Abstract
We report on optical photopolarimetric results of the radio-loud narrow line 
Introduction
Narrow-line Seyfert 1 (NLSy1) galaxies are a class of active galactic nuclei (AGNs).
Introduction
It is widely recognized that NLSy1 posses a relatively light central black hole (BH) of ∼ 10 6 − 10 7 M ⊙ accreting at a very high rate near the Eddington limit (e.g., Yuan et al. 2008) . Hence, NLSy1 is considered to be a young AGN growing toward a super massive BH which is believed to have a mass of 10 8 − 10 9 M ⊙ . Thus, studying a class of NLSy1 provides us with knowledge about BH evolution. NLSy1s are usually radio-quiet and only 7% of NLSy1 are radio-loud (RL) objects (Zhou et al. 2003; Komossa et al. 2006 ). Owing to the high sensitivity of Fermi LAT instrument, γ-ray emission from some of these RL-NLSy1s
Introduction
are detected and the presence of a third class of AGNs with relativistic jets are confirmed from multi-wavelength observation (Abdo et al. 2009 ) following blazars and radio galaxies.
Introduction
These discoveries opened interesting questions on the unified model of AGNs such as the development of the relativistic jet. Combined with the fact that their broadband spectra are quite similar to those of blazars, it is thought that at least these NLSy1s have powerful relativistic jets and they are directed toward us. Finding evidence of blazar-like behavior in RL-NLSy1 would help us to understand the evolution of relativistic jets.
Introduction
Polarized radiation is one of the evidences of synchrotron origin of jet emissions.
Introduction
Therefore, optical polarimetric observations provide a strong tool to probe jet structures (e.g., Marscher et al. 2008; Abdo et al. 2010a) . The radiation in the optical band is thought to be a superposition of synchrotron and disk emission from its spectral shape of the Fermi-detected NLSy1s which has enough multi-wavelength data. However, a timescale of micro variability with photopolarimetric observation in RL-NLSy1 has never been studied so far. It can provide an important and interesting tool to probe of the size and structure of the emission region in the jet and for distinguishing different models responsible for the variability. Several important and interesting results of micro variability for blazars are -4 -reported (e.g., Miller & Noble 1996; Fossati et al. 2008 Nolan et al. 2012; Beasley et al. 2002 ) is classified as a RL-NLSy1 and displays variable emission from radio to γ-ray bands, suggesting the presence of the relativistic jet (Zhou et al. 2003; Abdo et al. 2009 ). The optical polarization variability with monthly-timescale were detected in the past observations (Eggen et al. 2013 ). Additionally, a high polarization degree (PD) of 18.8% was also reported in April 2009 (Ikejiri et al. 2011) . Micro variability (timescale within a few hours) of optical flux was also observed on PMN J0948+0022 in 2009 (Liu et al. 2010) , and this result supports the fact that the object carries the relativistic jet with a small viewing angle. Hence, PMN J0948+0022 is a good target to study the difference of the relativistic jet between blazars and RL-NLSy1s.
Introduction
Recently, PMN J0948+0022 showed an extreme activity in the near-infrared and GeV γ-ray bands on December 2012 (Carrasco et al. 2012; D'Ammando & Orienti 2013) . In this paper, we present results of high-temporal-density optical monitoring observations of PMN J0948+0022 just after the December 18th 2012 (MJD 56279) near-infrared flare.
Observation
We performed the g', V and R C -band photometric observations of PMN J0948+0022 from 2012 December 20 to 2013 February 20, using the HOWPol (Hiroshima One-shot Wide field Polarimeter) installed to the 1.5m Kanata telescope located at the Higashi-Hiroshima Observatory, Japan (Kawabata et al. 2008 ) and the MITSuME (Multicolor Imaging
Observation
Telescopes for Survey and Monstrous Explosions) installed to the 1.05m Murikabushi telescope located at the Ishigaki island, Japan. Reductions of optical data were performed These values were added to the photometric errors of PMN J0948+0022 in each observation.
