Multi-band Photometric Monitoring Research Articles

Optical Continuum Reverberation Mapping of a Candidate IMBH in a Nearby Seyfert 1 Galaxy

To investigate the short-term variability and determine the size of the optical continuum emitting region of intermediate-mass black holes (IMBHs), we carried out high-cadence, multiband photometric monitoring of a Seyfert 1 galaxy J0249−0815 across two nights, together with a one-night single-band preliminary test. The presence of the broad Hα component in our target was confirmed by recent Palomar/P200 spectroscopic observations, 23 yr after the Sloan Digital Sky Survey, ruling out the supernovae origin of the broad Hα line. The photometric experiment was primarily conducted utilizing four-channel imagers MuSCAT 3 and 4 mounted on 2 m telescopes within the Las Cumbres Observatory Global Telescope Network. Despite the expectation of variability, we observed no significant variation (<1.4%) on timescales of 6–10 hr. This nondetection is likely due to substantial host galaxy light diluting the subtle active galactic nucleus (AGN) variability. Additionally, we cannot rule out that the target was in a relatively quiescent state without intranight variability during our monitoring, owing to the stochastic nature of AGN variations. To enhance the possibility of detecting subtle variability signals and lag in future IMBH reverberation campaigns, it may be beneficial to select targets with a higher AGN-to-host flux ratio, and conduct dual-band preliminary tests and tailored simulations.

Long-term multi-band photometric monitoring of Mrk 501

Aims. Radio-to-TeV observations of the bright nearby (z = 0.034) blazar Markarian 501 (Mrk 501), performed from December 2012 to April 2018, are used to study the emission mechanisms in its relativistic jet. Methods. We examined the multi-wavelength variability and the correlations of the light curves obtained by eight different instruments, including the First G-APD Cherenkov Telescope (FACT), observing Mrk 501 in very high-energy (VHE) gamma-rays at TeV energies. We identified individual TeV and X-ray flares and found a sub-day lag between variability in these two bands. Results. Simultaneous TeV and X-ray variations with almost zero lag are consistent with synchrotron self-Compton (SSC) emission, where TeV photons are produced through inverse Compton scattering. The characteristic time interval of 5−25 days between TeV flares is consistent with them being driven by Lense-Thirring precession.

Correcting for chromatic stellar activity effects in transits with multiband photometric monitoring: application to WASP-52

Context.The properties, distribution, and evolution of inhomogeneities on the surface of active stars, such as dark spots and bright faculae, significantly influence the determination of the parameters of an orbiting exoplanet. The chromatic effect they have on transmission spectroscopy, for example, could affect the analysis of data from future space missions such asJames WebbSpace Telescope and Ariel.Aims.To quantify and mitigate the effects of those surface phenomena, we developed a modelling approach to derive the surface distribution and properties of active regions by modelling simultaneous multi-wavelength time-series observables.Methods.We present an upgraded version of theStarSimcode, now featuring the capability to solve the inverse problem and derive the properties of the stars and their active regions by modelling time-series data. As a test case, we analyse ~600 days ofBV RImultiband photometry from the 0.8-mJoan Oró(TJO) and 1.2-m STELLA telescopes of the K2 V exoplanet host star WASP-52. From the results, we further simulated the chromatic contribution of surface phenomena on the observables of its transiting planet.Results.UsingStarSimwe are able to determine the relevant activity parameters of WASP-52 and reconstruct the time-evolving longitudinal map of active regions. The star shows a heterogeneous surface composed of dark spots with a mean temperature of 575 ± 150 K lower than the photospheric value, with filling factors ranging from 3 to 14%. We used the results to study the chromatic effects on the depths of exoplanet transits obtained at different epochs and corresponding to different stellar spot distributions. In the case of WASP-52, which has peak-to-peak photometric variations of ~7% in the visible, the residual effects of dark spots on the measured transit depth of its giant planet, after applying the calculated corrections, are about 10−4at 550 nm and 3 × 10−5at 6μm.Conclusions.We demonstrate that by using contemporaneous ground-based multiband photometry of an active star, it is possible to reconstruct the parameters and distribution of active regions over time, thus making it feasible to quantify the chromatic effects on the planetary radii measured with transit spectroscopy and mitigate them by about an order of magnitude.

