The stability of the optical flux variation gradient for 3C120

New $B$- and $V$-band monitoring in 2014 $-$ 2015 reveals that the Seyfert 1 Galaxy, 3C120, has brightened by a magnitude of $1.4$, compared to our campaign that took place in 2009 $-$ 2010. This allowed us to check for the debated luminosity and time-dependent color variations claimed for SDSS quasars. For our 3C120 data, we find that the $B/V$ flux ratio of the variable component in the bright epoch is indistinguishable from the faint one. We do not find any color variability on different timescales ranging from about $1$ to $1800$ days. We suggest that the luminosity and time-dependent color variability is an artifact caused by analyzing the data in magnitudes instead of fluxes. The flux variation gradients of both epochs yield consistent estimates of the host galaxy contribution to our 7.5" aperture. These results confirm that the optical flux variation gradient method works well for Seyfert galaxies.Comment: 8 pages, 6 figures. Accepted for publication in section 4. Extragalactic astronomy of Astronomy and Astrophysics v2: Language-Editor Versio

Circumstellar Disks revealed by HH/KK Flux Variation Gradients

The variability of young stellar objects (YSO) changes their brightness and color preventing a proper classification in traditional color-color and color magnitude diagrams. We have explored the feasibility of the flux variation gradient (FVG) method for YSOs, using $H$ and $K$ band monitoring data of the star forming region RCW\,38 obtained at the University Observatory Bochum in Chile. Simultaneous multi-epoch flux measurements follow a linear relation $F_{H}=\alpha + \beta \cdot F_{K}$ for almost all YSOs with large variability amplitude. The slope $\beta$ gives the mean $HK$ color temperature $T_{var}$ of the varying component. Because $T_{var}$ is hotter than the dust sublimation temperature, we have tentatively assigned it to stellar variations. If the gradient does not meet the origin of the flux-flux diagram, an additional non- or less-varying component may be required. If the variability amplitude is larger at the shorter wavelength, e.g. $\alpha < 0$, this component is cooler than the star (e.g. a circumstellar disk); vice versa, if $\alpha > 0$, the component is hotter like a scattering halo or even a companion star. We here present examples of two YSOs, where the $HK$ FVG implies the presence of a circumstellar disk; this finding is consistent with additional data at $J$ and $L$. One YSO shows a clear $K$-band excess in the $JHK$ color-color diagram, while the significance of a $K$-excess in the other YSO depends on the measurement epoch. Disentangling the contributions of star and disk it turns out that the two YSOs have huge variability amplitudes ($\sim 3-5$\,mag). The $HK$ FVG analysis is a powerful complementary tool to analyze the varying components of YSOs and worth further exploration of monitoring data at other wavelengths.Comment: 5 pages, 5 figures, accepted for publication in Astronomy and Astrophysic

Dust reverberation-mapping of the Seyfert 1 galaxy WPVS48

Using robotic telescopes of the Universitatssternwarte Bochum near Cerro Armazones in Chile, we monitored the z=0.0377 Seyfert 1 galaxy WPVS48 (2MASX J09594263-3112581) in the optical (B and R) and near-infrared (NIR, J and Ks) with a cadence of two days. The light curves show unprecedented variability details. The NIR variation features of WPVS48 are consistent with the corresponding optical variations, but the features appear sharper in the NIR than in the optical, suggesting that the optical photons undergo multiple scatterings. The J and Ks emission, tracing the hot (1600 K) dust echo, lags the B and R variations by on average 64 +/- 4 days and 71 +/- 5 days, respectively (restframe). WPVS48 lies on the known tau-M_V relationship. However, the observed lag is about three times shorter than expected from the dust sublimation radius r_sub inferred from the optical-UV luminosity, and explanations for this common discrepancy are searched for. The sharp NIR echos argue for a face-on torus geometry and allow us to put forward two potential scenarios: 1) as previously proposed, in the equatorial plane of the accretion disk the inner region of the torus is flattened and may come closer to the accretion disk. 2) The dust torus with inner radius r_sub is geometrically and optically thick, so that the observer only sees the facing rim of the torus wall, which lies closer to the observer than the torus equatorial plane and therefore leads to an observed foreshortened lag. Both scenarios are able to explain the factor three discrepancy between tau and r_sub. Longer-wavelength dust reverberation data might enable one to distinguish between the scenarios.Comment: 4 pages, 6 figures, Published in Astronomy and Astrophysic

