Study of Accretion Flow Dynamics of V404 Cygni during its 2015 Outburst

The 2015 Outburst of V404 Cygni is an unusual one with several X-ray and radio flares and rapid variation in the spectral and timing properties. The outburst occurred after $26$ years of inactivity of the black hole. We study the accretion flow properties of the source during its initial phase of the outburst using {\it Swift}/XRT and {\it Swift}/BAT data in the energy range of $0.5-150$ keV. We have done spectral analysis with the two-component advective flow (TCAF) model fits file. Several flow parameters such as two types of accretion rates (Keplerian disk and sub-Keplerian halo), shock parameters (location and compression ratio) are extracted to understand the accretion flow dynamics. We calculated equipartition magnetic field $B$ for the outburst and found that the highest $B \sim 900$~Gauss. Power density spectra (PDS) showed no break, which indicates no or very less contribution of the Keplerian disk component, which is also seen from the result of the spectral analysis. No signature of prominent quasi-periodic oscillations (QPOs) is observed in the PDS. This is due to the mismatch of the cooling timescale and infall timescale of the post-shock matter.


INTRODUCTION
Transient black hole candidates (BHCs) have two phases in their lives: quiescence phase and outbursting phase. They spend most of their lifetimes in the quiescence phase. Sudden rise in viscosity leads to an outburst when the X-ray intensity rises by a factor of thousands or more that of this process, gravitational potential energy is converted to heat and radiation. Black hole (BH) spectra generally consist of two components: a multicolour blackbody bump and a hard power-law tail.
The multicolour blackbody part is believed to originate from a Shakura-Sunyaev type standard thin disk (Shakura & Sunyaev 1973;Novikov & Thorne 1973). The power-law tail is believed to originate from a Compton corona (Sunyaev & Titarchuk 1980;Sunyaev & Titarchuk 1985). In the two component advective flow (TCAF) solution, the CENBOL or CENtrifugal pressure supported BOundary Layer (Chakrabarti 1995;Chakrabarti & Titarchuk 1995;Chakrabarti 1997) replaces the Compton corona used in other models such as disk-corona model (Zdziarski 1993;Haardt & Marschi 1993) or evaporated disk in ADAF (Narayan & Yi 1994;Esin et al. 1997). In this paper, we used the TCAF solution to study the accretion flow dynamics of V404 Cygni during its first outburst in 2015 after a long quiescent of ∼26 years.
V404 Cygni is one of the most studied black hole X-ray binary systems. It is also known as GS 2023+338. It was first identified as an optical nova in 1938 (Wachmann 1948). In 1956, another nova outburst was reported in this system (Ritcher 1989). In 1989, V404 Cygni went through another outburst.
The 1989 outburst was discovered with the all sky monitor onboard Ginga (Makino 1989). It is located at RA = 306 • .01 and Dec = 33 • .86. The 1989 outburst was studied extensively. On 2015 June 15, after long 26 years in quiescent, V404 Cygni went through a short but violent outburst. In Dec 2015, another short activity was observed (Barthelmy et al. 2015;Lipunov et al. 2015). The binary system V404 Cygni harbours a black hole of mass 9 − 12 M ⊙ at the centre with a K-III type companion of mass ∼ 1 M ⊙ (Casares et al. 1992;Shahbaz et al. 1994;Khargharia et al. 2010). The inclination angle of the binary system is ∼ 67 • (Shahbaz et al. 1994;Khargharia et al. 2010). The orbital period of the system is 6.5 days (Casares et al. 1992). The binary system is located at a distance of 2.39 kpc, measured by parallax method (Miller-Jones et al. 2009). V404 Cygni has a high spinning black hole with spin parameters a * > 0.92 (Walton et al. 2017).
