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Ryosuke Itoh, Yasuyuki T. Tanaka, Koji S. Kawabata, Makoto Uemura, Makoto Watanabe, Yasushi Fukazawa, Yuka Kanda, Hiroshi Akitaya, Yuki Moritani, Tatsuya Nakaoka, Miho Kawabata, Kensei Shiki, Michitoshi Yoshida, Yumiko Oasa, Jun Takahashi, A measurement of interstellar polarization and an estimation of Galactic extinction for the direction of the X-ray black hole binary V404 Cygni, Publications of the Astronomical Society of Japan, Volume 69, Issue 2, April 2017, 25, https://doi.org/10.1093/pasj/psw130
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Abstract
V404 Cygni is a well-known candidate for the black hole binary thought to have relativistic jets. It showed extreme outbursts in 2015 June, characterized by a large amplitude and short time variation of flux in the radio, optical, and X-ray bands. Not only disk emission but also synchrotron radiation from the relativistic jets was suggested by radio observations. However, it is difficult to measure the accurate spectral shape in the optical/near-infrared band because there are uncertainties of interstellar extinction. To estimate the extinction value for V404 Cygni, we performed photopolarimetric and spectroscopic observations of V404 Cygni and nearby field stars. Here, we estimate the Galactic extinction using interstellar polarization based on the observation that the origin of the optical polarization is the interstellar medium, and investigate the properties of interstellar polarization around V404 Cygni. We found a good correlation between the color excess and polarization degree in the field stars. We also confirmed that the wavelength dependence of the polarization degree in the highly polarized field stars was similar to that of V404 Cygni. Using the highly polarized field stars, we estimated the color excess for the (B − V) color and the extinction in the V band to be E(B − V) = 1.2 ± 0.2 and 3.0 < A(V) < 3.6, respectively. A tendency for a bluer peak of polarization (λmax < 5500 Å) was commonly seen in the highly polarized field stars, suggesting that the dust grains toward this region are generally smaller than the Galactic average. The corrected spectral energy distribution of V404 Cygni in the near-infrared and optical bands in our results indicated a spectral break between 2.5 × 1014 Hz and 3.7 × 1014 Hz, which might originate in the synchrotron self-absorption.
1 Introduction
The radiation from microquasars often suffers from the Galactic extinction and polarization by the interstellar medium, making it difficult to determine the nature of these objects. Polarization in the near-infrared (NIR) is one sign of synchrotron radiation in several microquasars (Shahbaz et al. 2008) and an important measurement used for investigating the environment of the jet and the origin of variability in the microquasars. However, the radiation is often contaminated by interstellar polarization (ISP). In addition, the Galactic extinction makes it difficult to measure the intrinsic spectral shape of microquasars in the optical and NIR bands. It is important to investigate the actual properties of the interstellar medium in order to determine the precise emission model for microquasars.
V404 Cygni (a.k.a, GS 2023+338) is a well-known X-ray black hole binary with 12 ± 3 M⊙ (Shahbaz et al. 1994) at a distance of 2.4 kpc (Miller-Jones et al. 2009). It showed extreme outbursts in 2015 June, with a large amplitude variability and a time scale of seconds to hours in the radio, optical, and X-ray bands (e.g., Kimura et al. 2016). In contrast to the large variation in the total flux, there was a very small variability of polarization in the optical or NIR band. Tanaka et al. (2016) reported no significant variability of polarization in the optical and NIR bands, while Shahbaz et al. (2016) and Lipunov et al. (2016) detected a very small polarization variation in limited time intervals which were not covered by Tanaka et al. (2016). This implies that disk or optically thick synchrotron emission (or both) dominated in the NIR regime.
To obtain an accurate spectrum in the optical/NIR band, a careful estimation of the Galactic extinction is needed. Casares et al. (1993) performed a high-resolution spectroscopic observation of V404 Cygni in 1991, and they estimated the spectral type of comparison star as K0(-1) III-IV. From this result, they determined the Galactic absorption of A(V) = 4.0. Shahbaz et al. (1994), however, suggested the Galactic absorption of A(V) < 3.3 by the NIR-band photometry of V404 Cygni and a tripped giant model.
