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A remarkable long-term light curve, and deep, low-state spectroscopy: Swift&XMM-Newton monitoring of the NLS1 galaxy Mkn 335

The Narrow-line Seyfert 1 galaxy (NLS1) Mkn 335 is remarkable because it has repeatedly shown deep, long X-ray low-states which show pronounced spectral structure. It has become one of the prototype AGN in deep minimum X-ray states. Here we report on the continuation of our ongoing monitoring campaign with Swift and the examination of the low state X-ray spectra based on a 200 ks triggered observation with XMM in June 2009. Swift has continuously monitored Mkn 335 since May 2007 typically on a monthly basis. This is one of the longest simultaneous UV/X-ray light curves so far obtained for an active galactic nucleus (AGN). Mkn 335 has shown strong X-ray variability even on time scales of hours. In the UV, it turns out to be one of the most variable among NLS1s. Long-term Swift monitoring allow us to examine correlations between the UV, X-rays and X-ray hardness ratios. We find no significant correlation or lag between the UV and X-ray variability; however, we do find distinct trends in the behavior of the hardness ratio variability. The hardness ratio and count rate are correlated in the low-flux state, but no correlation is seen in the high-state. The X-ray low-state spectra of the 2007 and 2009 XMM observations display significant spectral variability. We fit the X-ray spectra with a suite of phenomenological models in order to characterize the data. The broad band CCD spectrum can be fitted equally well with partial absorption and blurred reflection models. These more complicated models are explored in further detail in upcoming work.


