Long-term Light Curve Research Articles

K2 Study of the Magnetic Precataclysmic Variable V1082 Sagittarius

Abstract We present a long-term light curve of the precataclysmic variable (CV) V1082 Sgr obtained by the K2 mission over the course of 81 days. We analyze the entire complex light curve as well as explore several sections in detail with a sliding periodogram. The long data set allows the first detection of the orbital period in the light curve, as well as the confirmation of cyclical variability on a longer timescale of about a month. A portion of the light curve in deep minimum reveals a clean, near-sinusoidal variability attributed to the rotation of the spotted surface of the donor star. We model that portion of the light curve assuming that the donor star grossly under-fills its Roche lobe, has cool spots similar to a chromospherically active, slightly evolved early K-star, and might be irradiated by the X-ray beam from the magnetically accreting white dwarf. The fast variability of the object in the active phases resembles the light curves of magnetic CVs (polars).

Long-term variations in the X-ray activity of HR 1099

Context. Although timing variations in close binary systems have been studied for a long time, their underlying causes are still unclear. A possible explanation is the so-called Applegate mechanism, where a strong, variable magnetic field can periodically change the gravitational quadrupole moment of a stellar component, thus causing observable period changes. One of the systems exhibiting such strong orbital variations is the RS CVn binary HR 1099, whose activity cycle has been studied by various authors via photospheric and chromospheric activity indicators, resulting in contradicting periods. Aims. We aim at independently determining the magnetic activity cycle of HR 1099 using archival X-ray data to allow for a comparison to orbital period variations. Methods. Archival X-ray data from 80 different observations of HR 1099 acquired with 12 different X-ray facilities and covering almost four decades were used to determine X-ray fluxes in the energy range of 2−10 keV via spectral fitting and flux conversion. Via the Lomb-Scargle periodogram we analyze the resulting long-term X-ray light curve to search for periodicities. Results. We do not detect any statistically significant periodicities within the X-ray data. An analysis of optical data of HR 1099 shows that the derivation of such periods is strongly dependent on the time coverage of available data, since the observed optical variations strongly deviate from a pure sine wave. We argue that this offers an explanation as to why other authors derive such a wide range of activity cycle periods based on optical data. We furthermore show that X-ray and optical variations are correlated in the sense that the star tends to be optically fainter when it is X-ray bright. Conclusions. We conclude that our analysis constitutes, to our knowledge, the longest stellar X-ray activity light curve acquired to date, yet the still rather sparse sampling of the X-ray data, along with stochastic flaring activity, does not allow for the independent determination of an X-ray activity cycle.

Complex long-term activity of the post-nova X Serpentis

Aims. X Ser is a cataclysmic variable (CV) which erupted as a classical nova in 1903. In this work we use over 100 years of photometry to characterize the long-term light curve of X Ser, with the aim of interpreting the post-nova activity in X Ser in the context of behaviors in other old novae. Methods. This analysis of its long-term optical activity uses the data from the Digital Access to a Sky Century @ Harvard (DASCH), AAVSO, and Catalina Real-time Transient Survey databases, supplemented by the data of other authors. Results. We show that X Ser displays a strong complex activity with the characteristics of various CV types after the return to quiescence from its classical nova outburst. Both nova-like and dwarf nova (DN) features are present. The decaying branches of the individual post-nova outbursts display large mutual similarities and obey the Bailey law for outbursts of DNe. These outbursts of X Ser constitute a uniform group (inside-out outbursts), and their decaying branches can be explained by propagation of cooling front through the accretion disk. In the interpretation, X Ser rapidly transitioned to a thermal-viscous instability regime of the disk, initially only intermittently. The occurrence of the DN outbursts shortly after the end of the nova outburst suggests that the mass transfer rate into the disk was usually not sufficiently high to prevent a thermal-viscous instability of this post-nova. The very long orbital period, and hence large accretion disk of X Ser can contribute to this.