The 2015 outburst of V404 Cygni was discovered on June 15 simultaneously by Swift/BAT (Barthelmy et al. 2015) and MAXI/GSC (Negoro et al. 2015). During this outburst, the source was extensively observed in multi-wavelength bands, such as in radio (Mooley et al. 2015a;Trushkin et al. 2015b), optical (Gazeas et al. 2015) and X-ray ((Rodriguez et al. 2015); (Radhika et al. 2016)). INTEGRAL observation reported multiple X-ray flares during the outburst (Rodriguez et al. 2015). Several radio flares were also observed. The source showed rapid changes in the spectral properties in very short time (Motta et al. 2017). INTEGRAL observation detected e − − e + pair annihilation on June 20, 2015 (Siegert et al. 2016;Radhika et al. 2016). FERMI/LAT detected high energy γ-ray jet in the source on June 26, 2015 (Loh et al. 2016). King et al. reported detection of emission lines with Chandra-HETG, indicating strong disc wind emission (King et al. 2015) In this paper, we study the timing and the spectral properties of V404 Cygni with combined Swift XRT and BAT data in the broad energy range of 0.5 − 150 keV during the initial phase of the 2015 outburst. We have done spectral analysis with the TCAF model-based fits file to extract physical flow parameters. The nature of these model fitted accretion flow parameters allowed us to investigate physical reasons behind origin of the several flares, and their variability and turbulent features. We have also calculated equipartition The paper is organized in the following way. In Section2, we will briefly discuss the disk structure prescribed by TCAF and the way flow parameters decide on the spectral shape. In Section3, we discuss the observations and the data analysis procedure. In Section4, we present the results of our analysis. In Section5, we make a discussion based on our result, and finally, in Section6, we summarize our findings.

TCAF SOLUTION
TCAF configuration is based on the solution of a set of equations which govern viscous, transonic flows around a black hole (Chakrabarti 1990). In the TCAF solution, an accreting flow has two components: high viscous, high angular momentum, optically thick and geometrically thin Keplerian disk flow (ṁ d ) which accretes on the equatorial plane; and a weakly viscous, optically thin sub-Keplerian halo component (ṁ h ) with low angular momentum. The Keplerian disk is immersed within the sub-Keplerian flow. Due to rise in the centrifugal force close to the black hole, the halo matter slows down at the centrifugal barrier and forms an axisymmetric shock (Chakrabarti 1989). The post-shock region or CENBOL is a 'hot ' and 'puffed-up.' region. The Keplerian disk is truncated at the shock location. Multi-colour black body soft photons are generated in the Keplerian disk. A fraction of this soft photons are intercepted by the CENBOL. Depending on the temperature and size of the CENBOL, soft photons become hard photons via inverse-Comptonization at the CENBOL. Conversely, some Comptonized photons reflect from the Keplerian disk and produce a reflection hump. Thus in TCAF, reflection component is self-consistently incorporated. However, a Gaussian line may be required to add if an iron line is present. CENBOL is also considered to be the base of the jets or outflows (Chakrabarti 1999). Toroidal magnetic flux tubes are responsible for the collimation of jet . Oscillation of CENBOL can be triggered when the cooling and heating times inside CENBOL are similar and the emerging photons produce the quasi periodic oscillations (QPOs) (Molteni et al. 2016;Ryu et al. 1997;Chakrabarti et al. 2015); hereafter C15).
Transient BHCs generally show different spectral states during their outbursts. In TCAF, these observed spectral states are controlled by the flow parameters (two types of accretion rates and two shock parameters  Nandi et al. 2012;Debnath et al. 2015a). In the upper panel of Fig. 1 (adopted from (Chakrabarti 2018)), we show a cartoon diagram of the above four spectral states under the TCAF paradigm. In the lower panel, typical spectra of each spectral states correspond to the diagrams are shown. In the cartoon diagrams, brown, light green, dark green and grey region represent Keplerian disk, sub-Keplerian halo, CENBOL and jet, respectively.
Due to lower viscosity and angular momentum, the sub-Keplerian matter moves in with free-fall velocity, whereas the Keplerian flow moves in viscous time. When an outburst is triggered, the sub-Keplerian flow dominates in the accretion process since it moves faster than the Keplerian disk. show monotonically evolving type-C (low frequency) QPOs, and SIMS shows sporadic type-B or type-A QPOs due to shock oscillation. In soft sate these QPOs are absent, since CENBOL is absent. Here, EQPO means evolving QPO; SQPO means sporadic QPO; NQPO means no QPO. In the bottom panel, theoretical spectra corresponding to the top paneled spectral states are shown. These spectra are generated using five input parameters (M BH in M ⊙ ;ṁ d inṀ Edd ;ṁ h inṀ Edd ; X s in r s , R), whose values are marked inset.