Modelling photometric reverberation data -- a disk-like broad-line region and a potentially larger black hole mass for 3C120

We consider photometric reverberation mapping, where the nuclear continuum variations are monitored via a broad-band filter and the echo of emission line clouds of the broad line region (BLR) is measured with a suitable narrow-band (NB) filter. We investigate how an incomplete emission-line coverage by the NB filter influences the BLR size determination. This includes two basic cases: 1) a symmetric cut of the blue and red part of the line wings, and 2) the filter positioned asymmetrically to the line centre so that essentially a complete half of the emission line is contained in the NB filter. Under the assumption that the BLR size is dominated by circular Keplerian orbits, we find that symmetric cutting of line wings may lead to overestimating the BLR size by less than 5%. The case of asymmetric half-line coverage, similar as for our data of the Seyfert 1 galaxy 3C120, yields the BLR size with a bias of less than 1%. Our results suggest that any BLR size bias due to narrow-band line cut in photometric reverberation mapping is small and in most cases negligible. We used well sampled photometric reverberation mapping light curves with sharp variation features in both the continuum and the Hbeta light curves to determine the geometry type of the Hbeta BLR for 3C120. Modelling of the light curve, under the assumption that the BLR is essentially virialised, argues against a spherical geometry and favours a nearly face-on disk-like geometry with inclination i = 10 +/- 4 deg and extension from 22 to 28 light days. The low inclination may lead to a larger black hole mass than the derived when using the average geometry scaling factor f=5.5. We discuss deviations of Seyfert 1 galaxies from the M_BH - sigma relation.Comment: 9 pages, 11 figures, accepted for publication in Astronomy and Astrophysic

Cosmology and the Hubble Constant: On the Megamaser Cosmology Project (MCP)

The Hubble constant Ho describes not only the expansion of local space at redshift z ~ 0, but is also a fundamental parameter determining the evolution of the universe. Recent measurements of Ho anchored on Cepheid observations have reached a precision of several percent. However, this problem is so important that confirmation from several methods is needed to better constrain Ho and, with it, dark energy and the curvature of space. A particularly direct method involves the determination of distances to local galaxies far enough to be part of the Hubble flow through water vapor (H2O) masers orbiting nuclear supermassive black holes. The goal of this article is to describe the relevance of Ho with respect to fundamental cosmological questions and to summarize recent progress of the the `Megamaser Cosmology Project' (MCP) related to the Hubble constant.Comment: 10 pages, 7 postscript figures (8 ps files), IAU Symposium 287, uses iaus.cl

Multi-wavelength observations of IGR J17544-2619 from quiescence to outburst

In this paper we report on a long multi-wavelength observational campaign of the supergiant fast X-ray transient prototype IGR J17544-2619. A 150 ks-long observation was carried out simultaneously with XMM-Newton and NuSTAR, catching the source in an initial faint X-ray state and then undergoing a bright X-ray outburst lasting about 7 ks. We studied the spectral variability during outburst and quiescence by using a thermal and bulk Comptonization model that is typically adopted to describe the X-ray spectral energy distribution of young pulsars in high mass X-ray binaries. Although the statistics of the collected X-ray data were relatively high we could neither confirm the presence of a cyclotron line in the broad-band spectrum of the source (0.5-40 keV), nor detect any of the previously reported tentative detection of the source spin period. The monitoring carried out with Swift/XRT during the same orbit of the system observed by XMM-Newton and NuSTAR revealed that the source remained in a low emission state for most of the time, in agreement with the known property of all supergiant fast X-ray transients being significantly sub-luminous compared to other supergiant X-ray binaries. Optical and infrared observations were carried out for a total of a few thousands of seconds during the quiescence state of the source detected by XMM-Newton and NuSTAR. The measured optical and infrared magnitudes were slightly lower than previous values reported in the literature, but compatible with the known micro-variability of supergiant stars. UV observations obtained with the UVOT telescope on-board Swift did not reveal significant changes in the magnitude of the source in this energy domain compared to previously reported values.Comment: Accepted for publication on A&A. V2: few typos correcte