Discovery of deep eclipses in the cataclysmic variable IPHAS J051814.33+294113.0

Performing the photometric observations of the cataclysmic variable IPHAS J051814.33+294113.0, we discovered very deep eclipses. The observations were obtained over 14 nights, had a total duration of 56 hours and covered one year. The large time span, during which we observed the eclipses, allowed us to measure the orbital period in IPHAS J051814.33+294113.0 with high precision, Porb=0.20603098+/-0.00000025 d. The prominent parts of the eclipses lasted 0.1+/-0.01 phases or 30+/-3 min. The depth of the eclipses was variable in the range 1.8-2.9 mag. The average eclipse depth was equal to 2.42+/-0.06 mag. The prominent parts of the eclipses revealed a smooth and symmetric shape. We derived the eclipse ephemeris, which, according to the precision of the orbital period, has a formal validity time of 500 years. This ephemeris can be useful for future investigations of the long-term period changes. During the latter four observational nights in 2017 January, we observed the sharp brightness decrease of IPHAS J051814.33+294113.0 by 2.3 mag. This brightness decrease imitated the end of the dwarf nova outburst. However, the long-term light curve of IPHAS J051814.33+294113.0 obtained in the course of the Catalina Sky Survey during 8 years showed no dwarf nova outbursts. From this we conclude that IPHAS J051814.33+294113.0 is a novalike variable. Moreover, the sharp brightness decrease, which we observed in IPHAS J051814.33+294113.0, suggests that this novalike variable belongs to the VY Scl-subtype. Due to very deep eclipses, IPHAS J051814.33+294113.0 is suitable to study the accretion disc structure using eclipse mapping techniques. Because this novalike variable has the long orbital period, it is of interest to determine the masses of the stellar components from radial velocity measurements. Then, our precise eclipse ephemeris can be useful to the phasing of spectroscopic data.

Flux and spectral variation characteristics of 3C 454.3 at the GeV band

We analyze the long-term lightcurve of 3C 454.3 observed with Fermi/LAT and investigate its relation to flux in the radio, optical and X-ray bands. By fitting the 1-day binned GeV lightcurve with multiple Gaussian functions (MGF), we propose that the typical variability timescale in the GeV band is 1–10 d. The GeV flux variation is accompanied by the spectral variation characterized as flux-tracking, i.e., “harder when brighter.” The GeV flux is correlated with the optical and X-ray fluxes, and a weak correlation between γ-ray flux and radio flux is also observed. The γ-ray flux is not correlated with the optical linear polarization degree for the global lightcurves, but they show a correlation for the lightcurves before MJD 56000. The power density spectrum of the global lightcurve shows an obvious turnover at ∼ 7.7 d, which may indicate a typical variability timescale of 3C 454.3 in the γ-ray band. This is also consistent with the derived timescales by fitting the global lightcurve with MGF. The spectral evolution and an increase in the optical linear polarization degree along with the increase in γ-ray flux may indicate that the radiation particles are accelerated and the magnetic field is ordered by the shock processes during the outbursts. In addition, the nature of 3C 454.3 may be consistent with a self-organized criticality system, similar to Sagittarius A*, and thus the outbursts could be from plasmoid ejections driven by magnetic reconnection. This may further support the idea that the jet radiation regions are magnetized.

Long-Term and Rapid Radio Variability of the Blazar 3C 454.3 in 2010–2017

The article presents the results of observations of the blazar 3C 454.3 (J2253+1608), obtained in 2010–2017 on the RATAN-600 radio telescope of the Special Astrophysical Observatory at 4.6, 8.2, 11.2, and 21.7 GHz and on the 32-m Zelenchuk and Badary radio telescopes of the Quasar VLBI Network of the Institute of Applied Astronomy at 4.84 and 8.57 GHz. Long-term variability of the radio emission is studied, as well as variability on time scales of several days and intraday variability (IDV). Two flares were observed in the long-term light curve, in 2010 and in 2015–2017. The flux density at 21.7 GHz increased by a factor of ten during these flares. The delay in the maximum of the first flare at 4.85 GHz relative to the maximum at 21.7 GHz was six months. The time scale for variability on the descending branch of the first flare at 21.7 GHz was τvar = 1.2 yrs, yielding an upper limit on the linear size of the emitting region of 0.4 pc, corresponding to an angular size of 0.06 mas. The brightness temperature during the flare exceeded the Compton limit, implying a Doppler factor δ = 3.5, consistent with the known presence of a relativistic jet oriented close to the line of sight. No significant variability on time scales from several days to several weeks was found in five sets of daily observations carried out over 120 days. IDV was detected at 8.57 GHz on the 32-m telescopes in 30 of 61 successful observing sessions, with the presence of IDV correlated with the maxima of flares. The characteristic time scale for the IDV was from two to ten hours. A number of IDV light curves show the presence of a time delay in the maxima in the light curves for simultaneous observations carried out on the Badary and Zelenchuk antennas, which are widely separated in longitude. This demonstrates that the IDV most like arises in the interstellar medium.