X-ray flux dominates and hard state is observed. Compact jet is launched in this state from the CENBOL (Chakrabarti 1999). Evolving type-C QPO is produced in this state due to the resonance oscillation of the shock (Molteni et al. 2016).
The source enters in the HIMS after HS (Fig. 1b). The Keplerian disk accretion rate continues to rise and becomes comparable with the sub-Keplerian halo accretion rate. As a result, accretion rate ratio (ARR =ṁ h /ṁ d ) decrease. Due to rise of the Keplerian disk accretion rate, CENBOL becomes cooler, and the shock moves farther inward and the CENBOL shrinks. Shock strength decreases as the Compton cooling reduces the post-shock thermal pressure. Mass outflow rate to inflow rate ratio becomes maximum. Here also type-C QPOs are observed.
In the SIMS (Fig. 1c), the Keplerian rate keeps on increasing, although the sub-Keplerian flow rate started to decrease. This is because more and more sub-Keplerian flow becomes Keplerian by viscous transport. The shock becomes weak in this state. The shock further moves in, and the CENBOL becomes small. The soft X-ray flux increases and the hard X-ray flux decreases in this state due to rapid rise inṁ d and slow decrease inṁ h . Generally, type-A or type-B QPOs are observed sporadically in this state due to weak oscillation of the CENBOL (type-B) or due to oscillation of shock-less centrifugal barrier (type-A).
betweenṁ h andṁ d peaks gives us rough estimation of the viscous time scale of the source (Jana et al. 2016). In the SS (Fig. 1d), the Keplerian disk dominates and completely cools down the CENBOL. Soft X-ray flux dominates over hard X-ray flux. No shock is formed. As a result, the jet is completely quenched in this state (Chakrabarti 1999, Garain et al. 2012. No QPO is produced in the soft state.
The flow parameters evolve oppositely during the declining phase of the outburst. Starting from SS to SIMS transition day, both the Keplerian disk accretion rate and the sub-Keplerian halo accretion rate decreases, although, the Keplerian disk rate decreases faster. As a result, ARR increases. As in the SIMS of the rising phase, one may see sporadic type-B or 'A' QPOs in the declining SIMS. In the declining HIMS and HS, evolving type-C QPOs could be seen. Similar to the rising phase, one could observe compact jet in the HIMS and HS in the declining phase.
TCAF solution is implemented in XSPEC (Arnaud 1996) to analyze spectral properties around the black holes Debnath et al. 2015a) as an additive table model. TCAF model has input parameters (M BH ,ṁ d ,ṁ h , X s , R). Accretion flow dynamics around several black holes are studied quite successfully using TCAF model (Debnath et al. 2015b;Debnath et al. 2017;Debnath et al. 2020;Jana et al. 2016;Jana et al. 2020b;Chatterjee et al. 2016;Chatterjee et al. 2019;Chatterjee et al. 2020;Bhattacharjee et al. 2017;Shang et al. 2019). Frequencies of the dominating QPOs are predicted from TCAF model fitted shock parameters (Chatterjee et al. 2016). Masses of the black holes are estimated quite successfully from spectral analysis with the TCAF model (Molla et al. 2016;Chatterjee et al. 2016).
Jet contribution in the X-rays are also calculated using TCAF solution Jana et al. 2020a;Chatterjee et al. 2019). We used WT mode data for XRT observation. Cleaned event files were generated for XRT using xrtpipeline command. To reduce pileup effects, we used grade-0 data. For pileup correction, we chose an annular region around the source. We chose an outer radius of 30 pixels and a varying inner region, depending on the count rate. A background region is chosen far away from the source with 30 pixels radius.