Twisted quasar light curves: implications for continuum reverberation mapping of accretion disks

With the advent of high-cadence and multi-band photometric monitoring facilities, continuum reverberation mapping is becoming of increasing importance for the measurement of the physical size of quasar accretion disks. The method is based on measuring the time it takes for a signal to propagate from the center to the outer parts of the central engine, assuming the continuum light curve at a given wavelength has a time shift of the order of a few days with respect to light curves obtained at shorter wavelengths. We show that with high-quality light curves, this assumption is no longer valid and that light curves at different wavelengths are not only shifted in time, but also distorted: in the context of the lamp-post model and thin-disk geometry, the multi-band light curves are, in fact, convolved by a transfer function whose size increases with wavelength. We illustrate the effect with simulated light curves in the Large Synoptic Survey Telescope (LSST) ugrizy bands and examine the impact on the delay measurements when using three different methods, namely JAVELIN, CREAM, and PyCS. We find that current accretion disk sizes estimated from JAVELIN and PyCS are underestimated by ∼30% and that unbiased measurements are only obtained with methods that properly take the skewed transfer functions into account, as the CREAM code does. With the LSST-like light curves, we expect to achieve measurement errors below 5% with a typical two-day photometric cadence.

Complex Magnetospheric Accretion Flows in the Low Accretor CVSO 1335

Abstract Although the magnetospheric accretion model has been extensively applied to T Tauri Stars with typical mass accretion rates, the regime of very low accretion is still not fully explored. Here we report multi-epoch observations and modeling of CVSO 1335, a 5 Myr old star of solar mass that is accreting mass from the disk, as evidenced by redshifted absorption in the Hα profile, but with very uncertain estimates of mass accretion rate using traditional calibrators. We use the accretion shock model to constrain the mass accretion rate from the Balmer jump excess measured with respect to a non-accreting template, and we model the Hα profile, observed simultaneously, using magnetospheric accretion models. Using data taken on consecutive nights, we found that the accretion rate of the star is low, (4–9) × 10−10 M ⊙ yr−1, suggesting a variability on a timescale of days. The observed Hα profiles point to two geometrically isolated accretion flows, suggesting a complex infall geometry. The systems of redshifted absorptions observed are consistent with the star being a dipper, although multi-band photometric monitoring is needed to confirm this hypothesis.

Non-gray, Month-long Brightening of KIC 8462852 in the Immediate Aftermath of a Deep Dip

We present an analysis of the results of long-term, multi-band photometric monitoring of the enigmatic star, KIC 8462852. Observations in the B, g′, V, r′, and IC passbands have been acquired at University College London Observatory between 2017 May and 2018 September. We interrogate the wavelength dependence of the ∼month-long dimming and brightening exhibited by the target star over an 85 day interval, immediately following a days-long ∼5% drop in brightness on Julian Date (JD) 2,458,203. Between JD 2,458,215–300 we measure brightness variations which correspond to relative extinctions of , Ag′/AV = 1.16 ± 0.11, Ar′/AV = 0.80 ± 0.25 and AIc/AV = 0.49 ± 0.19, from which we infer an Ångström absorption coefficient of 1.33 ± 0.43 (). As with the days-long “dips”, the wavelength dependence of the longer-term brightness variations must also be associated with extinction arising from a dust distribution containing a substantial fraction of submicron-sized grains. This implies some common mechanism is responsible for the star’s variability over both these short and longer timescales.

Minute-scale Rapid Variability of Mrk 501 by Multi-band Photometric Monitoring from 2010 to 2017

Mrk 501 is one of our long-term photometric monitoring objects with the optical telescopes at Yunnan Astronomical Observatory (YAO). In this work, observations were performed from 2010 March 11 to 2017 June 7 in the BVRI bands. We detected minute-scale rapid variabilities in the optical multi-band light curves. Durations of these rapid variabilities are from 5 to 77 minutes. The minimal decay timescale is about 3 minutes (2.330 ± 1.572 minutes on 2012 May 22 and 3.172 ± 1.626 minutes on 2017 May 20). After subtracting the host-galaxy component, The minimal decay timescale was about 8 minutes on 2017 June 3, and the corresponding duration is about 10 minutes. This fast variability requires the strength of the co-moving magnetic field in the jet larger than 38.60 G (19.69 G for corrected results, with Doppler factor δD ∼ 3). The discrete correlation function (DCF) analysis between B and I band short-term light curves show significant correlated variability; however, no significant time lags are detected. The color–magnitude variation shows significant negative correlation on intranight timescales, which is due to the superposition of multiple components.