Oscillatory patterns in the light curves of five long-term monitored type 1 active galactic nuclei

A new combined data of 5 well known type 1 AGN are probed with a novel hybrid method in a search for oscillatory behavior. Additional analysis of artificial light curves obtained from the coupled oscillatory models gives confirmation for detected periods that could have physical background. We find periodic variations in the long-term light curves of 3C 390.3, NGC 4151, NGC 5548 and E1821+643, with correlation coefficients larger than 0.6. We show that oscillatory patterns of two binary black hole candidates NGC 5548 and E1821+643 corresponds to qualitatively different dynamical regimes of chaos and stability, respectively. We demonstrate that absence of oscillatory patterns in Arp 102B could be due to a weak coupling between oscillatory mechanisms. This is the first good evidence that 3C 390.3 and Arp 102B, categorized as double-peaked Balmer line objects, have qualitative different dynamics. Our analysis shows a novelty in the oscillatory dynamical patterns of the light curves of these type 1 AGN.

NuSTAR view of the central region of M31

Our neighbouring large spiral galaxy, the Andromeda galaxy (M 31 or NGC 224), is an ideal target to study the X-ray source population of a nearby galaxy. NuSTAR observed the central region of M 31 in 2015 and allows studying the population of X-ray point sources at energies higher than 10 keV. Based on the source catalogue of the large XMM-Newton survey of M 31, we identified counterparts to the XMM-Newton sources in the NuSTAR data. The NuSTAR data only contain sources of a brightness comparable (or even brighter) than the selected sources that have been detected in XMM-Newton data. We investigate hardness ratios, spectra and long-term light curves of individual sources obtained from NuSTAR data. Based on our spectral studies we suggest four sources as possible X-ray binary candidates. The long-term light curves of seven sources that have been observed more than once show low (but significant) variability.

Identification of high-mass X-ray binaries selected from XMM–Newton observations of the LMC

Abstract The Large Magellanic Cloud (LMC) currently hosts around 23 high-mass X-ray binaries (HMXBs) of which most are Be/X-ray binaries. The LMC XMM–Newton survey provided follow-up observations of previously known X-ray sources that were likely HMXBs, as well as identifying new HMXB candidates. In total, 19 candidate HMXBs were selected based on their X-ray hardness ratios. In this paper we present red and blue optical spectroscopy, obtained with Southern African Large Telescope and the South African Astronomical Observatory 1.9-m telescope, plus a timing analysis of the long-term optical light curves from OGLE to confirm the nature of these candidates. We find that nine of the candidates are new Be/X-ray binaries, substantially increasing the LMC Be/X-ray binary population. Furthermore, we present the optical properties of these new systems, both individually and as a group of all the BeXBs identified by the XMM–Newton survey of the LMC.

Long-term photometric observations in the field of the star formation region NGC7129

AbstractWe present results from long-term optical photometric observations of the Pre-Main Sequence (PMS) stars, located in the star formation region around the bright nebula NGC 7129. Using the long-term light curves and spectroscopic data, we tried to classify the PMS objects in the field and to define the reasons for the observed brightness variations. Our main goal is to explore the known PMS stars and discover new, young, variable stars. The new variable PMS star 2MASS J21403576+6635000 exhibits unusual brightness variations for very short time intervals (few minutes or hours) with comparatively large amplitudes (ΔI = 2.65 mag).