OBSERVATION AND DATA ANALYSIS
Then, we obtained .pha and background files using these cleaned event files in XSELECT v2.4. A scaling factor was applied to the source and background with BACKSCAL. Spectral data were re-binned to 20 counts per bin using grppha command. 0.5 − 10 keV 0.01 sec lightcurves were generated in XSELECT v2.4 using cleaned source and background event files. We followed standard procedures to generate BAT spectra and lightcurves. Detector plane images (dpi) were generated using the task batbinevt. For appropriate detector quality, we used batdetmask task. Noisy detectors were found, and a quality map mode data. A systematic error was applied to the BAT spectra using batphasyserr. Ray-tracing was corrected using batupdatephakw task. Then a response matrix for the spectral file was generated using batdetmask. BAT lightcurves of 0.01 sec were obtained using batbinevt for 15 − 150 keV.
Here, we used the TCAF model-based fits file for the spectral analysis. We also used combined 'diskbb' (DBB) and 'powerlaw' (PL) models to get rough estimation about the thermal and the non-thermal fluxes where a reflection component is often required to find the best fits. TCAF model-based fits do not require any additional component for reflection since the reflection component is already incorporated while generating a spectrum. We required a Gaussian model to incorporate F e-k α emission line. We used phabs model for interstellar absorption and pcf abs model for partial absorption. With the TCAF, we extracted physical parameters such as the mass of the black hole (M BH ) in solar mass (M ⊙ ), Keplerian disk rate (ṁ d ) in Eddington rate (Ṁ Edd ), sub-Keplerian halo rate (ṁ d ) in Eddington rate (Ṁ Edd ), shock location (X s ) (i.e., size of the Compton cloud) in Schwarzschild radius (r s ) and the shock compression ratio (R = ρ + /ρ − with ρ + and ρ − are post-and pre-shock density respectively). In TCAF, N depends on the distance and inclination angle of the source and is just a constant factor between the emitted flux and observed flux by a given instrument. However, it can vary if any physical processes are present other than the accretion. Since the current version of TCAF model fits file does not include jets, for instance, a variation of normalization is observed if they are present Chatterjee et al. 2019). We first analyzed the spectra after keeping the mass of the black hole as a free parameter. We obtained the mass of the black hole in the range of 9.5 − 11.5 M ⊙ or average value of 10.6 M ⊙ . This measured mass range agrees very well with previously reported values by many authors (Casares et al. 1992;Khargharia et al. 2010). Then, we refitted all the spectra after keeping the mass of the black hole frozen at 10.6 M ⊙ . The result based on the later analysis is presented here.
We achieved best-fittings using steppar command. After obtaining a best-fit based on χ 2 red (∼ 1) with TCAF, we ran steppar to verify fitted parameter values. The 'steppar' command ran for pair of parametersṁ d -ṁ h and X s -R. We also calculated uncertainties with the steppar. In Figs

RESULTS
We present the results of spectral and temporal analysis of the source in 0.5 − 150 keV energy band using combined XRT+BAT or only BAT (in 15 − 150 keV) or only XRT (in 0.5 − 10 keV) data. In Fig In Fig. 6a, we show the evolution of BAT and XRT fluxes. In Fig. 6(b-c), we show the variation of the Keplerian disk rate (ṁ d ), the sub-Keplerian halo rate (ṁ h ) with day (in MJD). In Fig. 4d, we show the evolution of accretion rate ratio (ARR =ṁ h /ṁ d ). In Fig. 7a, we show the variation of the equipartition magnetic field with the day (see, below for details). In Fig. 5(b-d), we show the variation of the shock location (X s ), the shock compression ratio (R) and TCAF model normalization with the day. In Fig showed almost flat spectral slope with broad-band noise. The noise decreased as the outburst progressed during which we found two slopes in the PDS, steep powerlaw slope in the lower frequency and flat slope at a higher frequency. In some PDS, we observed that power diminished very rapidly. We did not find any break in the PDS. Similar nature is also observed in BAT PDS.