Study on Variability and Spectral Properties of Blazar 3C 273 with Long-term Multi-band Optical Monitoring from 2006 to 2015

We present long-term optical multi-band photometric monitoring of blazar 3C 273, from 2006 May 19 to 2015 March 31 with high temporal resolution in the BVRI bands. The source is in a steady state and showed very small variability, with the values of the fractional variability amplitude of , , and in B, V, R and I, respectively. The intra-night point-to-point fractional variability (Fpp) in each band is below 1.0%, and the Fpp variation amplitude increase from the B-band to the I-band. We find a variability with the timescale of 5.8 ± 2.9 minutes in the I-band on 2009 March 11. This fast variability requires the comoving magnetic field strength in the jet above 18 G with a Doppler factor . Using the discrete correlation function (DCF), the B- and I-band light curves are examined for correlation on whole campaign. Low significance (∼99.73 percent confidence) correlations with the I-band lags the B-band variations are observed. The spectral behaviors in the different variability episodes are studied. “Bluer-when-brighter” spectral behavior is presented for the whole campaign, while there is an opposite tendency when . The weak of the correlation between B- and I-band and the spectrum analysis indicate that the optical radiation consists of two variable components.

Intra-Night Variability of OJ 287 with Long-Term Multiband Optical Monitoring

We present long-term optical multi-band photometric monitoring of the blazar OJ 287 from 6 March 2010 to 3 April 2016, with high temporal resolution in the V R I -bands. The flux variations and colour-magnitude variations on long and short timescales were investigated to understand the emission mechanisms. In our observation, the major outbursts occurred in January 2016, as predicted by the binary pair of black holes model for OJ 287, with F v a r of 1.3∼2.1%, and variability amplitude (Amp) of 5.8∼9.0%. The intra-night variability (IDV) durations were from 18.5 to 51.3 min, and the minimal variability timescale was about 4.7 min. The colour-magnitude variation showed a weak positive correlation on the long timescale with Pearson’s r = 0 . 450 , while a negative correlation was found on intra-night timescales. We briefly discuss the possible physical mechanisms that are most likely to be responsible for the observed flux and colour-magnitude correlation variability.

The relationship between variable and polarized optical spectral components of luminous type 1 non-blazar quasars

Abstract Optical spectropolarimetry by Kishimoto et al. (2004, MNRAS, 354, 1065) has shown that several luminous type 1 quasars show a strong decrease of the polarized continuum flux in the rest-frame near-ultraviolet (UV) wavelengths of λ &amp;lt; 4000 Å. In the literature, this spectral feature is interpreted as evidence of the broadened hydrogen Balmer absorption edge imprinted on the accretion disk thermal emission due to the disk atmospheric opacity effect. On the other hand, quasar flux variability studies have shown that the variable continuum component in UV–optical spectra of quasars, which is considered to be a good indicator of the intrinsic spectral shape of the accretion disk emission, generally has a significantly flat spectral shape throughout the near-UV to optical spectral range. To examine whether the disk continuum spectral shapes revealed as the polarized flux and as the variable component spectra are consistent with each other, we carry out multi-band photometric monitoring observations for a sample of four polarization-decreasing quasars of Kishimoto et al.'s (4C 09.72, 3C 323.1, Ton 202, and B2 1208+32) to derive the variable component spectra and compare the spectral shape of them with that of the polarized flux spectra. Contrary to expectation, we confirm that the two spectral components of these quasars have totally different spectral shapes, in that the variable component spectra are significantly bluer compared to the polarized flux spectra. This discrepancy between two spectral shapes may imply either (1) the decrease of polarization degree in the rest-frame UV wavelengths is not indicating the Balmer absorption edge feature but is induced by some unknown (de)polarization mechanisms, or (2) the UV–optical flux variability is occurring preferentially at the hot inner radii of the accretion disk and thus the variable component spectra do not reflect the whole accretion disk emission.

Is the β Pictoris member GJ 3039AB a physical binary? What the rotation periods tell us?

We have carried out a multi-band photometric monitoring of the close visual binary GJ 3039, consisting of a M4 primary and a fainter secondary component, and likely member of the young stellar association β Pictoris (24-Myr old). From our analysis we found that both components are photometric variables and, for the first time, we detected two micro-flare events. We measured from periodogram analysis of the photometric time series two rotation periods P = 3.355 d and P = 0.925 d, that we could attribute to the brighter GJ 3039A and the fainter GJ 3039B components, respectively. A comparison of these rotation periods with the period distribution of other β Pictoris members further supports that GJ 3039A is a member of this association. We find that also GJ 3039B could be a member, but the infrared magnitude differences between the two components taken from the literature and the photometric variability, which is found to be comparable in both stars, suggest that GJ 3039B could be a foreground star physically unbound to the primary A component.