Introduction
While AGN typically vary in X-rays by factors of a few (e.g. Grupe et al. 2001), some AGN show dramatic drops in their X-ray flux, accompanied by changes in their spectrum. These AGN are known to be typically X-ray bright, but for some time they display very low flux states which makes them different from AGN such as PHL 1811 which are intrinsically X-ray weak (Leighly et al. 2007).
Some recent examples of such deep minimum states observed in AGN are PG 2112+059, PG 1535+547, PG 1543+489, RX J2217.9-5941, Mkn 335, PHL 1092, and PG 0844+349 (Schartel et al. 2010(Schartel et al. , 2007Ballo et al. 2008;Vignali et al. 2008;Grupe et al. 2004a;Grupe et al. 2007bGrupe et al. , 2008aMiniutti et al. 2009;Gallo et al. 2011). Absorption has always been considered to play an important role in explaining AGN X-ray spectra. Variability through a variable absorber may play a much more important role in AGN than previously thought (e.g. Turner & Miller 2009). Some of the best examples of variable absorbers are e.g. NGC 1365 (Risaliti et al. 2009), Mkn 766 (Miller et al. 2007Turner et al. 2007), 1 H0557-385 (Longinotti et al. 2009), andNGC 3516 Turner et al. (2011). However, this picture is far from being complete and clear. Besides absorption, a popular explanation of the X-ray low states in AGN are reflection models such as proposed by Fabian et al. (2002Fabian et al. ( , 2004Fabian et al. ( , 2009) for 1H 0707-495, or by Gallo (2006) for NLS1s in general. Both, reflection and partial covering absorber models produce relatively similar X-ray spectra (see e.g. discussion in Grupe et al. 2008a). To make things even more complex, as shown by e.g. Chevalier et a. (2006) and Merloni et al. (2006), the X-ray spectrum can be dominated in the high state by reflection and is then modified by a partial covering absorber when the AGN is in a low state. Even for MCG-6-30-15, which has been the poster-child for reflection models after the ASCA detection of a broad red wing of the Fe Kα line by Tanaka et al. (1995), Miller et al. (2008, 2009 argued that its X-ray spectra can be consistently explained by partial covering absorption. Bright AGN in deep low-states with wellcovered light curves and good low-state spectra are essential for further exploring the physics which are responsible for the structures and features seen in AGN X-ray spectra, and especially in low-states, where spectral complexity is most pronounced. The NLS1 Mkn 335 (α 2000 = 00 h 06 m 19. s 5, δ 2000 = +20 • 12 ′ 11. ′′ 0) is such an AGN: it goes repeatedly into deep low-states, is relatively X-ray bright even in those states, and has been monitored by us for years to identify these low-states. It shows interesting spectral structures, and one possible interpretation has been that it exhibits an unusually broad Fe line (Longinotti et al. 2007a). It is nearby (z=0.0258), well studied in the optical spectral band, and has a well-measured BH mass of 1.4 × 10 7 M ⊙ from reverberation mapping (Peterson et al. 2004;Grier et al. 2012).
Mkn 335 has been known to be an X-ray bright AGN for decades, starting with UHURU (Tananbaum et al. 1978) and EINSTEIN (Halpern 1982). EXOSAT and BBXRT observations suggested a strong soft X-ray excess in the Xray spectrum of Mkn 335 (Pounds et al. 1987;Turner et al. 1993, respectively) while Nandra & Pounds (1994) reported on the presence of a warm absorber in this source. During the ROSAT All Sky-Survey and pointed observations Mkn 335 appeared to be X-ray bright, and was modeled with a strong soft X-ray excess (Grupe et al. 2001). From the 1993 observations by ASCA (George et al. 2000), Leighly (1999) concluded that the X-ray spectrum was affected by the presence of a warm absorber, while Ballantyne et al. (2001) interpreted the spectral shape by Xray reflection on the accretion disk. Mkn 335 was also observed by XMM-Newton in 2000 and 2006 (Gondoin et al. 2002;Longinotti et al. 2007a;Longinotti et al. 2007;O'Neill et al. 2007;Arevalo et al 2008) and Suzaku in June 2006 (Larsson et al. 2008). In all cases Mkn 335 was X-ray bright, with the exception of one EXOSAT observation in 1983(Pounds et al. 1987. However, when Mkn 335 was observed by Swift (Gehrels et al. 2004) in 2007 May as part of a Swift fill in project to study the spectral energy distributions in AGN (Grupe et al. 2010) it appeared to be dramatically fainter in X-rays than expected from previous X-ray observations (Grupe et al. 2007b). In order to investigate the nature of the low-state in more detail we initiated a Target-of-Opportunity (ToO) observation with XMM-Newton, which was executed on 2007 July 10 ( Grupe et al. 2008a). During this 22 ks XMM-Newton observation we discovered strong soft Xray emission lines of H and He-like ions such as OVII (Grupe et al. 2008a;Longinotti et al. 2008) in the Reflection Grating Spectrometer (RGS; den Herder et al. 2001). These lines are only visible during an extreme X-ray low-state and they can provide information of the physical conditions of the gas surrounding the central black hole. Because the 22ks XMM-Newton observation was too short to obtain any reliable line ratios, we applied for a 200ks XMM-Newton observation which would be triggered by a low state seen by Swift. When Swift started the monitoring campaign again in May 2009 after Mkn 335 came out of the sun constraint, it appeared to be again in an extreme low state. We therefore triggered our approved XMM-Newton observation and observed Mkn 335 for 200 ks in 2009 June 11th to 14th. Mkn 335 also became a target of our Swift Guest Investigator program in 2008 in which we monitored the AGN on a weekly basis in X-rays and all 6 UVOT filters.
Here we report on the results of the Swift monitoring campaign of Mkn 335 and the continuum short term light curve measured by XMM-Newton during the 200 ks triggered observation. This first paper in a sequence focusses on presenting the rich data sets, on simple modeling, and on revealing spectral trends. In-depth modeling of the multicomponent warm absorber based on the RGS data, and a detailed investigation of blurred reflection models will each be presented in follow-up work. This Paper is organized as follows: in Section 2 we describe the observations and the data reduction of the Swift and XMM-Newton observations. In Section 3 we present the long-and short-term light curves and the analysis of the X-ray spectra obtained by XMM-Newton. The results of this analysis are discussed in Section 4. Throughout the paper spectral indices are denoted as energy spectral indices with F ν ∝ ν −α . Luminosities are calculated assuming a ΛCDM cosmology with Ω M =0.27, Ω Λ =0.73 and a Hubble constant of H 0 =75 km s −1 Mpc −1 corresponding to a luminosity distance D=105 Mpc. All errors are 90% confidence unless stated otherwise.