Evolution of the Spectral Properties
We have done the spectral analysis using 0.5 − 150 keV combined Swift/XRT and Swift/BAT data between 2015 June 15 and 26. (Radhika et al. 2016) analyzed the same data set using phenomenological diskbb and powerlaw models. We analyzed the data with combined diskbb and powerlaw models and have found similar results as in (Radhika et al. 2016). In general, we used 'phabs * pcf * (diskbb + powerlaw + gaussian)' model to estimate thermal and non-thermal fluxes. While analyzing with phenomenological models, we did not require diskbb component on a regular basis. Diskbb component was required only in 9 observations out of a total 19 observations. During the entire period, PL photon index varied between 0.60 emission along with the 'diskbb + powerlaw' model. Detailed results of the phenomenological model are given in Table I. In the present paper, our main goal is to study the accretion flow dynamics of the source from spectral analysis with the physical TCAF model. For this purpose, we used 'phabs * pcf * (T CAF + gaussian)' model. Detailed results using this model is presented in Table II After MJD = 57198.02,ṁ d was obtained in a narrow range of 0.13 − 0.16Ṁ Edd (see Fig. 6b).
A strong shock (R = 2.56) was found far away from the black hole (X s = 334 r s ) on the first day (MJD=57188.77) of our observation (see Fig. 7b & 7c). The shock remained strong for the next five days.
After that, the shock was found to move closer to the black hole as the Keplerian disk rate increased. The shock was found at 126 r s on MJD = 57194.16 with R = 1.92. The shock did not move closer than this.
After that, shock moved away from the black hole. Again we found that the shock was moving inward after MJD=57197.21. On the last day of our observation, we found the shock to be at 143 r S .
We add a Gaussian profile along with the TCAF solution to incorporate the contribution of the F e emission line in XRT data. Fe-line varied within 6.08 keV and 6.97 keV. In some observation, we required the line width of > 1 keV. We used phabs models for the interstellar absorption. We did not freeze n H at a particular value. Rather, we kept it free. In our analysis, we observed it to vary between 0.54 × 10 22 to 1.49 × 10 22 cm −2 . We also used pcf abs model to incorporate for partial absorption in XRT data. In some observations, the covering required as high as 95%. In general, it varied between 50% and 95%. For covering absorption, n H varied between 2.2 × 10 22 and 28.9 × 10 22 cm −2 .

Time Resolved BAT Spectra
To study the evolution of spectral nature in short time intervals, we analyzed the time-resolved BAT spectra.
Rapid variation of the accretion rates and other physical flow parameters from observation to observation motivated us to make this study. Here, we analyzed time-resolved BAT spectra for four observations on June 26, 2015 (MJD = 57199.52). We found rapid variation in the BAT spectra within very short period of time even in one observation. In Table III, TCAF model fitted parameters for time-resolved spectra are presented. For example, on 2015 June 15, within total BAT exposure of 1202 sec (Fig. 8)

Estimation of Magnetic Field
Observation of presence of high magnetic field on 2015 June 25 by (Dallilar et al. 2017) motivated us to estimate magnetic field strength at the 'hot' Compton cloud region (here CENBOL) for the BHC V404 Cygni during its 2015 outburst. We made some simple assumptions as mentioned below to calculate the equipartition value of the magnetic field. At the shock location, energy conservation leads to the following equation (Chakrabarti 1990), where, n is the polytropic index of the flow, a is sound speed, V is particle velocity. '+ ′ and '− ′ signs indicate the values at post-and pre-shock region respectively. The electron number density (n e ) is given by, For our calculation, we assume CENBOL shape as cylindrical. m p is the mass of the proton. H shk is the shock height. The shock height could be calculated from the TCAF model fitted shock parameters (Debnath et al. 2015a) using standard vertical equilibrium (Chakrabarti 1989), Now, we can calculate pressure at CENBOL using the following equation, P gas = a 2 + m e n e n .
We consider equipartition magnetic field (B) as, where, P rad is the radiation pressure. The radiation pressure is negligible compared to the gas pressure.
Using this equation, we calculated the maximum possible value of the equipartition magnetic field for this source (see , Table IV). We show the variation of the equipartition magnetic field in Fig. 7a (Molteni et al. 2016;Ryu et al. 1997;Chakrabarti et al. 2015). Thus to check if the resonance condition is satisfied or not, we have calculated both cooling and infall time scales during the outburst. Generally in low mass X-ray binaries, Compton cooling is the primary process of cooling. We also considered the synchrotron cooling since the magnetic field was present.