The broad-line region and dust torus size of the Seyfert 1 galaxy PGC50427

We present the results of a three years monitoring campaigns of the $z = 0.024$ type-1 active galactic nucleus (AGN) PGC50427. Through the use of Photometric Reverberation Mapping with broad and narrow band filters, we determine the size of the broad-line emitting region by measuring the time delay between the variability of the continuum and the H$\alpha$ emission line. The H$\alpha$ emission line responds to blue continuum variations with an average rest frame lag of $19.0 \pm 1.23$ days. Using single epoch spectroscopy we determined a broad-line H$\alpha$ velocity width of 1020 km s$^{-1}$ and in combination with the rest frame lag and adoption a geometric scaling factor $f = 5.5$, we calculate a black hole mass of $M_{BH} \sim 17 \times 10^{6} M_{\odot}$. Using the flux variation gradient method, we separate the host galaxy contribution from that of the AGN to calculate the rest frame 5100\AA~ luminosity at the time of our monitoring campaign. The rest frame lag and the host-subtracted luminosity permit us to derive the position of PGC50427 in the BLR size -- AGN luminosity diagram, which is remarkably close to the theoretically expected relation of $R \propto L^{0.5}$. The simultaneous optical and NIR ($J$ and $K_{s}$) observations allow us to determine the size of the dust torus through the use of dust reverberation mapping method. We find that the hot dust emission ($\sim 1800K$) lags the optical variations with an average rest frame lag of $46.2 \pm 2.60$ days. The dust reverberation radius and the nuclear NIR luminosity permit us to derive the position of PGC50427 on the known $\tau - M{V}$ diagram. The simultaneus observations for the broad-line region and dust thermal emission demonstrate that the innermost dust torus is located outside the BLR in PGC50427, supporting the unified scheme for AGNs. (Abstract shortened, see the manuscript.)Comment: 11 pages, 23 figures, accepted for publication in Astronomy and Astrophysic

Magnetic field geometry and chemical abundance distribution of the He-strong star CPD -57 3509

The magnetic field of CPD -57 3509 was recently detected in the framework of the BOB (B fields in OB stars) collaboration. We acquired low-resolution spectropolarimetric observations of CPD -57 3509 with FORS2 and high-resolution UVES observations randomly distributed over a few months to search for periodicity, to study the magnetic field geometry, and to determine the surface distribution of silicon and helium. We also obtained supplementary photometric observations at a timeline similar to the spectroscopic and spectropolarimetric observations. A period of 6.36d was detected in the measurements of the mean longitudinal magnetic field. A sinusoidal fit to our measurements allowed us to constrain the magnetic field geometry and estimate the dipole strength in the range of 3.9-4.5kG. Our application of the Doppler imaging technique revealed the presence of He I spots located around the magnetic poles, with a strong concentration at the positive pole and a weaker one around the negative pole. In contrast, high concentration Si III spots are located close to the magnetic equator. Further, our analysis of the spectral variability of CPD -57 3509 on short time scales indicates distinct changes in shape and position of line profiles possibly caused by the presence of beta Cep-like pulsations. A small periodic variability in line with the changes of the magnetic field strength is clearly seen in the photometric data.Comment: 11 pages, 5 tables, 7 figures, accepted for publication in MNRA