2.
Observations and data reduction

Swift observations
Swift started monitoring Mkn 335 on 2007 May 17 and still continues with this campaign on at least a monthly basis (Table 1), except for the period of February to May when Mkn 335 is in sun constraint for Swift. As part of a Swift Guest Investigator program the cadence was changed to once per week starting in June 2008. The X-Ray Telescope (XRT, Burrows et al. 2005) ob-servations were performed in Photon Counting mode (PC mode Hill et al. 2004). X-ray data were reduced with the task xrtpipeline version 0.12.1. Source and background photons were extracted with XSELECT version 2.4, from circles with radii of 47 ′′ and 189 ′′ , respectively when the source count rate was less than 0.4 counts s −1 . However, during some parts of our monitoring campaign the count rates were significantly higher than 0.4 counts s −1 which means that the data were affected by pileup. In order to avoid the effects of pileup we excluded the inner part of the Point Spread Function, depending on the brightness of the AGN. The spectral data were re-binned with at least 20 photons per bin using grppha version 3.0.0. The 0.3-10.0 keV spectra were analyzed with XSPEC version 12.3.1x (Arnaud 1996). The auxiliary response files were created with xrtmkarf and corrected using the exposure maps, and the standard response matrices swxpc0to12s0 20010101v011.rmf and swxpc0to12s6 20010101v011.rmf were used for the observations before and after the XRT substrate voltage change in August 2007, respectively (Godet et al. 2009).
The UV-optical Telescope (UVOT, Roming et al. 2005) covers the range between 1700-6500Å and is a sister instrument of XMM-Newton's OM. Although it has a similar set of filters to the OM (Mason et al. 2001;Roming et al. 2005), the UVOT UV throughput is a factor of about 10 higher than that of the OM. The UVOT data were coadded for each segment in each filter with the UVOT task uvotimsum version 1.3. Source photons in all filters were selected in a circle with a radius of 5 ′′ . UVOT magnitudes and fluxes were measured with the task uvotsource version 3 based on the most recent UVOT calibration as described in Poole et al. (2008) Mason et al. 2001). The data are used to measure the optical-to-X-ray spectral energy distribution of Mkn 335 during the XMM-Newton observation. Due to slew problems at the beginning of the observations, V-filter observations were not obtained. All OM observations were performed in a science-user defined configuration with a 7 ′ × 7 ′ observing window.
The XMM-Newton data were processed in the standard way using the XMMSAS version xmmsas 20100423 1803-10.0.0. The EPIC pn data were checked for episodes of high particle background. At the end of the first orbit (odsID 0600540601) the pn data were strongly affected by high particle background. Times with a background at energies E> 10 keV was larger than 10 counts s −1 were screened and not used for spectral analysis. This left an effective observing time of 99036s. During the second orbit (ObsID 0600540501) there were only very short episodes of high particle background. The total screened exposure time during this orbit was 69339s.
The source X-ray photons in the EPIC pn and MOS were selected in a circular region with a radius of 1 ′ . Likewise, background photons were selected from a nearby, source-free region with the same radius. Only single and double events (PATTERN.le.4) and single to quadruple events (PATTERN.le.12) were selected for the pn and MOS data, respectively. The spectra were rebinned with the XMMSAS task specgroup with an oversampling of 3 of the resolution elements at the energy of the bin. The redistribution matrices and the auxiliary response files were created by the XMMSAS tasks rmfgen and arfgen, respectively. We included also the 2007 XMM-Newton pn data in our analysis. Note, however, that we also applied specgroup to rebin this spectrum and that the results may slightly differ from those presented in Grupe et al. (2008a). For comparison purposes, we also display the 2006 XMM-Newton high-state data in form of a light curve and hardness ratios. RGS spectra and response matrices were created by the standard RGS XMMSAS tool rgsproc. The RGS spectra were rebinned with 10 photons per bin using grppha. Spectral fits to the EPIC pn and MOS, and RGS spectra were performed with XSPEC version 12.3.1x (Arnaud 1996). The OM data were processed with the XMMSAS task omichain. The magnitudes and fluxes of Mkn 335 were taken from the source lists created by the omichain task. For the count rate to flux conversion we used the conversion factors given in the OM Calibration document XMM-SOC-CAL-TN-0019.

Xinglong optical spectroscopy
The optical spectrum of Mkn 335 displays strong high-ionization iron coronal lines. In order to search for changes in the broad emission lines, and in the coronal lines, we have triggered an optical spectroscopic observation of Mkn 335 with the 2.16m Xinglong telescope quasi-simultaneous with the 2009 XMM-Newton observation.
The data were acquired on 2009 July 31 with the Opto-Mechanics Research (OMR) spectrograph equipped with a 600 line mm −1 grating and the 2 ′′ slit. This setup produces a resolution of 5Å. The spectrum of Mrk 335 was taken with 3600s exposure.
Data reduction was done following standard procedures using IRAF. The CCD reductions included bias subtraction, flat-field correction, and cosmic-ray removal. Wavelength and flux calibration were performed.
We find that within the uncertainties there is no variability in the coronal lines, similar to our previous result (Grupe et al. 2008a).