We calculated both cooling time scales (synchrotron and Compton) during the entire period of our observations and compared that with the infall times. If cooling and infall timescales are roughly comparable, then we may say that the resonance condition for oscillation of the shock is satisfied. For simplicity, here we assume that the matter is moving radially at the post-shock region with speed V + . This allowed us to calculate infall time using the following equation, Similar to the magnetic field calculation, here we also assume CENBOL as a cylindrical in shape. Now the total thermal energy of electron content within the CENBOL is, where γ e is the Lorentz factor. It is given by, γ e = 1/ 1 − β 2 e , where β e = v/c, v being electron velocity and c is the velocity of light. The synchrotron cooling rate is (Rybicki & Lightman 1979), where σ T is Boltzman constant. U B is magnetic energy density and given by, U B = B 2 /8π. The synchrotron cooling time is given by, We calculated cooling time due to synchrotron using Eqn. 9. Note, we use γ e =10 for our calculation.
We also checked the Compton cooling timescale (t Comp ). To calculate Compton cooling, we use the same method as described in C15. The Compton cooling timescale is given by Eqn. 6 of C15, Here, Λ Comp is Compton cooling rate (for more details, see, C15). Here we find that the Compton cooling is much faster than the synchrotron cooling (see , Table IV). Thus we may assume that inverse-Comptonization is the primary process for cooling. Now if we compare t inf with t syn or with t Comp , in both the cases, the ratio deviates largely from unity. Thus we may say that during The TCAF model fitted spectral analysis allowed us to understand the nature of this violent outburst from a physical perspective. The source exhibited this outburst after a long 26 years of quiescent. During this outburst, many radio and X-ray flares were also observed. The presence of a strong magnetic field is considered to be one of the reasons behind this unusual outburst of V404 Cygni. In the following sub-Sections, we discuss those in details.

Magnetic Field
A magnetic field is brought in by the accretion flow. The shear, convection and advection amplify the predominantly toroidal flux tubes. They are expelled from the CENBOL due to the magnetic buoyancy  in the direction towards the pressure gradient force. If the buoyancy timescale is larger than the shear amplification timescale, then the magnetic flux tubes are amplified within the dynamical timescale until they reach equipartition where gas pressure matches with the magnetic pressure. In general, the magnetic field is not found to contribute to the accretion disk spectra of black holes since TCAF alone fits the data very well. However, it is necessary for acceleration and collimation of jets and outflow.
Here, we calculated the equipartition magnetic field by equating magnetic pressure with the local gas pressure at the shock. We found B eq = 97 G on the first observation day. After that, it increased gradually with the increase of the accretion rate. We found the maximum value of the magnetic field to be B eq = 923 We found it B eq = 500±18 G on that day (MJD = 57198.15). However, our estimated value of the magnetic field is much lower than the previously estimated values for other Galactic black holes. Previously, magnetic field was found to be about ∼ 10 5 − 10 7 G for Cygnus X-1 (Del Santo et al. 2013); ∼ 5 × 10 4 G for XTE J1550-564 (Chatty et al. 2011; and ∼ 1.5 × 10 4 G for GX 339-4 (Cutri et al. 2003). However, the magnetic field was calculated in a jet for these sources; thus, they showed much higher values.

Power Density Spectra
No where f is the frequency and β is powerlaw index. In the most PDS, we found β ∼ 0, i.e., flat spectra. It This also indicates no or minimal contribution of the Keplerian disk accretion. Precisely, this is found from the spectral analysis.

Absence of QPOs
We believe that the oscillation of shock is responsible for the QPOs (Molteni et al. 2016). Generally strong type-C QPOs are observed if the resonance condition is satisfied, i.e., when the infall time of the post-shock matter roughly matches with the cooling time inside the CENBOL. The shock oscillation could also be observed when Rankine-Hugoniot conditions are not satisfied to form a stable shock (Ryu et al. 1997).
According to C15, the type-B or type-A QPOs mainly occur due to weak resonance phenomenon in CENBOL (type-B) or in shock-less centrifugal barrier (type-A). The cooling process could be via inverse-Compton scattering, bremsstrahlung or synchrotron emission. If the magnetic field is high enough, one can expect the dominance of synchrotron cooling.