Very-High-Energy γ-Ray Observations of the Blazar 1ES 2344+514 with VERITAS

We present very-high-energy $\gamma$-ray observations of the BL Lac object 1ES 2344+514 taken by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) between 2007 and 2015. 1ES 2344+514 is detected with a statistical significance above background of $20.8\sigma$ in $47.2$ hours (livetime) of observations, making this the most comprehensive very-high-energy study of 1ES 2344+514 to date. Using these observations the temporal properties of 1ES 2344+514 are studied on short and long times scales. We fit a constant flux model to nightly- and seasonally-binned light curves and apply a fractional variability test, to determine the stability of the source on different timescales. We reject the constant-flux model for the 2007-2008 and 2014-2015 nightly-binned light curves and for the long-term seasonally-binned light curve at the $> 3\sigma$ level. The spectra of the time-averaged emission before and after correction for attenuation by the extragalactic background light are obtained. The observed time-averaged spectrum above 200 GeV is satisfactorily fitted (${\chi^2/NDF = 7.89/6}$) by a power-law function with index $\Gamma = 2.46 \pm 0.06_{stat} \pm 0.20_{sys} $ and extends to at least 8 TeV. The extragalactic-background-light-deabsorbed spectrum is adequately fit (${\chi^2/NDF = 6.73/6}$) by a power-law function with index $\Gamma = 2.15 \pm 0.06_{stat} \pm 0.20_{sys} $ while an F-test indicates that the power-law with exponential cutoff function provides a marginally-better fit ($\chi^2/NDF $ = $2.56 / 5 $) at the 2.1$\sigma$ level. The source location is found to be consistent with the published radio location and its spatial extent is consistent with a point source.

Long-term and rapid variability of the radio source J1603+1105

We present the long-term light curve of the radio source J1603+1105 and results of the study of its variability on timescales from several days to several weeks. From 2007, a flare with the maximum in 2010 was observed for the object that earlier showed no significant variations of flux density. Three flares with a successively decreasing amplitude were detected at an active phase in the long-term light curve. The characteristic time of the first one was 2.5 yrs. In five sets of daily observations of 95 to 120 days, the flux density variability on scales from 9 to 32 days in 2011, 2012, 2015, and 2016 was detected; in 2015 it was detected at three frequencies simultaneously. In 2011, the variability was found at a single frequency of 4.8 GHz; in 2012—at two frequencies, 4.8 and 7.7 GHz; in 2015—at 4.6, 8.2, and 11.2 GHz.We present instant spectra of the source at different flare phases showing that the dynamics of the flare development is consistent with the model, in which the variability is the result of the shock wave evolution in the radio source jet.

Tandem Swift and INTEGRAL Data to Revisit the Orbital and Superorbital Periods of 1E 1740.7–2942

Abstract The black hole candidate 1E 1740.7−2942 is one of the strongest hard X-ray sources in the Galactic Center region. No counterparts in longer wavelengths have been identified for this object yet. The presence of characteristic timing signatures in the flux history of X-ray sources has been shown to be an important diagnostic tool for the properties of these systems. Using simultaneous data from NASA’s Swift and ESA’s INTEGRAL missions, we have found two periodic signatures at 12.61 ± 0.06 days and 171.1 ± 3.0 days in long-term hard X-ray light curves of 1E 1740.7−2942. We interpret those as the orbital and superorbital periods of the object, respectively. The reported orbital period is in good agreement with previous studies of 1E 1740.7−2942 using NASA’s RXTE data. We present here the first firm evidence of a superorbital period for 1E 1740.7−2942, which has important implications for the nature of the binary system.