Results
3.1. Long-term light curve observed by Swift Figure 1 displays the Swift XRT count rate and hardness ratio light curves as well the the UVOT light curves in each of the 6 filters. Note that after the end of the Swift GI program for Mkn 335 in January 2009, we limited the UVOT observations to W2 in order to reduce the UVOT filter wheel rotations. The vertical lines in Figure 1 mark the times of the 2007 and 2009 XMM-Newton observations. The XRT count rates, hardness ratios and UVOT magnitudes for these light curves are summarized in When we started monitoring Mkn 335 in May 2007 it was in its historical low state as we reported in Grupe et al. (2007b). However, it became significantly brighter starting from September 2007 and Mkn 335 remained in this intermediate state throughout 2008 ( Figure 1). Due to a failure of one of the Swift gyros (Grupe et al. 2007b) the UVOT was turned off in September and October 2007. When we started monitoring Mkn 335 again after it came out of the Swift sunconstraint in May 2009, we found Mkn 335 back in a low state. This low state was the reason why we triggered our pre-approved XMM-Newtonobservation. Mkn 335 remained in a low state throughout 2009 with a slight increase towards the end of that monitoring episode. In 2010 Mkn 335 has been in a low state for most of the time. After emerging from the Swift sun-constraint in May 2011, it shows again a very low state with a Swift XRT count rate of 0.08 counts s −1 and even displayed an all-time low state on 2011 August 28 with a count rate of 0.042±0.010 counts s −1 (right panel in Figure 2). However, recently in November 2011 Mkn 335 went into a high state peaking at about 1 count s −1 in the XRT. We therefore changed our observing strategy to a four day cadence. Mkn 335 is now showing a very rapid variability behavior suggesting that it switches into a high state. Currently (January 2012) we are ob-serving Mkn 335 daily. On 2012 January 11 it displayed the highest XRT count rate measured since January 2009 with 1.3 counts s −1 . This Xray flux is comparable to the 2006 XMM-Newton observation shown in Figure 6.
One question regarding a highly variable source like Mkn 335 is, does the spectral shape change with X-ray flux? As we have shown already in Grupe et al. (2008a) the X-ray spectra of Mkn 335 look completely different in the low and high states. The low number of photons in the Swift XRT spectra, however, does not allow us to perform a detailed spectral analysis. Still, a hardness ratio provides some clues about the changes in the X-ray spectrum. Figure 3 shows the relation between the count rate and hardness ratio in the Swift data 1 . Figure 3 suggests that the AGN becomes softer with increasing count rate. This results is a linear correlation coefficient r l = −0.580 and a Spearman rank order correlation coefficient For both correlations the probability of a random results is P< 10 −6 .
As shown in Figure 1, Mkn 335 also shows variability in all 6 UVOT filters. During the 2009 XMM-Newton observation, Mkn 335 appeared to be slightly fainter by about 0.2-0.3 mag in the OM B, U, W1, M2, and W2 Filters (Table 2) Table 3, in the UVOT W2 filter Mkn 335 was about 0.3 magnitudes brighter during the May 2007 X-ray low state compared with the 2009 May/June low-state. UV variability by 0.3 mag is quite common among NLS1s (Grupe et al. 2010). However, Mkn 335 exhibited a remarkable change in W2 in 2010 as displayed in the right panel of Figure 2 when after a sudden drop from 13.25 to 13.54 in September 2010 it became brighter by 0.79 mag over a period of just three and a half months. As shown in the right panel of Figure 2, it shows an even stronger drop between June and September 2011. On 2011 June 11 it reached its brightest UV state seen during our entire Swift monitoring campaign with UVW2=12.69 mag. This brightening, how-ever, was followed by a continuous fading in the W2 filter with the lowest measurement with 13.63 mag on 2011 September 09. This is a drop in W2 by almost 1 magnitude within three months, equivalent to an increase in flux by a factor of 2.5one of the strongest changes in the UV observed in our entire AGN sample (Grupe et al. 2010), which even exceeds the UV variability seen in WPVS 007 (Grupe et al. 2007a(Grupe et al. , 2008b. These drops in the UV flux are extremely rare in NLS1s, which tend to show no or only little UV variability (e.g. Grupe et al. 2010). Now in January 2012 Mkn 335 displays a very high flux in the UV again, peaking on January 18 with a magnitude of 12.67 in UV W2. Figure 4 displays the relation between the XRT count rate and the UVOT W2 magnitude. This plot shows that Mkn 335 is only found to be faint in the UV when the AGN is faint in X-rays. However, it appears to be bright in the UV independently of the XRT count rate. We found a linear correlation coefficient r l = −0.376 with a probability P=0.00020 of a random result. A Spearman rank order test results in a correlation coefficient r s = −0.347, T s = 3.50 with a probability P=0.00072 of a random result. However, there is not a direct correlation with the source being bright in the UV, when also being bright in X-rays. There is a large scatter in the UV W2 magnitude when the AGN appears to be X-ray faint. Note that the faintest and the brightest UV W2 magnitudes are both measured when Mkn 335 has a count rate less than 0.5 count s −1 . The large scatter may suggest that the X-ray and UV emission generally do not vary together.
To explore this in more detail, we investigated potential lags in the variability of the Swift XRT count rate and the UVOT W2 filter data during the time period 2007 May tp 2011 July. Using the Bayesian framework (e.g. Gregory 2005; Albert 2009, for an introduction to Bayesian analysis) we created "synthetic" model count rates from the observed W2 magnitudes. We first transformed the magnitudes into relative fluxes and then applied a Gaussian bandpass filter in order to only retain variability on specific time scales of interest. For every time scale that was tested the same filter was also applied to the observed count rates. The filtered "synthetic" time series was free to be rescaled and shifted in flux, as well as trans-lated in time. Overall, our model has 4 free parameters: time lag, flux offset and linear scaling factor, and width of the Gaussian used for the timescale of the bandpass filter. We assigned uniform prior probabilities within sensible parameter ranges for all parameters. Assuming normally distributed uncertainties, we then calculated the likelihood of obtaining the observed count rate data, given a particular set of parameter values for our model. Using the nested sampling code Multi-Nest (Feroz et al. 2009), we calculated the posterior probability distributions for all 4 parameters, as well as the Bayesian evidence. For comparison, we also calculated the Bayesian evidence for a reference model with constant flux (i.e., no information from the W2 magnitudes is used except for the temporal sampling). Our calculations show that the constant flux model is preferred over the model created from the W2 magnitudes. Therefore, we conclude that at present there is not enough evidence for lags between the XRT count rate and W2 filter for Mkn 355. A more detailed description of the method used, and its application to the Mkn 355 data, will be provided in an upcoming paper (Gruberbauer et al., in preparation).