We did not observe any prominent type-C QPOs during the 2015 outburst of V404 Cygni. Nonsatisfaction of the resonance condition could be responsible for it. To verify this assertion, we calculated synchrotron and Compton cooling times for this source. We found that the Compton cooling rate is much faster than the synchrotron cooling rate. Thus Compton cooling dominated the cooling process. We compared both types of cooling times with the infall time. We found that the infall time and cooling time were not comparable at all (see Table IV). C15 showed that QPO would be generated if the ratio of the infall and the cooling times is between 0.5 and 1.5, , i.e., within 50% of unity either way. Since the resonance condition was not satisfied, we were not supposed to see any strong type-C QPOs. Huppenkothen et al. 2017 reported of detection of mHz QPO with Chandra, Swift/XRT and Fermi observations (Huppenkothen et al. 2017). They reported simultaneous detection of 18 mHz QPO with Swift/XRT and Fermi/GBM. They classified this QPO as a new type of low-frequency QPO. However, Radhika et al. argued that they did not find any clear signature of this QPO (Radhika et al. 2016). Chandra/ACIS observation revealed signatures of 73 mHz and 1.03 Hz QPOs. However, they did not seem to be type-C QPOs.
Thus the resonance condition was not behind the origin of these QPOs. This could be due to non-satisfaction of the Rankine-Hugoniot conditions at the shock front.

Spectral and Temporal Evolution
V404 Cygni showed complex behaviour during the 2015 outburst. We required two absorption models to fit the spectra: phabs model for interstellar absorption and pcf abs for the partial covering absorption. The later absorption may be due to disk wind emission or outflow emission (King et al. 2015;Radhika et al. 2016).
After MJD = 57198, the dust halo started to dominate in the field of view; hence we studied the spectral and the timing properties up to this day.
Some observations of V404 Cygni could not be fitted with simple disk blackbody and powerlaw models since a significant reflection component was present. In the first six observations, DBB component was not required to fit the spectra (till MJD = 57193.56). After that, it was required occasionally. PL photon ing the entire period of our observation. We often observed a 'hump' region after ∼ 8 keV in the XRT data. This 'reflection hump' was extended up to ∼ 20 keV in the BAT data. Radhika  of 400 − 600 keV. They found that Comptonization temperature was ∼ 40 keV with seed photon temperature ∼ 7 keV. This is very high for the disk emission. They concluded that this emission could be of different origins, such as synchrotron emission from the jet.
We extracted the physical parameters of the accretion flows from each fit. We first fitted each spectrum by keeping all model input parameters, including the mass of the black hole as free. We found a variation of M BH in a narrow range of 9.5 − 11.5 M ⊙ with an average value of 10.6 +0.9 −1.1 M ⊙ . This estimated mass of V404 Cygni agrees well with other reported values in the range of 9 − 12 M ⊙ (Casares et al. 1992;Shahbaz et al. 1994;Khargharia et al. 2010). We then re-fitted all the spectra by keeping M BH frozen at its most probable value (= 10.6 M ⊙ ) to extract values of other physical flow parameters during the outburst. also occurred after about 26 years, showed peculiar behaviour with several flares and outflow activity. Thus it is possible that a huge amount of matter accumulated at a large pileup distance over a long time. With a sudden enhancement of viscosity, the outburst is triggered and leads to a violent and non-settling activity

Evolution of the Spectral Properties with Flares
The present outburst of V404 Cygni did not behave like any other typical outburst of a classical transient black hole. V404 Cygni showed a strong jet associated with several flares. The flares were observed in multi-wavebands, from X-ray, optical, IR to radio (Rodriguez et al. 2015;Gandhi et al. 2016;Trushkin et al. 2015a;Trushkin et al. 2015b;Tetarenko et al. 2017). Eighteen X-ray flares were reported with the INTEGRAL and SWIFT/BAT observations between June 20, 2015 (MJD = 57193) and June 25, 2015 (MJD = 57198). The magnetic field was the strongest during this phase of the outburst. The magnetic field could be responsible for this flaring activity.