Here, we estimated the color excess of V404 Cygni by measuring interstellar polarization and color excess for the surrounding field stars. The distinctive feature of our method is that we do not require any assumption of intrinsic luminosity and color of comparison stars in microquasars. In this paragraph, we introduce the basic principles of our method. The relation between color excess originating in the Galactic extinction and ISP has been well studied (e.g., Fosalba et al. 2002). In general, the polarization degree (PD) of ISP increases with color excess. It is also known that the wavelength dependence of ISP in the UV–NIR bands is well described by the Serkowski law (Serkowski et al. 1975; details also described in subsection 3.1). A wavelength at peak PD is related to the ratio of extinction to color excess, defined as RV = A(V)/E(B − V), where A(V) is the extinction in the V band and E(B − V) is the color excess for the (B − V) color. To estimate the contribution of interstellar extinction and polarization for V404 Cygni, we performed optical and NIR polarimetric observations of V404 Cygni and its nearby stars. First, we measured the color excess and ISP for the field stars around V404 Cygni and clarified the relations between these parameters. Then, we estimated the color excess of V404 Cygni from its PD value. From a measurement of the wavelength dependence of V404 Cygni, we estimated the RV value with the Serkowski law. Using these two parameters, E(B − V) and RV, we estimated the Galactic extinction for V404 Cygni.
2 Observation
2.1 Photopolarimetric observation
We performed V-, RC-, IC-, J-, H-, and Ks-band photopolarimetric observation of V404 Cygni and field stars on 2015 June 22 using the Hiroshima Optical and Near-Infrared camera (HONIR: Akitaya et al. 2014) installed on the 1.5 m Kanata telescope, located at Higashi-Hiroshima Observatory in Japan. We also performed long-term photopolarimetric monitoring of V404 Cygni in the RC band from 2015 June 17 to September 27 with a multi-spectral imager (MSI: Watanabe et al. 2012) installed on the 1.6 m Pirka telescope at Nayoro Observatory of Hokkaido University in Japan. Each observation consisted of a sequence of exposures at four position angles of the achromatic half-wave plates, 0|${^{\circ}_{.}}$|0, 22|${^{\circ}_{.}}$|5, 45|${^{\circ}_{.}}$|0, and 67|${^{\circ}_{.}}$|5. The field of view of HONIR for polarimetric observation consisted of five rectangles (0|${^{\prime}_{.}}$|8 × 10΄) separated on the side by 0|${^{\prime}_{.}}$|8. We confirmed that instrumental polarization of HONIR was less than 0.1% in the optical band and 0.2% in the NIR band by the observation of unpolarized standard stars (HD 212311 and G191B2B; Schmidt et al. 1992). We also corrected the instrumental depolarization by the measurement of an artificially 100% polarized star, with a wire-grid polarizer for each band. The origins of the polarization angle were calibrated with the strong polarized stars, HD 150193 and HD 19820 (Whittet et al. 1992). Each measured PD and position angle (PA) after all calibrations were consistent with catalog values of strong polarized stars, with uncertainties of ΔPD ∼ 0.1% and ΔPA ∼ 2°, respectively.
We used archival data of the Swift observation for the field stars around V404 Cygni on MJD 54947. We used the B- and V-band photometric data taken by the UV and Optical Telescope (UVOT). UVOT data were reduced following the standard procedure for CCD photometry. Counts were extracted from an aperture of 5΄ radius for all filters and annulus background regions were sampled 27΄ away from object stars, and then converted to a flux using the standard zero points (Poole et al. 2008).
2.2 Spectroscopic observation
To determine the spectral type of the field stars, we performed low-resolution spectroscopic observations of field stars with Hiroshima One-shot Wide-field Polarimeter (HOWPol; 0.41–0.94 μm, R = λ/Δλ ∼ 400; Kawabata et al. 2008) on the Kanata Telescope from 2015 July 24 to 30. To investigate the relation between PD and color excess, we selected four bright field stars, with a range of from the weak to the strong polarization (0.1% < PD < 6% in the RC band), for the optical spectroscopic observation. In addition, we intensively performed the spectroscopic observation of all field stars which have a strong polarization (PD > 7% in the RC band) in order to make secure comparisons with V404 Cygni. In total we obtained seven spectroscopic observations of field stars with a range of from the weak to the strong polarization (0.1% < PD < 8% in the RC band). Table 1 shows the positions of the field stars. The typical total exposure time for each star was about 30 min. The wavelength calibration was performed with atmospheric emission lines for each frame. The flux was calibrated using observations of a spectrophotometric standard star (HR 7596) obtained on the same night.