Short-term variability observed by
XMM-Newton Figure 5 shows the XMM-Newton EPIC pn count rate and hardness ratio and OM W2 light curves. The pn light curve was binned in 1000s bins. The W2 bins are typically 4400 s as listed in Table 2. Overall, Mkn 335 appeared to be brighter during the 2009 June observation compared with the 2007 July observation (Grupe et al. 2007b). The overall trend is that the AGN becomes softer when the overall count rate increases from the beginning to the end of the XMM-Newton observations. Also note that the 'flares' appear to be soft. These 'flares' show doubling times of roughly 3 hours. This 'flaring' is similar to what had been reported by O'Neill et al. (2007) during the 133 ks 2006 XMM-Newton high-state observation. The light curve from the 2006 XMM-Newton observation is displayed in Figure 6 for comparison.
The XMM-Newton 2009 pn light curve shown in Figure 5 suggests a dependence of the hardness ratio and therefore the shape of the X-ray spectrum on count rate. Figure 7 displays the count rate vs. hardness ratios in the 2009 and 2006 XMM-Newton observations in the left and right panels, respectively. Clearly, there is a strong correlation between count rate and hardness ratio in the low-state 2009 observation. A Spearman rank order test results in a correlation coefficient r s = −0.73 with a Student's T-test value T s = −15.3 with a probability of P< 10 −4 of a random result. However, the high state data from 2006 give a completely different picture. Here we only see a marginal trend that the source becomes softer with increasing count rate. A Spearman rank order test results is r s = −0.20, T s = −2.3, and a probability P=0.023.
The Swift long-term monitoring data confirm that there is only a strong correlation between the count rate and hardness ratio when the AGN is in the low state. This result is similar to the one found in the XMM-Newton data. During the high state (like in the 2006 XMM-Newton data) we do not see a dependence of the X-ray spectral shape with X-ray flux.

General remarks
As shown in the previous subsection, Mkn 335 displays a strong dependence of the shape of its X-ray spectrum on its X-ray flux: the X-ray spectrum appears to be harder when Mkn 335 is in a low state and softer when in a bright state. Therefore performing and interpreting the spectral fits to the average data set will be limited. We therefore split the data into 'bright' and faint' states, defining 'bright' as phases when the pn count rate was > 4 counts s −1 and 'faint' when the count rate was < 3 counts s −1 , corresponding to 0.2-10 keV fluxes of 1.6×10 −14 and 1.2×10 −14 W m −2 , respectively. This results in four spectra, one 'bright' and 'faint' in each of the two orbits. However, most relevant are the spectra of the 'faint' state of the first orbit and the 'bright' state of the second orbit. The other two spectra do not have enough quality to allow detecting significant spectral changes. Therefore for the remainder of the paper we focus on these two epochs of the 2009 observation. The 2009 first orbit 'faint' state and the second orbit 'bright' state data are denoted (I) and (II) in Table 4. In addition, we have also included the low-state spectrum from July 2007 (Grupe et al. 2008a) to our analysis in order to investigate, if and how the overall spectral shape changed over those two years. This spectrum is marked as (III) in Table 4.
In order to see how much the spectra have changed between July 2007 when Mkn 335 was in its low state (Grupe et al. 2008a) and 2009 June, as a first step, we fitted the new data with the 2007 best-fit absorption model. The 2009 first orbit 'faint' and second orbit 'bright' mode were therefore described with a power law model with Galactic and intrinsic partial covering absorption, fixed to the parameters determined for the 2007 low-state data with N H,pc = 15.1 × 10 22 cm −2 , f pc =0.94, and α X =1.78 (Grupe et al. 2008a). The absorber at z=0 was fixed to the Galactic value (3.96 × 10 20 cm −2 ; Dickey & Lockman 1990) which we used for all fits. This fit is shown in the left panel of Figure 8 with the first orbit 'faint' spectrum displayed in black, the 'bright' second orbit spectrum in red and the 2007 low state spectrum in green. Clearly there is a strong deviation of the 2009 data from the neutral partial covering absorber model used for the 2007 spectrum. This result suggests that the absorber and/or the intrinsic continuum spectrum must have changed significantly.
Therefore, in order to start exploring which model can best characterize the observed changes in the spectra, and which models can be safely ruled out, in a second step, we applied some simple spectral models to the new data; and we continue to compare with the previous data (note that those did not have simultaneous deep RGS observations). In these models we fix and tie as many parameters as possible and then thaw them in order to study systematically the influence on the spectral fits of each of these parameters. There are strong residuals around 0.5, 0.9 and 6.4 keV which are likely due to well-localized X-ray emission lines and absorption edges. In order to constrain the broadband continuum parameters, we excluded the energy ranges 0.45-0.6, 0.7-1.1, and 5.5-6.7 keV at first from further analysis at this point. These energy bands correspond to the OVII and OVIII emission lines and absorption edges, and the Fe Kα emission line complex. This strategy will keep the number of free parameters low and we can focus on the continuum properties first.

