MOMO. IV. The Complete Swift X-Ray and UV/Optical Light Curve and Characteristic Variability of the Blazar OJ 287 during the Last Two Decades

We have been carrying out a dense monitoring of the blazar OJ 287 with Swift since late 2015 as part of our project MOMO (Multiwavelength Observations and Modeling of OJ 287). This is the densest existing monitoring of OJ 287 involving X-ray/UV data. In this latest publication of a sequence, we characterize the multiwavelength variability of OJ 287 based on >4000 Swift single-wave-band data sets including archival data since 2005. A structure function analysis reveals a characteristic timescale of ∼5 days in the optical–UV at epochs of low-level activity and larger during outbursts. The discrete correlation function shows zero lag between optical and UV, with τ = 0 ± 1 day at the epoch of densest cadence. During outbursts (in 2016/17 and 2020) the X-rays follow the UV with near-zero lags. However, during quiescence, the delay is 7–18 days with X-rays leading or lagging, interpreted as due to a different X-ray component dominated by inverse Compton emission. Scaling relations are used to derive the characteristic length scales of the broad-line region and torus in OJ 287. A remarkable, symmetric UV–optical deep fade is identified in late 2017, lasting 2 months. We rule out occultation from the passage of a dusty cloud and a model where the secondary black hole deflects the jet between the primary and observer. We speculate about a temporary dispersion or jet swing event in the core or in a bright quasi-stationary jet feature. The deep fade reveals an additional, spatially distinct X-ray component. The epoch 2020.9–2021.1 was searched for precursor flare activity predicted by the binary black hole model of OJ 287.

Classical Novae at Radio Wavelengths

We present radio observations (1–40 GHz) for 36 classical novae, representing data from over five decades compiled from the literature, telescope archives, and our own programs. Our targets display a striking diversity in their optical parameters (e.g., spanning optical fading timescales, t 2 = 1–263 days), and we find a similar diversity in the radio light curves. Using a brightness temperature analysis, we find that radio emission from novae is a mixture of thermal and synchrotron emission, with nonthermal emission observed at earlier times. We identify high brightness temperature emission (T B > 5 × 104 K) as an indication of synchrotron emission in at least nine (25%) of the novae. We find a class of synchrotron-dominated novae with mildly evolved companions, exemplified by V5589 Sgr and V392 Per, that appear to be a bridge between classical novae with dwarf companions and symbiotic binaries with giant companions. Four of the novae in our sample have two distinct radio maxima (the first dominated by synchrotron and the later by thermal emission), and in four cases the early synchrotron peak is temporally coincident with a dramatic dip in the optical light curve, hinting at a common site for particle acceleration and dust formation. We publish the light curves in a machine-readable table and encourage the use of these data by the broader community in multiwavelength studies and modeling efforts.

THE GRB PROMPT OPTICAL EMISSION FEATURES ON THE EXAMPLE OF GRB160625B

This article discusses a model for the occurrence of quasiperiodic oscillations in the optical light curve of a gamma-ray burst(GRB). The model is based on the assumption that GRB occurs in a binary system with strong stellar wind. Progenitor is a small core of helium star which emits a strong stellar wind that perturbs by the compact companion (neutron star).There is a calculation of model parameters by using the GRB160625B as an example, for which the MASTER global network received an optical curve with high time resolution.

The dual nature of blazar fast variability: Space and ground observations of S5 0716+714

ABSTRACT Blazar S5 0716+714 is well-known for its short-term variability, down to intraday time-scales. We here present the 2-min cadence optical light curve obtained by the TESS space telescope in 2019 December–2020 January and analyse the object fast variability with unprecedented sampling. Supporting observations by the Whole Earth Blazar Telescope Collaboration in B, V, R, and I bands allow us to investigate the spectral variability during the TESS pointing. The spectral analysis is further extended in frequency to the UV and X-ray bands with data from the Neil Gehrels Swift Observatory. We develop a new method to unveil the shortest optical variability time-scales. This is based on progressive de-trending of the TESS light curve by means of cubic spline interpolations through the binned fluxes, with decreasing time bins. The de-trended light curves are then analysed with classical tools for time-series analysis (periodogram, autocorrelation, and structure functions). The results show that below 3 d there are significant characteristic variability time-scales of about 1.7, 0.5, and 0.2 d. Variability on time-scales $\lesssim 0.2$ d is strongly chromatic and must be ascribed to intrinsic energetic processes involving emitting regions, likely jet substructures, with dimension less than about 10−3 pc. In contrast, flux changes on time-scales $\gtrsim 0.5$ d are quasi-achromatic and are probably due to Doppler factor changes of geometric origin.