Name* . | Coordinates† . |
---|---|
FS1 | 20h24m19|${^{s}_{.}}$|5 +33°52΄42|${^{\prime\prime}_{.}}$|3 |
FS2 | 20h24m11|${^{s}_{.}}$|9 +33°49΄11|${^{\prime\prime}_{.}}$|5 |
FS3 | 20h23m56|${^{s}_{.}}$|0 +33°53΄28|${^{\prime\prime}_{.}}$|7 |
FS4 | 20h23m57|${^{s}_{.}}$|1 +33°52΄39|${^{\prime\prime}_{.}}$|0 |
FS5 | 20h23m56|${^{s}_{.}}$|4 +33°48΄16|${^{\prime\prime}_{.}}$|9 |
FS6 | 20h23m49|${^{s}_{.}}$|2 +33°50΄08|${^{\prime\prime}_{.}}$|2 |
FS7 | 20h23m49|${^{s}_{.}}$|7 +33°48΄23|${^{\prime\prime}_{.}}$|5 |
V404 Cygni | 20h24m03|${^{s}_{.}}$|8 +33°52΄02|${^{\prime\prime}_{.}}$|2 |
Name* . | Coordinates† . |
---|---|
FS1 | 20h24m19|${^{s}_{.}}$|5 +33°52΄42|${^{\prime\prime}_{.}}$|3 |
FS2 | 20h24m11|${^{s}_{.}}$|9 +33°49΄11|${^{\prime\prime}_{.}}$|5 |
FS3 | 20h23m56|${^{s}_{.}}$|0 +33°53΄28|${^{\prime\prime}_{.}}$|7 |
FS4 | 20h23m57|${^{s}_{.}}$|1 +33°52΄39|${^{\prime\prime}_{.}}$|0 |
FS5 | 20h23m56|${^{s}_{.}}$|4 +33°48΄16|${^{\prime\prime}_{.}}$|9 |
FS6 | 20h23m49|${^{s}_{.}}$|2 +33°50΄08|${^{\prime\prime}_{.}}$|2 |
FS7 | 20h23m49|${^{s}_{.}}$|7 +33°48΄23|${^{\prime\prime}_{.}}$|5 |
V404 Cygni | 20h24m03|${^{s}_{.}}$|8 +33°52΄02|${^{\prime\prime}_{.}}$|2 |
*Name in this paper. Their positions are shown in figure 2.
†Right ascension and declination (J2000.0).
Name* . | Coordinates† . |
---|---|
FS1 | 20h24m19|${^{s}_{.}}$|5 +33°52΄42|${^{\prime\prime}_{.}}$|3 |
FS2 | 20h24m11|${^{s}_{.}}$|9 +33°49΄11|${^{\prime\prime}_{.}}$|5 |
FS3 | 20h23m56|${^{s}_{.}}$|0 +33°53΄28|${^{\prime\prime}_{.}}$|7 |
FS4 | 20h23m57|${^{s}_{.}}$|1 +33°52΄39|${^{\prime\prime}_{.}}$|0 |
FS5 | 20h23m56|${^{s}_{.}}$|4 +33°48΄16|${^{\prime\prime}_{.}}$|9 |
FS6 | 20h23m49|${^{s}_{.}}$|2 +33°50΄08|${^{\prime\prime}_{.}}$|2 |
FS7 | 20h23m49|${^{s}_{.}}$|7 +33°48΄23|${^{\prime\prime}_{.}}$|5 |
V404 Cygni | 20h24m03|${^{s}_{.}}$|8 +33°52΄02|${^{\prime\prime}_{.}}$|2 |
Name* . | Coordinates† . |
---|---|
FS1 | 20h24m19|${^{s}_{.}}$|5 +33°52΄42|${^{\prime\prime}_{.}}$|3 |
FS2 | 20h24m11|${^{s}_{.}}$|9 +33°49΄11|${^{\prime\prime}_{.}}$|5 |
FS3 | 20h23m56|${^{s}_{.}}$|0 +33°53΄28|${^{\prime\prime}_{.}}$|7 |
FS4 | 20h23m57|${^{s}_{.}}$|1 +33°52΄39|${^{\prime\prime}_{.}}$|0 |
FS5 | 20h23m56|${^{s}_{.}}$|4 +33°48΄16|${^{\prime\prime}_{.}}$|9 |
FS6 | 20h23m49|${^{s}_{.}}$|2 +33°50΄08|${^{\prime\prime}_{.}}$|2 |
FS7 | 20h23m49|${^{s}_{.}}$|7 +33°48΄23|${^{\prime\prime}_{.}}$|5 |
V404 Cygni | 20h24m03|${^{s}_{.}}$|8 +33°52΄02|${^{\prime\prime}_{.}}$|2 |
*Name in this paper. Their positions are shown in figure 2.