Orbital variations in intensity and spectral properties of the highly obscured sgHMXB IGR J16318−4848

IGR J16318-4848 is an X-ray binary with the highest known line of sight absorption column density among all known X-ray binary systems in our galaxy. In order to investigate the reason behind such a large absorption column, we looked at the variations in the X-ray intensity and spectral parameters as a function of the tentatively discovered $\sim$ 80 day orbit of this source. The orbital period is firmly confirmed in the long term ($\sim$ 12 year) Swift BAT lightcurve. Two peaks about half an orbit apart, one narrow and small, and the other broad and large are seen in the orbital intensity profile. We find that while most orbits show enhanced emissions at these two peaks, the larger peak in the folded longterm lightcurve is more a result of randomly occurring large flares spread over $\sim$ 0.2 orbital phase. As opposed to this, the smaller peak is seen in every orbit as a regular increase in intensity. Using archival data spread over different phases of the orbit and the geometry of the system as known from previously published infrared observations, we present a possible scenario which explains the orbital intensity profile, bursting characteristics and large column density of this X-ray binary.

Discovery of a 26.2 day period in the long-term X-ray light curve of SXP 1323: a very short orbital period for a long spin period pulsar

Context. About 120 Be/X-ray binaries (BeXBs) are known in the Small Magellanic Cloud (SMC); about half of them are pulsating with periods from a few to hundreds of seconds. SXP 1323 is one of the longest-period pulsars known in this galaxy. Aims. SXP 1323 is in the field of view of a large set of calibration observations that we analyse systematically, focusing on the time analysis, in search of periodic signals. Methods. We analyse all available X-ray observations of SXP 1323 from Suzaku, XMM-Newton, and Chandra, in the time range from 1999 to the end of 2016. We perform a Lomb-Scargle periodogram search in the band 2.5-10 keV on all observations to detect the neutron star spin period and constrain its long-term evolution. We also perform an orbital period search on the long-term light curve, merging all datasets. Results. We report the discovery of a 26.188+-0.045 d period analysing data from Suzaku, XMM-Newton, and Chandra, which confirms the optical period derived from the Optical Gravitational Lensing Experiment (OGLE) data. If this corresponds to the orbital period, this would be very short with respect to what is expected from the spin/orbital period relationship. We furthermore report on the spin period evolution in the last years. The source is spinning-up with an average rate of Pdot/P of 0.018 yr-1, decreasing from 1340 to 1100 s, in the period from 2006 to the end of 2016, which is also extreme with respect to the other Be/X-ray pulsars. From 2010 to the end of 2014, the pulse period is not clearly detectable, although the source was still bright. Conclusions. SXP 1323 is a peculiar BeXB due to its long pulse period, rapid spin-up for several years, and short orbital period. A continuous monitoring of the source in the next years is necessary to establish the long-term behaviour of the spin period.

Long-term photometric behavior of the eclipsing cataclysmic variable V729 Sagittarii

Abstract We present the analysis results of an eclipsing cataclysmic variable (CV) V729 Sgr, based on our observations and American Association of Variable Star Observers data. Some outburst parameters are determined, such as outburst amplitude (An) and recurrence time (Tn), and then the relationship between An and Tn is discussed. A cursory examination of the long-term light curves reveals that there are small-amplitude outbursts and dips present, which is similar to the behavior seen in some novalike CVs (NLs). More detailed inspection suggests that the outbursts in V729 Sgr may be Type A (outside-in) with a rise time ∼1.76 d. Further analysis also shows that V729 Sgr is an intermediate between dwarf nova and NLs, and we constrain its mass transfer rate to $1.59\times 10^{-9}<\skew5\dot{M}_{2}<5.8\times 10^{-9}\,M_{\odot }\:$yr−1 by combining the theory for Z Cam type stars with observations. Moreover, the rapid oscillations in V729 Sgr were detected and analyzed for the first time. Our results indicate that the oscillation at ∼25.5 s is a true dwarf nova oscillation (DNO), being associated with the accretion events. The classification of the oscillations at ∼136 and 154 s as longer-period DNOs (lpDNOs) is based on the relation between PlpDNOs and PDNOs. Meanwhile, quasi-periodic oscillations with periods of hundreds of seconds are also detected.