Simple spectral models
The results of 5 spectral fits are summarized in Table 4. We start with those simple models that have been routinely applied to essentially all AGN observed so far: a single power law, a broken power law, a power law with soft excess, and a power law with absorption. First of all, we find that a single absorbed power law model does not result in an acceptable fit for any of these spectra. Although a broken power law model does significantly improve the fits, it is not an acceptable model for any of the spectra, either. The same holds true for a power law plus black-body-type soft excess. Therefore, other models are required to describe the data, and we continue with the next most obvious addition: an ionized absorption component.

X-ray continuum fits with partial covering absorber models
As a first step in characterizing the 2009 spectra, we used a power law with neutral partial covering absorber, as we successfully applied it to the 2007 low state data. The absorption column density and the covering fraction were left free to vary. However, this model does not yield acceptable results for the 2009 data, neither when fitting the model to the single spectra, nor when fitting it simultaneously to the 'faint state' of the first orbit and the 'bright state' during the second orbit. The next step was to fit the spectra with an ionized partial covering absorber (zxipcf in XSPEC as described by Reeves et al. 2008) and a power law model. As shown in Table 4, the underlying intrinsic continuum spectrum can not be modeled by a single power law and requires an extra component, but addition of a black body component improves the fit. All spectra can be basically fitted with an intrinsic spectrum with the same blackbody temperature and a hard X-ray spectral slope α X =1.0. When all parameters of the ionized partial covering absorber are left free to vary, all 2009 spectra show very similar covering fractions and ionization parameters. If we fix the covering fraction to 51% and the ionization parameter to logξ=1.91 [10 −5 W m, ergs s −1 cm], then the differences in the spectra are mostly due to changes in the absorption column density of the ionized partial covering absorber.
In a final step, we fitted the 2009 'faint' and 'bright' spectra and the 2007 low state spectrum simultaneously, again with an ionized partial coverer. As shown in Table 4, the 2007 data can be fitted by the ionized partial covering absorber model. However, they are fully consistent with a neutral partial covering absorber model as well.
Given the possible degeneracy of the blackbody component and the parameters of the ionized absorber, further modeling and an accurate parameter determination of the absorber parameters, is not possible with the CCD-type spectra discussed here. Therefore, no further modeling is presented here. In fact, our RGS analysis (in prep) suggests that a multi-component absorber is preferred to fully characterize the ionized medium. We have demonstrated that fitting the CCD spectrum with a single warm absorber is sufficient and more complex models are not warranted statistically, and would not yield meaningful results.

Fits with Refection Models
Although the continuum can be fitted by an ionized absorber model quite well, the previous spectral data of Mrk 335 could also be described in terms of the blurred reflection model (e.g. Ross & Fabian 2005). For completeness, here, we briefly show that such a model can also explain the new 2009 data; but we leave a study of the full parameter space of possible models to a dedicated future study (Gallo et al., in preparation).
The initial model is the double reflector model used to interpret the 2007 X-ray weak state of Mrk 335 (Grupe et al. 2008a). The model considered the possibility of having the disc illuminated by two different primary emitters; for example a compact emitter located close to the black hole and a second, more extended corona illuminating the disc at larger distances. In the 2007 low-state the spectrum was described as being reflection dominated where the direct emission from the power law component was significantly suppressed relative to the reflection component. An additional component required in the 2007 lowstate was emission from a distant ionized emitter (Grupe et al. 2008a;Longinotti et al. 2008). This was modeled using the vmekal model in XSPEC for a hot, diffuse gas. There were no obvious absorption features in the 2007 spectrum.
Here, the 2007 data and the 2009 data are fitted together with the model described above. We find that the primary difference in the continuum between 2007 and 2009 is the level of the power law emitter. That is, the power law is more dominant in the 2009 date than in the 2007 low-state. The ionized emitter remains constant in all three spectra and is consistent with being emitted from large distances. The apparent weakness of the emission spectrum in 2009 is attributed to the increased fraction of the power law component in the X-ray band.
Residuals remain in the fit at approximately 1.5 keV. Considering contribution from a warm absorber as is evident in the RGS analysis (Longinotti et al., in preparation) improves the fit. In an upcoming work we are examining this model in much greater detail by considering also the variability of the source during each observation.