†Right ascension and declination (J2000.0).
3 Results
3.1 Interstellar polarization
Tanaka et al. (2016) reported no temporal variability of optical linear polarization for V404 Cygni, although it showed extreme flares. Figure 1 shows the RC-band light-curve and temporal measurement of polarization for V404 Cygni from 2015 June to September. In order to investigate the variability of PD, we adopted the constant fitting for the temporal variability of PD. The average value of PD is 7.8% ± 0.1% with a value of |$\chi ^2_{\nu }/{\rm d.o.f} = 9.40/16$| which corresponds to a p value of 0.89. This implies that there is no significant variability in PD between the active and quiescent states.
Band . | Wavelength [Å] . | PD [%] . | PA [°] . |
---|---|---|---|
V | 5504 | 8.9 ± 0.1 | 6.3 ± 0.3 |
R C | 6587 | 8.0 ± 0.1 | 7.6 ± 0.3 |
I C | 8059 | 6.6 ± 0.1 | 8.1 ± 0.1 |
J | 12149 | 3.5 ± 0.1 | 8.4 ± 0.1 |
H | 16539 | 2.3 ± 0.1 | 9.5 ± 0.6 |
K s | 21555 | 1.4 ± 0.1 | 10.6 ± 0.7 |
Band . | Wavelength [Å] . | PD [%] . | PA [°] . |
---|---|---|---|
V | 5504 | 8.9 ± 0.1 | 6.3 ± 0.3 |
R C | 6587 | 8.0 ± 0.1 | 7.6 ± 0.3 |
I C | 8059 | 6.6 ± 0.1 | 8.1 ± 0.1 |
J | 12149 | 3.5 ± 0.1 | 8.4 ± 0.1 |
H | 16539 | 2.3 ± 0.1 | 9.5 ± 0.6 |
K s | 21555 | 1.4 ± 0.1 | 10.6 ± 0.7 |
Band . | Wavelength [Å] . | PD [%] . | PA [°] . |
---|---|---|---|
V | 5504 | 8.9 ± 0.1 | 6.3 ± 0.3 |
R C | 6587 | 8.0 ± 0.1 | 7.6 ± 0.3 |
I C | 8059 | 6.6 ± 0.1 | 8.1 ± 0.1 |
J | 12149 | 3.5 ± 0.1 | 8.4 ± 0.1 |
H | 16539 | 2.3 ± 0.1 | 9.5 ± 0.6 |
K s | 21555 | 1.4 ± 0.1 | 10.6 ± 0.7 |
Band . | Wavelength [Å] . | PD [%] . | PA [°] . |
---|---|---|---|
V | 5504 | 8.9 ± 0.1 | 6.3 ± 0.3 |
R C | 6587 | 8.0 ± 0.1 | 7.6 ± 0.3 |
I C | 8059 | 6.6 ± 0.1 | 8.1 ± 0.1 |
J | 12149 | 3.5 ± 0.1 | 8.4 ± 0.1 |
H | 16539 | 2.3 ± 0.1 | 9.5 ± 0.6 |
K s | 21555 | 1.4 ± 0.1 | 10.6 ± 0.7 |
For comparison, we also adopted a simple power-law fitting for the PD dependence of wavelength, which is related to an analog of the “IR polarization excess” found in the Galactic ISP at longer wavelengths (e.g., Nagata 1990), described as in figure 3. The wavelength dependence of PD is well represented with β = 1.2 ± 0.1 and |$\chi ^2_{\nu }/{\rm d.o.f} = 110.96/4$|. We note that the value of β = 1.2 obtained for the region of V404 Cygni is slightly shallow compared with the typical value of β = 1.5–2.0 for Galactic sources.