LONG-TERM MULTI-BAND PHOTOMETRIC MONITORING OF BLAZAR S5 0716+714

We present long-term optical multi-band photometric monitoring of blazar S5 0716+714, from 2004 January 11 to 2012 November 4, with high temporal resolution of approximately 15 minutes in the BVRI bands. The source was in an active state during the whole monitoring campaign, showing intraday variability in 11 of 72 days. The average magnitudes in each band were B = 14.398, V = 13.821, R = 13.255, and I = 12.885. The overall variability amplitudes were , , , and . The structure function showed that typical timescales for intraday variability were between approximately 2 and 7.5 hr. The intraday variability amplitudes were from a few percent to approximately 30%. We found typical variation rates of approximately 0.05 mag hr−1 in both the rising and falling phases, with a minimal variability timescale of 130 minutes. A 10 day period short-term variability was observed simultaneously in the BVRI bands. The discrete correlation function suggests that there is significant correlated variability between the B- and I-band light curves. However, no significant time lags were detected. The spectral behaviors in the different variability episodes were studied, and our observations show bluer-when-brighter behavior on long, short, and intraday timescales for the blazar S5 0716+714. The variability and relevant spectral trends can be explained by the shock-in-jet scenario.

Outburst activity in comets - II. A multiband photometric monitoring of comet 29P/Schwassmann-Wachmann 1

We have carried out a continuous multi-band photometric monitoring of the nuclear activity of comet 29P/Schwassmann-Wachmann 1 from 2008 to 2010. Our main aim has been to study the outburst mechanism on the basis of a follow-up of the photometric variations associated with the release of dust. We used a standardized method to obtain the 10 arc-sec nucleus photometry in the V, R, and I filters of the Johnson-Kron-Cousins system, being accurately calibrated with standard Landolt stars. Production of dust in the R and I bands during the 2010 Feb. 3 outburst has been also computed. We conclude that the massive ejection of large (optically-thin) particles from the surface at the time of the outburst is the triggering mechanism to produce the outburst. Ulterior sublimation of these ice-rich dust particles during the following days induces fragmentation, generating micrometer-sized grains that increase the dust spatial density to produce the outburst in the optical range due to scattering of sun light. The material leaving the nucleus adopts a fan-like dust feature, formed by micrometer-sized particles that are decaying in brightness as it evolved outwards. By analyzing the photometric signal measured in a standardized 10-arcsec aperture using the Phase Dispersion Minimization technique we have found a clear periodicity of 50 days. Remarkably, this value is also consistent with an outburst frequency of 7.4 outbursts/year deduced from the number of outbursts noticed during the effective observing time.

Variability and periodicity of field M dwarfs revealed by multichannel monitoring

We present simultaneous, multiband photometric monitoring of 19 field dwarfs covering most of the M spectral sequence (M2–M9). Significant variability was found in seven objects in at least one out of the three channels I, R and G. Periodic variability was tested with a CLEAN power spectral analysis. Two objects, LHS370 (M5V) and 2M1707+64 (M9V), show periods of and h respectively. On account of the agreement with the typical values of published for M dwarfs (Mohanty & Basri 2003, ApJ, 583, 451), we claim these to be the objects' rotation periods. Three further objects show possible periods of a few hours. Comparing the variability amplitude in each channel with predictions based on the synthetic spectra of Allard et al. (2001, ApJ, 556, 357), we investigated the source of variability in LHS370 and 2M1707+64. For the latter, we find evidence for the presence of magnetically-induced cool spots at a temperature contrast of , with a projected surface coverage factor of less than 0.075. Moreover, we can rule out dust clouds (as represented by the COND or DUSTY models) as the cause of the variability. No conclusion can be drawn in the case of LHS370. Comparing the frequency of occurrence of variability in this and various L dwarf samples published over the past few years, we find that variability is more common in field L dwarfs than in field M dwarfs (for amplitudes larger than on timescales of 0.5 to 20 h). Using the homogeneous data sets of this work and Bailer-Jones & Mundt (2001, A&A, 367, 218), we find fractions of variable objects of among field M dwarfs and among field L dwarfs (and , respectively if we take into account a larger yet more inhomogeneous sample). This is marginally significant ( deviation) and implies a change in the physical nature and/or extent of surface features when moving from M to L dwarfs.