The dipper light curve of V715 Persei: is there dust in the magnetosphere?

Context. The dipper optical light curves in young stellar objects are commonly interpreted as partial or total occultation of the stellar radiation by dust surrounding the star. Aims. In this work, we analyze the amplitude of the optical light curve of V715 Per, located in the young star forming region IC 348. Observations gathered over the years suggest that the light curve can be explained by dust extinction events. Methods. In our model, the dust is distributed inside the magnetosphere according to the strength of the stellar magnetic field. The dust distribution is modulated by the vertical component of the field whose axis is misaligned with respect to the rotational axis. We include a model for evaporation of the dust reaching the magnetosphere in order to consistently calculate its distribution. Results. For V715 Per, there is dust in the optically thick warp at the disk truncation radius. We suggest that the optical light curve is explained by extinction caused by dust reaching inside the magnetosphere. The dust distribution is optically thin, and it cannot survive for a long time because of the high temperature and low density. However, as the grains rapidly move towards the stellar surface and the sublimation is not instantaneous, there is a layer of dust covering the magnetosphere responsible for the extinction. Conclusions. Dust surviving the harsh conditions of the magnetospheric accretion flow may be responsible for some of the dipper light curves.

Multiwaveband quasi-periodic oscillation in the blazar 3C 454.3

ABSTRACT We report the detection (>4σ) of a quasi-periodic oscillation (QPO) in the gamma-ray light curve of 3C 454.3 along with a simultaneous marginal QPO detection (>2.4σ) in the optical light curves. Periodic flux modulations were detected in both of these wavebands with a dominant period of ∼47 d. The gamma-ray QPO lasted for over 450 d (from MJD 56800 to 57250), resulting in over nine observed cycles which is among the highest number of periods ever detected in a blazar light curve. The optical light curve was not well sampled for almost half of the gamma-ray QPO span due to the daytime transit of the source, which could explain the lower significance of the optical QPO. Autoregressive Integrated Moving Average (ARIMA) modelling of the light curve revealed a significant, exponentially decaying, trend in the light curve during the QPO, along with the 47 d periodicity. We explore several physical models to explain the origin of this transient quasi-periodic modulation and the overall trend in the observed flux with a month-like period. These scenarios include a binary black hole system, a hotspot orbiting close to the innermost stable circular orbit of the supermassive black hole, and precessing jets. We conclude that the most likely scenario involves a region of enhanced emission moving helically inside a curved jet. The helical motion gives rise to the QPO and the curvature (∼0.05○ pc−1) of the jet is responsible for the observed trend in the light curve.

An investigation of 56Ni shells as the source of early light curve bumps in type Ia supernovae

An excess of flux (i.e. a bump) in the early light curves of type Ia supernovae has been observed in a handful of cases. Multiple scenarios have been proposed to explain this excess flux. Recently, it has been shown that for at least one object (SN 2018oh) the excess emission observed could be the result of a large amount of 56Ni in the outer ejecta (∼0.03 M⊙). We present a series of model light curves and spectra for ejecta profiles containing 56Ni shells of varying masses (0.01, 0.02, 0.03, and 0.04 M⊙) and widths. We find that even for our lowest mass 56Ni shell, an increase of >2 magnitudes is produced in the bolometric light curve at one day after explosion relative to models without a 56Ni shell. We show that the colour evolution of models with a 56Ni shell differs significantly from those without and shows a colour inversion similar to some double-detonation explosion models. Furthermore, spectra of our 56Ni shell models show that strong suppression of flux between ∼3700–4000 Å close to maximum light appears to be a generic feature for this class of model. Comparing our models to observations of SNe 2017cbv and 2018oh, we show that a 56Ni shell of 0.02–0.04 M⊙ can match shapes of the early optical light curve bumps, but the colour and spectral evolution are in disagreement. Our models also predict a strong UV bump that is not observed. This would indicate that an alternative origin for the flux excess is necessary. In addition, based on existing explosion scenarios, producing such a 56Ni shell in the outer ejecta as required to match the light curve shape, without the presence of additional short-lived radioactive material, may prove challenging. Given that only a small amount of 56Ni in the outer ejecta is required to produce a bump in the light curve, such non-monotonically decreasing 56Ni distributions in the outer ejecta must be rare, if they were to occur at all.