Name . | Spectral type* . | Flux [mag]† . | PD [%]‡ . | R C − IC§ . | E(RC − IC)‖ . | E(B − V)‖ . |
---|---|---|---|---|---|---|
FS1 | K7–M2 | 15.5 | 7.6 ± 0.3 | 1.77 ± 0.02 | 0.8 ± 0.2 | 1.0 ± 0.3 |
FS2 | K0–K4 | 16.2 | 7.5 ± 0.7 | 1.49 ± 0.02 | 1.0 ± 0.1 | 1.6 ± 0.5 |
FS3 | A5–A9 | 14.6 | 2.9 ± 0.2 | 0.67 ± 0.02 | 0.5 ± 0.1 | 0.8 ± 0.1 |
FS4 | G8–K4 | 16.1 | 7.3 ± 0.5 | 1.75 ± 0.02 | 1.3 ± 0.1 | 0.8 ± 0.6 |
FS5 | F0–F7 | 12.3 | 0.16 ± 0.05 | 0.31 ± 0.02 | 0.1 ± 0.1 | 0.2 ± 0.1 |
FS6 | M0–M5 | 13.6 | 5.1 ± 0.5 | 2.47 ± 0.02 | 1.6 ± 0.5 | 1.3 ± 0.1 |
FS7 | K0–K4 | 14.5 | 7.6 ± 0.2 | 1.60 ± 0.02 | 1.1 ± 0.1 | 1.5 ± 0.1 |
Name . | Spectral type* . | Flux [mag]† . | PD [%]‡ . | R C − IC§ . | E(RC − IC)‖ . | E(B − V)‖ . |
---|---|---|---|---|---|---|
FS1 | K7–M2 | 15.5 | 7.6 ± 0.3 | 1.77 ± 0.02 | 0.8 ± 0.2 | 1.0 ± 0.3 |
FS2 | K0–K4 | 16.2 | 7.5 ± 0.7 | 1.49 ± 0.02 | 1.0 ± 0.1 | 1.6 ± 0.5 |
FS3 | A5–A9 | 14.6 | 2.9 ± 0.2 | 0.67 ± 0.02 | 0.5 ± 0.1 | 0.8 ± 0.1 |
FS4 | G8–K4 | 16.1 | 7.3 ± 0.5 | 1.75 ± 0.02 | 1.3 ± 0.1 | 0.8 ± 0.6 |
FS5 | F0–F7 | 12.3 | 0.16 ± 0.05 | 0.31 ± 0.02 | 0.1 ± 0.1 | 0.2 ± 0.1 |
FS6 | M0–M5 | 13.6 | 5.1 ± 0.5 | 2.47 ± 0.02 | 1.6 ± 0.5 | 1.3 ± 0.1 |
FS7 | K0–K4 | 14.5 | 7.6 ± 0.2 | 1.60 ± 0.02 | 1.1 ± 0.1 | 1.5 ± 0.1 |
*Estimated spectral type.
†Observed magnitude in the RC band.
‡Observed optical polarization degree in the RC band.
§Observed (RC − IC) color.
‖Estimated color excess.
Name . | Spectral type* . | Flux [mag]† . | PD [%]‡ . | R C − IC§ . | E(RC − IC)‖ . | E(B − V)‖ . |
---|---|---|---|---|---|---|
FS1 | K7–M2 | 15.5 | 7.6 ± 0.3 | 1.77 ± 0.02 | 0.8 ± 0.2 | 1.0 ± 0.3 |
FS2 | K0–K4 | 16.2 | 7.5 ± 0.7 | 1.49 ± 0.02 | 1.0 ± 0.1 | 1.6 ± 0.5 |
FS3 | A5–A9 | 14.6 | 2.9 ± 0.2 | 0.67 ± 0.02 | 0.5 ± 0.1 | 0.8 ± 0.1 |
FS4 | G8–K4 | 16.1 | 7.3 ± 0.5 | 1.75 ± 0.02 | 1.3 ± 0.1 | 0.8 ± 0.6 |
FS5 | F0–F7 | 12.3 | 0.16 ± 0.05 | 0.31 ± 0.02 | 0.1 ± 0.1 | 0.2 ± 0.1 |
FS6 | M0–M5 | 13.6 | 5.1 ± 0.5 | 2.47 ± 0.02 | 1.6 ± 0.5 | 1.3 ± 0.1 |
FS7 | K0–K4 | 14.5 | 7.6 ± 0.2 | 1.60 ± 0.02 | 1.1 ± 0.1 | 1.5 ± 0.1 |
Name . | Spectral type* . | Flux [mag]† . | PD [%]‡ . | R C − IC§ . | E(RC − IC)‖ . | E(B − V)‖ . |
---|---|---|---|---|---|---|
FS1 | K7–M2 | 15.5 | 7.6 ± 0.3 | 1.77 ± 0.02 | 0.8 ± 0.2 | 1.0 ± 0.3 |
FS2 | K0–K4 | 16.2 | 7.5 ± 0.7 | 1.49 ± 0.02 | 1.0 ± 0.1 | 1.6 ± 0.5 |
FS3 | A5–A9 | 14.6 | 2.9 ± 0.2 | 0.67 ± 0.02 | 0.5 ± 0.1 | 0.8 ± 0.1 |
FS4 | G8–K4 | 16.1 | 7.3 ± 0.5 | 1.75 ± 0.02 | 1.3 ± 0.1 | 0.8 ± 0.6 |
FS5 | F0–F7 | 12.3 | 0.16 ± 0.05 | 0.31 ± 0.02 | 0.1 ± 0.1 | 0.2 ± 0.1 |
FS6 | M0–M5 | 13.6 | 5.1 ± 0.5 | 2.47 ± 0.02 | 1.6 ± 0.5 | 1.3 ± 0.1 |
FS7 | K0–K4 | 14.5 | 7.6 ± 0.2 | 1.60 ± 0.02 | 1.1 ± 0.1 | 1.5 ± 0.1 |
*Estimated spectral type.
†Observed magnitude in the RC band.
‡Observed optical polarization degree in the RC band.
§Observed (RC − IC) color.
‖Estimated color excess.
3.2 Estimation of color excess