Fe Kα emission
So far, the energy bins including the Fe Kα emission line energy range had been excluded from spectral fitting. The rest frame 6.4 keV Fe Kα line is present in all spectra. The width of the line is about σ=140 eV. In order to determine the flux and equivalent width of the Fe Kα line during each observation, we fitted the spectra with a single power law plus redshifted Gaussian line in the 2-10 keV energy range. We found that the line fluxes determined from the 2009 'faint and 'bright' state and 2007 low state spectra are (9.6±2.2)×10 −17 W m −2 , (12.9±2.4)×10 −17 W m −2 , and (13.2±3.3)×10 −17 W m −2 , respectively. These fluxes suggest that the line has been constant regardless in which state the AGN is. The equivalent widths were 220±75, 200±60, and 310±110 eV, respectively. The values for the narrow Fe Kα line are very similar when the reflection model is applied to the data.

Discussion and Conclusions
We presented the results from our long-term monitoring campaign with Swift and the shortterm light-curve and X-ray spectral analysis of the highly variable NLS1 Mkn 335 using a dedicated, triggered 200 ks observation with XMM-Newton. Mkn 335 is one of the best examples of a typically bright AGN that goes through states of low X-ray fluxes. Another example is the Seyfert 1 PG 0844+349 for which we recently reported on an XMM-Newton observation during its deep low X-ray flux state (Gallo et al. 2011). After Mkn 335 was discovered in an extremely low Xray flux state in May 2007 we discovered in the 20 ks XMM-Newton observation from July 2007 that it showed strong soft X-ray emission lines (Grupe et al. 2008a). This observation however was too short to put constraints on the ionized gas properties and to model the continuum shape of the low-state in detail. Therefore, we triggered the deep XMM-Newton observation discussed here, which also led to the first detection of narrow absorption lines in Mkn 335 with RGS (Longinotti et al., in preparation).

Continuum spectrum
As we saw previously for the 2007 low state XMM-Newton observations of Mkn 335, partial covering absorber and reflection models yield a similar quality of the spectral fits. The continuum spectrum is highly variable and complex. When Mkn 335 was observed by XMM-Newton in June 2009 it varied very fast on timescales of just hours with the spectrum becoming softer with increasing X-ray flux. The 2009 data require that the absorber has to be ionized. Since there were no signs of intrinsic absorption features in the 2000 and 2006 XMM-Newton and Suzaku data, the presence of the absorber is not permanent, but transient.
How is a partial coverer model consistent with the long-term variability behavior of this NLS1 seen by Swift ? As shown in Sections 3.1 and 3.2 the X-ray spectra become softer with increasing Xray flux. At first glance, an increase in flux could mean intrinsically an increase of the accretion rate and therefore luminosity and L/L Edd which would result in a steeper X-ray spectrum (Grupe 2004;Grupe et al. 2010;Shemmer et al. 2008). However, as we have seen from the shape of the X-ray spectrum, this simple picture can not explain the X-ray spectrum which is much more complicated than a simple power law model. A variable partial covering absorber, however, can explain the X-ray light curves seen on long as well and short time scales: When the absorber becomes stronger and the observed X-ray flux lower, the spectrum becomes harder. On the other hand, when the absorber becomes more transparent or even dis-appears the spectrum becomes soft.

X-ray variability
As we have shown in Figures 1, 5, and 6 Mkn 335 is highly variable in X-rays on long and short time scales. Our long-term Swift monitoring has shown that Mkn 335 varies in X-rays by factors of about 40 even within months. Beside this strong flux variability we also observe a strong spectral variability when the AGN is in a low state. Above a certain threshold we only see X-ray flux variability with no significant changes in the hardness ratio. This behavior appears on short as well as on long time scales (see Figures 3 and 7). Such a behavior has also been reported by Turner et al. (2008Turner et al. ( , 2011 for NGC 3516. If we assume the partial covering absorber picture, then the spectral changes in the hardness ratio at lower X-ray fluxes can be explained in terms of changes in the absorber properties, such as the column density, covering fraction and ionization parameter.
Indeed, we find that the flux in the Fe Kα line is constant, regardless of the state of the AGN. This constant line flux indicates an underlying continuum component that is not variable.