Observation
We corrected the data for the Galactic extinction of A g ′ =0.305 mag, A V =0.253 mag and
Observation
We also performed temporally high-densed photopolarimetric observations of PMN J0948+0022 with the HOWPol on 6 nights. In order to study a relation between the optical flux and the micro variability, we performed high-dense photopolarimetric observations in several flux levels. Thanks to the double-Wollaston prism installed to the HOWPol, we could obtain both the Stokes Q and U parameters in only a single exposure. Therefore short-interval (100-310 s) photopolarimetric observations were available. A detail photopolarimetric observation log is given in Table 1 . The bad quality frame (cloudy sky, suffering from cosmic ray and so on) are excluded from our analysis. Polarimetry with the HOWPol suffers from large instrumental polarization (∆PD ∼ 4%) caused by the reflection of the incident light on the tertiary mirror of the telescope. The instrumental polarization was modeled as a function of the declination of the object and the hour angle at the observation, and we subtracted it from the observed value. The polarization angle 1 PYRAF is a product of the Space Telescope Science Institute, which is operated by AURA for NASA. http://www.stsci.edu/institute/software hardware/pyraf 2 http://ned.ipac.caltech.edu/ -6 -(PA) is defined in the standard manner as measured from north to east. The PA was calibrated with two polarized stars, HD183143 and HD204827 (Schulz & Lenzen 1983) .
Observation
Because the PA has an ambiguity of ±180 • × n (where n is an integer), we selected n which gives the least angle difference from the previous data, assuming that the PA would change smoothly. We confirmed that the error of PD in the instrumental polarization correction is smaller than 0.5% and the error of PA is smaller than 2
Observation
• from observations of unpolarized and polarized stars. The typical standard deviations of the Stokes parameters of the comparison star during the observation were ∆Q ∼ 0.02 and ∆U ∼ 0.02 which corresponds to ∆PD ∼ 3%. These values were added to the polarimetric errors of PMN J0948+0022 in each observation.
Observation
3. Results index α is defined between two brightness measurements F 1 , F 2 at frequencies ν 1 and ν 2 , respectively, as α = ln(F 1 /F 2 )/ ln(ν 1 /ν 2 ). We use the mean wavelengths of the filters, 658.8 nm for the R C band, 550.5 nm for the V band and 485.8 nm for the g' band. We calculate the spectral index with the R C and the V band data except the data on MJD 56282, on which the spectral index is calculated with the R C and the g' band data. The error bars include both statistical and systematic ones. Large variation has been detected in the 
Observation
where PF C represents an assumed constant level underlying the outburst, PF long and PF short measures the amplitude of the flares, t long and t short describe approximately the time of the peak, T rlong and T rshort measure the rise time and T dlong and T dshort measure the decay time.
Observation
Suffixes mean the long-term and the short-term component. We obtained T rlong = 16 ± 3 min and T dlong = 39 ± 4 min for the long-term component and T rshort = 2.4 ± 1.5 min and T dshort = 3.0 ± 1.6 min for the short-term component. The characteristic flare duration can be estimated as T r + T d . There is a significant difference of variability timescale between two flares. In contrast, PA maintains its value during the outburst around 19.6 UT with an average value of 15 ± 1 deg. We also obtained the upper limit of PF C < 0.2 × 10 −12 erg cm −2 s −1 . -12 -Except the MJD 56281, there is no significant micro variability of total flux and PD while the daily light curve indicates significant variability from MJD 56283 to 56284. Data on MJD 56283, 56284 and 56295 show a low PD value of ∼ 5%. It seems that there is micro variability of PAs during these periods, but it could be due to low statistics. In MJD 56342 and 56343, a relatively increased total flux was observed with no significant micro variability. During these periods, a total flux is constant but higher than that of MJD 56281. There are also no significant micro variability of PAs on MJD 56342 and 56343 but a gradual increasing of PA is implied. PD is also higher than that on MJD 56283 and 56284. The average of PAs is 36 ± 3 and 51 ± 2 deg in MJD 56342 and 56343, respectively, and these are slightly different from that on MJD 56281 (PA is 15 ± 1 deg).
Discussion
Thanks to one-shot photopolarimetry of the HOWPol, we obtained the highly variable light curve of polarized flux on MJD 56281. In particular, the light curve in the latter night of the same day (around 19-21 UT) consisted of two major pulses whose variability timescales (rise and decay times) are of the order of minute and hour. No significant time lags between flux and PD would indicate that the short-duration pulse occurred in the shock-compressed region with highly ordered magnetic field. This minute-scale variability suggests a very compact emission region of the order of R ∼ cδt var = 1 × 10 14 (δ/10) (t var /320 s) cm, where R is typical size of the emission region, c is the speed of light, δ is beaming factor, and t var is the observed variability timescale. In addition, the very high PD of up to ∼36% observed during the short-duration pulse requires highly ordered magnetic field inside the very compact emission region.