Figure 4 shows the optical spectra of field stars FS1–7 obtained with HOWPol. Comparing the patterns of dominant spectral features (e.g., H α, Ca IR triplet, TiO bands) with the template spectral atlas of standard stars (Silva & Cornell 1992), we estimated the spectral type of FS1–7. For comparison, some template spectra of standard stars are also shown in figure 4. Then, we measure the color excess E(RC − IC) = (R − I)observed − (R − I)intrinsic, using (RC − IC)observed obtained from our observations and (RC − IC)intrinsic from well-studied field stars which have the same spectral type as the stars in FS1–7 (Wenger et al. 2000). The same methods were used for the derivation of E(B − V) with UVOT data. The uncertainties of color excess depended on the classification of spectral type. Finally, we adopted the correction of extinction for the optical spectra based on the estimated color excess. The measurements are summarized in table 3.
Figure 5 shows a scatter plot for PD, color (RC − IC), estimated color excess E(RC − IC), and E(B − V) for V404 Cygni and field stars. We also plotted all of the data points of field stars (not FS stars) in a scatter plot for color and polarization, using black data points. In this figure, we can see that there is a general increase in PD with color and color excess.
The field stars FS1, FS2, FS4, and FS7 show similar properties of PD and color excess (see table 3). From these results, we estimated the color excess of V404 Cygni. Using the average value of color excess of the field stars FS1, FS2, FS4, and FS7, we estimated color excesses of E(RC − IC) = 1.0 ± 0.2 and E(B − V) = 1.2 ± 0.3 for the PD value of PD = 8.0% ± 0.1% for V404 Cygni. These values are consistent with the values derived by spectroscopic observations of a comparison star for V404 Cygni (Casares et al. 1993). We note that these color excess values are derived based on the PD value during the outburst.
4 Discussion
The aligned polarization angle and wavelength dependence of polarization imply that the polarization observed in V404 Cygni originated in the interstellar medium (this is also discussed in Tanaka et al. 2016). On the other hand, Shahbaz et al. (2016) reported that the PD value in the quiescent state is PD = 7.41% ± 0.32% on 2016 May 26. This value is slightly low compared with the PD value in the 2015 outburst, and several short time variabilities of PD, which probably originated in the jet during the 2015 outburst, were also reported (e.g., Shahbaz et al. 2016; Lipunov et al. 2016). However, the variability of PD is a relatively rare phenomenon during the outburst and the amplitude of the PD variability is small (ΔPD ∼ 1%–2%). Therefore, in this paper, we assumed that most of the polarization originated in ISP. In this section, we estimate the Galactic extinction and discuss the dust properties of V404 Cygni.