In general, we see a decreasing ARR in an observation immediately after a flare, indicating softening of the spectra (see Fig. 6). For example, an X-ray flare was observed on MJD = 57194.31 (Rodriguez et al. 2015). This could be due to the high magnetic field on MJD = 57194.16. Immediately after the flare, on MJD = 57194.54, we found that the ARR decreased slightly from its previous observation (21.6 to 20.2). On June 18, 2015 (MJD = 57191) we observed that the Keplerian disk rate suddenly rose to 0.12Ṁ Edd from 0.06Ṁ Edd within ∼ 30 mins. This could be associated with the radio flare observed on MJD = 57191.09 with AMI-LA observation (Mooley et al. 2015a). This is expected since a large amount of mass was ejected from the CENBOL during a flare and inflowing matter rapidly moved inward to fill the vacant space. This led to the softening of the spectrum as the CENBOL size was reduced. However, this was not observed after every flare. It is possible that those flares were not localized and the disk was unstable. Around MJD = 57198, we found that B eq decreased sharply, although N increased very rapidly. This could be due to the high magnetic field which produced flare and outflow. This flare and the outflow was responsible for the rapid rise of N .
The X-ray jet flux can be calculated based on the deviation of the constancy of the TCAF model normalization Jana et al. 2020a;Chatterjee et al. 2019). However, to calculate jet X-ray flux by this model, we must have at least one observation where the effects of the jet were negligible. In that observation, the entire observed X-ray should be contributed only from the inflowing matter of the accretion disk and CENBOL. However, for V404 Cygni, a strong jet was present in all the observations. Thus we were not able to separate the jet X-ray contribution from the total X-ray. Random variation in normalization may be due to the presence of fluctuating magnetic field or unsettling disk, which led to the flaring activity of the source.

SUMMARY
The first epoch of the 2015 outburst of the Galactic black hole V404 Cygni was an unusual and violent outburst. It did not behave like other typical outbursts of Galactic transient black hole candidates. Rapid variations were observed in both spectral and timing properties in a very short time scales, ranging from a few minutes to hours. We have used 0.5 − 150 keV combined Swift/XRT and Swuft/BAT data to study the accretion flow properties of the source. Spectral analysis was done using the TCAF model-based fits file in XSPEC. The model fitted/derived flow parameters allowed us to understand the evolution of accretion flow parameters of this violent outburst. We have also calculated the equipartition magnetic field for the outburst. This is also confirmed form the spectral analysis. The presence of white noise in higher frequencies in the power density spectra indicates the presence of a highly turbulent disk. The strong magnetic field could be the reason behind it. It is also responsible for the flares. We find that the Compton cooling process is much faster than the synchrotron cooling process. Since the resonance condition between cooling and infall time scales inside the CENBOL is not satisfied, we did not expect any sharp low frequency QPO. Indeed the object did not show any signature of prominent type-C QPO. All fluxes are in the units of 10 −9 ergs cm −2 s −1 .
nH 1 and nH 2 are in the unit of 10 22 cm −2 . nH 1 is Hydrogen column density for interstellar absorption.nH 2 is Hydrogen column density for partial covering absorption.
The Fe emission line energy and σ are mentioned in Col. 15 & 16.
Best fitted values of χ 2 and degrees of freedom are mentioned in Col. 17 as χ 2 /dof .  nH 1 and nH 2 are in the unit of 10 22 cm −2 . nH 1 is Hydrogen column density for interstellar absorption.nH 2 is Hydrogen column density for partial covering absorption.
TCAF model fitted/derived parameters are mentioned in Cols. 7-12. The Fe emission line energy and σ are mentioned in Col. 13 & 14.
Best fitted values of χ 2 and degrees of freedom are mentioned in Col. 15 as χ 2 /dof .
Note: Mass of the black hole was kept frozen at 10.6 M⊙ during spectral fitting with the TCAF model fits file.
The average values of 90% confidence ± values obtained using steppar command in XSPEC, are placed as superscripts of fitted parameter values. Note: First row in each spectra are TCAF model fitted spectral analysis results when entire data exposure including gaps are used.
The mass of the BH is frozen at 10.6 M⊙ during the fitting.