The nearly periodic fluctuations of blazars in long-term X-ray light curves

By means of structure function method, we have analyzed 20 blazars observed by the Rossi X-Ray Timing Explorer (RXTE) to search for possible variability period. There are the year-like periodic values in 11 of these blazars. The year-like periodicities in AO 0235+164, Mrk 501, 3C 273 and PKS 1510-089 are also found by the discrete correlation function method, the Jurkevich method and the power spectrum method. AO 0235+164, Mrk 501 and 3C 273 have possible nearly periodic values of about 370 days, 630 days and 370 days, respectively. PKS 1510-089 has two nearly periodic components, about 230 and 330 days. The year-like periodicities could be explained in the framework of a binary black hole system. The turbulence behind a shock may also give rise to the year-like periodicities, dominated by the dominant eddies’ turnover times. Also, the year-like periodicities may be from the interplay of a shock with essentially helical structures wrapping around a relativistic jet.

General Model for Light Curves of Chromospherically Active Binary Stars∗

Abstract The starspots on the surface of many chromospherically active binary stars concentrate on long-lived active longitudes separated by 180°. Shifts in activity between these two longitudes, the “flip-flop” events, have been observed in single stars like FK Comae and binary stars like σ Geminorum. Recently, interferometry has revealed that ellipticity may at least partly explain the flip-flop events in σ Geminorum. This idea was supported by the double-peaked shape of the long-term mean light curve of this star. Here we show that the long-term mean light curves of 14 chromospherically active binaries follow a general model that explains the connection between orbital motion, changes in starspot distribution, ellipticity, and flip-flop events. Surface differential rotation is probably weak in these stars, because the interference of two constant period waves may explain the observed light curve changes. These two constant periods are the active longitude period and the orbital period . We also show how to apply the same model to single stars, where only the value of P act is known. Finally, we present a tentative interference hypothesis about the origin of magnetic fields in all spectral types of stars.

The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674

We present $NuSTAR$ X-ray observations of the active galactic nucleus (AGN) in NGC 7674. The source shows a flat X-ray spectrum, suggesting that it is obscured by Compton-thick gas columns. Based upon long-term flux dimming, previous work suggested the alternate possibility that the source is a recently switched-off AGN with the observed X-rays being the lagged echo from the torus. Our high-quality data show the source to be reflection-dominated in hard X-rays, but with a relatively weak neutral Fe K$\alpha$ emission line (equivalent width [EW] of $\approx$ 0.4 keV) and a strong Fe XXVI ionised line (EW $\approx$ 0.2 keV). We construct an updated long-term X-ray light curve of NGC 7674 and find that the observed 2-10 keV flux has remained constant for the past $\approx$ 20 years, following a high flux state probed by $Ginga$. Light travel time arguments constrain the minimum radius of the reflector to be $\sim$ 3.2 pc under the switched-off AGN scenario, $\approx$ 30 times larger than the expected dust sublimation radius, rendering this possibility unlikely. A patchy Compton-thick AGN (CTAGN) solution is plausible, requiring a minimum line-of-sight column density ($N_{\rm H}$) of 3 $\times$ 10$^{24}$ cm$^{-2}$ at present, and yields an intrinsic 2-10 keV luminosity of (3-5) $\times$ 10$^{43}$ erg s$^{-1}$. Realistic uncertainties span the range of $\approx$ (1-13) $\times$ 10$^{43}$ erg s$^{-1}$. The source has one of the weakest fluorescence lines amongst {\em bona fide} CTAGN, and is potentially a local analogue of bolometrically luminous systems showing complex neutral and ionised Fe emission. It exemplifies the difficulty of identification and proper characterisation of distant CTAGN based on the strength of the neutral Fe K$\alpha$ line.

Long-term photometric behavior of the PMS stars V977 Cep and V982 Cep in the vicinity of NGC 7129**Based on observations obtained at Rozhen National Astronomical Observatory, Bulgaria.

Long-term BV RI photometric light curves of the pre-main sequence stars V977 Cep and V982 Cep during the period from 2000 October to 2016 August are presented. The stars are located in the vicinity of the reflection nebula NGC 7129. Our photometric data show that both stars exhibit strong photometric variability in all optical passbands, which is typical for Classical T Tauri stars. Using our observational data we analyze the reasons for the observed brightness variations. In the case of V977 Cep we identify previously unknown periodicity in its light curve.