Discussion
NLSy1s are in general believed to have relatively smaller central BH mass compared to typical blazars (see §1). If we assume that the variability timescale is determined -13 -by the activity of central BH, its lower limit is the light crossing time of BH horizon t lc ∼ r g /c ∼ 3.2 × 10 2 (M/10 7.5 M ⊙ ) s, where r g is the Schwarzshild radius and M is the mass of central BH. This is in good agreement with the observed timescale, which may indicate that the central BH mass of PMN J0948+0022 is indeed small as was estimated by Yuan et al. (2008) under the conical structure assumption. On the other hand, However, larger BH mass cannot be ruled out if the conical assumption is not held and the emission region size is estimated to be larger by the Doppler factor than the observed time scale. We finally note that such an minute-scale variability has recently been detected by Paliya et al. (2013) , too and hence it is robust that PMN J0948+0022 shows this kind of rapid variability in the optical band.
Discussion
High spatial resolution radio images toward the core of PMN J0948+0022 taken with VLBI revealed presence of parsec-scale jet and the jet is directed at ∼30 deg (Doi et al. 2006 ). Since the magnetic field direction is usually assumed to be perpendicular to the PA, the observed optical PA of ∼15 deg implies that the magnetic field direction at the emission site is roughly transverse to the jet direction. This reminds us of well-known internal shock scenario (Hughes & Miller 1991) to explain the outburst. If this is the case, relativistic shells emitted from central BH intermittently with an interval of r g /c collide at r ∼ r g (Γ/10) 2 ≈ 1.5 × 10 16 cm and internal transverse shock is generated there.
Discussion
Compression of magnetic field by the transverse shock would generate highly ordered magnetic field inferred from the observed high PD. The variability timescale for hour-scale suggests the size of the emission region of R ∼ cδt var = 1 × 10 15 (δ/10) (t var /3300 s) cm, which is consistent with the standard conical jet assumption (R ∼ rθ) of the opening angle of θ ∼ 1/Γ. We speculate that the transverse shock generated relatively compact emission region of R ∼ 10 15 cm with highly ordered magnetic field.
Discussion
If the minute-scale pulse is indeed radiated by very compact and different blob from Hagen-Thorn et al. 2008; Sasada et al. 2008; Itoh et al. 2013) . But it should be noted that these blazars not always shows the correlation between total flux and PD in the micro variability. In addition, simultaneous short-term photopolarimetric observations have been performed in several blazars (Andruchow et al. 2003; Cellone et al. 2007; Andruchow et al. 2011) , and only a few blazar show the correlation between the total flux and PD. In some blazars, the multi-zone model which have several emission regions are applied to explain these randomly correlations between the total flux and PD in micro variability (e.g., Uemura et al. 2010; Rafle et al. 2012) . On the other hand, our results indicates the model with a single emission region in the jet of PMN J0948+0022 as mentioned above.
Discussion
This assumption might be reflecting a difference in the formation rate of blobs between -15 -NLSy1s and blazars. Namely, a pure synchrotron radiation from intermittent emergence of single blob can be seen in RL-NLSy1 jets. The low values of PD in quiescent states and no significant underlying component in MJD 56281 (PF C < 0.2 × 10 −12 erg cm −2 s −1 ) support this scenario. Of course there is only one short-term photopolarimetric observation of PMN J0948+0022, thus more photopolarimetric observations of RL-NLSy1s in the short time variability will be needed to study the relation between RL-NLSy1 jets and blazar jets.
Discussion
We also found that the total flux increased by a factor of ∼2 on MJD 56342, 61 days after the first observation (see also Figure 1 ). Motivated by this brightening, we conducted continuous polarimetric observation over the subsequent two nights. As shown in Figure 2 and Figure 3 , temporal profiles of the total flux, PD and PA did not show significant variation during the exposures, but we note that PD was somewhat increased from < 10% to ∼15% compared to the previous observations on MJD 56283, 56284, and 56295. These results would be understood in terms of the large-scale emission region of R cδt var = 3 × 10 16 (δ/10) (t var /10 5 s) cm, by substituting the variability timescale greater than 10 5 s. In addition, PDs on MJD 56342 are lower than that on MJD 56281. No significant micro variability except the data on MJD 56281 indicates that there are various timescales of variability. In general, the size of emission region is related with the location of emission region in the jet. These differences of emission regions give weak correlation between the total flux and PD on long-term variability.
Discussion
In conclusion, we performed optical polarimetric observation of RL-NLSy1 PMN J0948+0022 using Kanata/HOWPol and Murikabushi/MITSuME on December 2012
Discussion
to February 2013 after the near-infrared and γ-ray flux enhancements. Our findings are 

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