1 day observed quiescent fluxes of dwarf novae and LMXBs
(DHL). areconstantordecreasing(however,seeWu et al.2016).
A description of the rise to outburst can be found We observe a long-term trend in the optical magnitude
in Sect. 5.1 of DHL. We discuss our results accord- of V404, a 0.1 mag decrease followed by a 0.1 mag in-
ing to this latest version of the DIM. The delay rep- crease. The source is known to show year-to-year opti-
resents the difference between the time t , when the cal modulation changes of similar intensity (see Fig. 1
V
outburst starts at some R(V) of the truncated disk and in Zurita et al. 2004). Most of the variability we detect
the time t ,when the moving-ininner diskedge reaches above the orbital modulation is likely due to accretion
X
the radius R(X), where R(V) > R(X), attaining the activity and occurs at all orbital phases (Fig. 2).
temperature that allows emission of X-rays. The X- It is known that accretion is happening at a low level
ray delay (∆tV−X) corresponds to the difference be- in quiescence (short term variability has been seen in
tween the two radii. From Eq. 16 of DHL we can es- X-ray, optical and radio). Small accretion rate changes
timate R(V) if we assume that the 7 days X-ray delay from year to year, likely explain the long-term optical
corresponds to the viscous time (tvis = ∆tV−X). We variations. The rise in the optical flux from April 2015
get ∆tV−X = 15.3M110/2α−0.12T5−1(cid:16)R110/2(V)−R110/2(X)(cid:17) minatyhreefloeucttbaurrsetc.enTthiencdriesaksebiencoMm˙ eesvepnrtougarlelysscivuelmlyinhaottitnegr
days, where M10 = MBH/10M⊙, α0.2 = α/0.2 is the as matter builds up and when the temperature reaches
hot branch viscosity parameter, R = (R/1010cm) is the ionization level, the inside-out heating wave quickly
10
the disk radius, and T = (T/105K) is the midplane propagates from the trigger site, close to the inner edge
5
temperature, where T & 3−4×104 K at the start of of the truncated disk, through the whole disk (in both
the outburst (see Lasota, Dubus & Kruk 2008). We use directions). Then, the inner edge of the truncated disk
T =30000−50000K,α=0.1−0.2,andR(X)=5×108 moves inwards, on the longer viscous timescale, and a
cm(asinDHL),andwederivethattheinstabilitycorre- week after the optical precursor,when Rin <6200Rs (a
sponding to the outburst precursor (MJD 57181.5) may factor of ∼3 lower than in quiescence), it is hot enough
have been triggered at R(V) ∼ 0.9 −2.2 ×109cm (∼ togenerateX-rayemissionandweobservethefirstX-ray
flare.
340−830R ). We notice that the disk size is ∼1012cm
s
We also derive a direct constraint on R(V) from the
ACKNOWLEDGMENTS
HWZI of Hα. At MJD 57188.208, 13 hours before the
firstX-raydetection(∼6.5daysaftertheopticalprecur- The FTN is maintained and operated by Las Cum-
sor detection), R(V) < 6200R . This is an upper limit bres Observatory Global Telescope Network. JPL was
s
toR(X),sinceR(X)<R(V),andsoitrepresentsacon- supported by the CNES. JC acknowledges support by
straintonthesizeoftheinneredgeofthetruncateddisk MINECO under grants AYA2013-42627 and PR2015-
(R ), close to the X-rayoutburst onset. We notice that 00397
in
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