Luminous supernovae associated with ultra-long gamma-ray bursts from hydrogen-free progenitors extended by pulsational pair-instability

We show that the luminous supernovae associated with ultra-long gamma-ray bursts can be related to the slow cooling from the explosions of hydrogen-free progenitors that are extended by pulsational pair-instability. We have recently shown that some rapidly-rotating hydrogen-free gamma-ray burst progenitors that experience pulsational pair-instability can keep an extended structure caused by pulsational pair-instability until the core collapse. These types of progenitors have large radii exceeding 10 R⊙ and they sometimes reach beyond 1000 R⊙ at the time of the core collapse. They are, therefore, promising progenitors of ultra-long gamma-ray bursts. Here, we perform light-curve modeling of the explosions of one extended hydrogen-free progenitor with a radius of 1962 R⊙. The progenitor mass is 50 M⊙ and 5 M⊙ exists in the extended envelope. We use the one-dimensional radiation hydrodynamics code STELLA in which the explosions are initiated artificially by setting given explosion energy and 56Ni mass. Thanks to the large progenitor radius, the ejecta experience slow cooling after the shock breakout and they become rapidly evolving (≲10 days), luminous (≳1043 erg s−1) supernovae in the optical even without energy input from the 56Ni nuclear decay when the explosion energy is more than 1052 erg. The 56Ni decay energy input can affect the light curves after the optical light-curve peak and make the light-curve decay slowly when the 56Ni mass is around 1 M⊙. They also have a fast photospheric velocity above 10 000 km s−1 and a hot photospheric temperature above 10 000 K at around the peak luminosity. We find that the rapid rise and luminous peak found in the optical light curve of SN 2011kl, which is associated with the ultra-long gamma-ray burst GRB 111209A, can be explained as the cooling phase of the extended progenitor. The subsequent slow light-curve decline can be related to the 56Ni decay energy input. The ultra-long gamma-ray burst progenitors we proposed recently can explain both the ultra-long gamma-ray burst duration and the accompanying supernova properties. When the gamma-ray burst jet is off-axis or choked, the luminous supernovae could be observed as fast blue optical transients without accompanying gamma-ray bursts.

HP Cet and Swift J0820.6–2805: two candidate intermediate polars observed by XMM–Newton

ABSTRACT We report on the XMM–Newton observation of HP Cet and Swift J0820.6–2805, two X-ray photon sources that are candidates to be members of the intermediate polar class of cataclysmic variables. If the historical optical light curve of HP Cet shows a periodic feature at ≃96 min, a clear identification of such a signature in the high energy band (apart for a variability on a time-scale of ≃8 min as detected by the ROSAT satellite) is lacking. By using XMM–Newton archive data, we clearly identify a feature (at ≃88 min) which is marginally consistent with one of the binary system orbital periods reported in the literature. We also found a signature of a periodic features on the time-scale of ≃5.6 min. In the case of Swift J0820.6–2805, the intermediate polar nature was previously unclear and the orbital and the white dwarf spin periods were unknown. Here, the 0.3–10 keV data undoubtedly reveal an orbital period and a white dwarf spin of ≃87.5 and ≃27.9 min, respectively. The spectral analysis showed that both HP Cet and Swift J0820.6–280 are members of the underluminous IP subclass since their luminosity in the 0.3–10 keV band is estimated to be ≃5 × 1030 and ≃3.8 × 1029 erg s−1, respectively.