UV variability
The UV light curve of Mkn 335 is quite remarkable. While NLS1s typically vary by only 0.3 mag in the UV (Grupe et al. 2010), Mkn 335 shows variability by about 1 mag over a time scale of a few months as seen between 2010 September and December and 2011 June and September. (The strongest UV variability we have seen from the Swift observations listed in Grupe et al. (2010), was from Seyfert 1.5 galaxies). In Mkn 335, we did not find any clear correlation between the X-ray and UV flux changes during the time period 2007 May to 2011 July. This result may suggest that the changes in the UV flux are not directly linked to changes in the X-ray flux. However, as shown in the XMM-Newton short-term light curve Figure 5 the brightening in the OM W2 light curve at the end could be seen as a response to the 'flare' at 225 ks in X-rays. Currently we do not have the temporal resolution to exclude that there is not a direct connection between the UV and Xrays. Our investigation of possible lags between the UV and X-ray emission is certainly affected by the under-sampling in the Swift light curves. As we have seen from the XMM-Newton 2009 light curve, Mkn 355 is highly variable even on time scales of hours. Our current Swift light curve systematically under-samples these time scales.

Mkn 335 and AGN in deep minimum X-ray flux states
Mkn 335 is one of the best examples of an AGN that used to be typically in an X-ray bright flux state but then suddenly becomes dramatically fainter. The most extreme of these cases is the Narrow-Line Seyfert 1 galaxy (NLS1) WPVS 007 (Grupe et al. 1995) which dropped by a factor of more than 400 between its ROSAT All-Sky Survey (RASS; Voges et al. 1999) and ROSAT pointed observations about three years later. FUSE UV spectroscopy and Swift X-ray observations revealed the presence of strong UV absorption line troughs and a partial covering absorber in X-rays (Leighly et al. 2009;Grupe et al. 2008b). In the case of WPVS 007 our interpretation is that this is a low-luminosity, low-redshift analogon of a Broad Absorption Line Quasar (BAL QSO) as suggested by Leighly et al. (2009). Because the black hole mass in WPVS 007 is only a few 10 6 M ⊙ , the timescales in this AGN are hundreds of times shorter than in a typical BAL QSO. Mkn 335 could be just another example of such a BAL QSO analogons. As pointed out by (Brandt & Gallagher 2000) and Boroson (2002), BAL QSOs and NLS1s are both AGN with high L/L Edd Eddington ratios, but with significantly different central black hole masses. WPVS 007 as well as Mkn 335 may be the link between these two AGN classes.

Conclusions
We presented the results from a more than four year long monitoring campaign with Swift of the Narrow Line Seyfert 1 galaxy Mkn 335, one of the longest with simultaneous X-ray and UV measurements so far obtained for an AGN. We also presented a 200 ks observation with XMM-Newton triggered at low flux state. We found that • Mkn 335 continues to be highly variable in X-rays on long and short time scales. The total amplitude of variability in count rate in the Swift XRT data (peak to dip between 2007 and 2011) is a factor of 24. The lowest count rate was seen on 2011 August 28 with 0.042 counts s −1 and the highest count rate on 2007 October 18 with 1.277 counts s −1 .
The fastest doubling timescale we saw from the XMM-Newton observation in 2009 was about 2 hours during the first orbit making it one of the most rapidly varying AGN in X-rays.
• The X-ray and UV variability is not strongly correlated. However, during X-ray bright states, the faintest UV states do not occur, while during X-ray low-states the amplitude of UV variability is highest.
• With a variability of about 1 mag in the UV within a few months as seen between 2011 June and September, Mkn 335 is one of the most variable NLS1 known in the UV.
• Its X-ray hardness ratio shows distinct variability patterns in high-and low-state. During the low-states, there is a clear correlation of hardness ratio with count rate. However, this pattern disappears in high-state; hardness ratio is independent of count rate, and occasionally shows some abrupt changes on short time scales.
• Formally, both ionized absorbers and blurred reflectors do provide successful spectral fits to the XMM low-state data.
While the presence of ionized absorption is confirmed by the RGS data (in prep.), the number and the properties of all spectral components present at any given time will be addressed by in-depth follow-up modeling.
Mrk 335 continues to be one of the few AGN which are still bright enough in their low-states for spectral analysis, and therefore hold the best hopes of understanding AGN spectral components, spectral complexity, and mechanisms of variability. We continue to monitor Mrk 335, in order to identify pronounced high-and low-states.
3 The black body temperature T BB is given in units of eV. 4 The column density of the ionized partial covering absorber is given in unites of 10 22 cm −2 .
5 The ionization parameter ξ is given in units 10 −5 W m, or ergs s −1 cm.
6 The covering fraction of the ionized absorber pegged.
7 Covering fraction and ionization parameter fixed to the values of the low state. When left as free parameters log ξ results in the same value, but fpc is not constrained.