From optical spectroscopic observations, we identified the spectral type and determined the color excess of the field stars around V404 Cygni. The color excess and PD showed a good correlation, and we estimated the color excess of V404 Cygni as E(RC − IC) = 1.0 ± 0.2 and E(B − V) = 1.2 ± 0.3, which corresponds to A(V) = 3.7 with RV = 3.1. We also measured the upper limit of peak wavelength for multi-band PD as λmax < 5500 Å. It is known that a relation between λmax and RV is described with the empirical formula RV = 5.5 × 10−4λmax (Serkowski et al. 1975). We obtained the upper limit of RV < 3.0 and A(V) < 3.6 with λmax < 5500 Å. In addition, using another empirical relation of the maximum value of P(V)/A(V) < 0.03 mag−1 (e.g., Voshchinnikov & Das 2008), where P(V) is the PD value at the V band, we obtained the lower limit of 3.0 < A(V). Finally, we obtained an extinction value of 3.0 < A(V) < 3.6. This extinction value is consistent with the value of 2.2 < A(V) < 3.3 reported in Shahbaz et al. (2003), but slightly low compared with the value of A(V) = 4.0 reported in Hynes et al. (2009). From X-ray observations, we also estimate the hydrogen column density via spectral fitting. Using the relation between the hydrogen column density NH and E(B − V) of NH = (6.86 ± 0.27) × 1021E(B − V) (Güver & Özel 2009), NH = (8 ± 2) × 1021 cm−2 is estimated with E(B − V) = 1.2, and this value is also consistent with the X-ray observation of the integrated hydrogen column density of NH = 6–12 × 1021 cm−2 during the outburst in 2015 (Radhika et al. 2016). In general, measurements of the Galactic extinction for X-ray binary systems are model-dependent (e.g., Shahbaz et al. 2003). On the other hand, our method does not include the uncertainties caused by the emission model of a comparison of stars and disks, as it is independent from the accurate intrinsic luminosity of binary systems. Our method is applicable to other Galactic transients that have no intrinsic polarization in the optical band.
The typical value of the interstellar PA within 3° of V404 Cygni is about 35° ± 35° (stellar polarization catalogs: Heiles 2000). Compared with this perspective trend of polarization in the Galactic plane, it is implied that the local ISP around V404 Cygni is not irrelevant to the global Galactic ISP. On the other hand, the maximum PD of the ISP in the Galactic plane within 3° for V404 Cygni is about PD = 3.69 ± 0.18 for HD 331976 at λ ∼ 5400 Å. Of course, this may be due to a lack of measurement of stellar polarization in this region (we have only 22 sources in the stellar polarization catalog within 3° of V404 Cygni). However, the measured local ISP of PD = 8.9% ± 0.1% for V404 Cygni in the V band and the color excess of E(B − V) = 1.2 are among the highest values in the Galaxy (Fosalba et al. 2002). The tendency of the bluer peak of polarization (λmax < 5500 Å) suggested that the dust grains toward the V404 Cygni region are generally smaller than the Galactic average.
Figure 6 shows the quasi-simultaneous spectral energy distribution (SED) of V404 Cygni, with a correction for Galactic extinctions with several A(V) and RV values. The SED data were taken with the HONIR within 30 minutes on MJD 57194. For comparison, we showed the SED with A(V) = 2.2–4.4 and RV = 3.1, which are often used for correction for V404 Cygni, and the SED with no correction in figure 6. There is a break between the J and IC bands (corresponding to 2.5 × 1014 Hz and 3.7 × 1014 Hz, respectively) for the SED with A(V) = 3.0, RV = 3.0. In the optical band (>2.5 × 1014 Hz), the SED with A(V) = 3.0 and RV = 3.0 shows a flat spectrum in the νFν regime. With the synchrotron emission model in the NIR and optical bands, this break indicates the break frequency due to synchrotron self-absorption (SSA, defined as νSSA) which is an important value for estimating the magnetic field strength in the jet. The estimated νSSA value of 2.5 × 1014 < νSSA < 3.7 × 1014 Hz is consistent with the νSSA value assumed in Tanaka et al. (2016).
Acknowledgments
This work is supported by JSPS KAKENHI Grant Numbers 24000004. This work is also supported by JSPS and NSF under the JSPS-NSF Partnerships for International Research and Education (PIRE). This work is also supported by the Optical and Near-infrared Astronomy Inter-University Cooperation Program by the Ministry of Education, Culture, Sports, Science and Technology of Japan. M.W. was supported by a Grant-in-Aid for Young Scientists (A) (25707007) from JSPS.
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