Light Curve Variations Research Articles

The Lyα Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

The hydrogen Lyα (H i Lyα) emission during solar flares has rarely been studied in spatially resolved images, and its physical origin has not been fully understood. In this paper, we present novel Lyα images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-wave-band and multiperspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Lyα emission has a good temporal correlation with the thermal emissions at 1–8 Å and 5–7 keV, indicating that the flaring Lyα is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Lyα at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Lyα emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Lyα emission shows fairly similar features to the He ii λ304 emission in light curve and spatiotemporal variation, along with small differences. These observational results improve our understanding of the Lyα emission in solar flares and also provide some insights for investigating the Lyα emission in stellar flares.

Assigning Degrees of Stochasticity to Blazar Light Curves in the Radio Band Using Complex Networks.

We focus on characterizing the high-energy emission mechanisms of blazars by analyzing the variability in the radio band of the light curves of more than a thousand sources. We are interested in assigning complexity parameters to these sources, modeling the time series of the light curves with the method of the Horizontal Visibility Graph (HVG), which allows us to obtain properties from degree distributions, such as a characteristic exponent to describe its stochasticity and the Kullback–Leibler Divergence (KLD), presenting a new perspective to the methods commonly used to study Active Galactic Nuclei (AGN). We contrast these parameters with the excess variance, which is an astronomical measurement of variability in light curves; at the same time, we use the spectral classification of the sources. While it is not possible to find significant correlations with the excess variance, the degree distributions extracted from the network are detecting differences related to the spectral classification of blazars. These differences suggest a chaotic behavior in the time series for the BL Lac sources and a correlated stochastic behavior in the time series for the FSRQ sources. Our results show that complex networks may be a valuable alternative tool to study AGNs according to the variability of their energy output.

Physical properties of near-Earth asteroid (2102) Tantalus from multiwavelength observations

ABSTRACT Between 2010 and 2017, we have collected new optical and radar observations of the potentially hazardous asteroid (2102) Tantalus from the ESO NTT and Danish telescopes at the La Silla Observatory, and from the Arecibo planetary radar. The object appears to be nearly spherical, showing a low-amplitude light-curve variation and limited large-scale features in the radar images. The spin-state is difficult to constrain with the available data; including a certain light-curve subset significantly changes the spin-state estimates, and the uncertainties on period determination are significant. Constraining any change in rotation rate was not possible, despite decades of observations. The convex light curve-inversion model, with rotational pole at λ = 210° ± 41° and β = −30° ± 35°, is more flattened than the two models reconstructed by including radar observations: with prograde (λ = 36° ± 23°, β = 30° ± 15°), and with retrograde rotation mode (λ = 180° ± 24°, β = −30 ± 16°). Using data from WISE, we were able to determine that the prograde model produces the best agreement in size determination between radar and thermophysical modelling. Radar measurements indicate possible variation in surface properties, suggesting one side might have lower radar albedo and be rougher at the centimetre-to-decimetre scale than the other. However, further observations are needed to confirm this. Thermophysical analysis indicates a surface covered in fine-grained regolith, consistent with radar albedo, and polarisation ratio measurements. Finally, geophysical investigation of the spin-stability of Tantalus shows that it could be exceeding its critical spin-rate via cohesive forces.

Analytic model for photometric variation due to starspots on a differentially rotating star

Abstract We present an analytic model of the light-curve variation for stars with non-evolving starspots on a differentially rotating surface. The Fourier coefficients of the harmonics of the rotation period are expressed in terms of the latitude of the spot, ℓs, and the observer’s line-of-sight direction, ℓo, including the limb-darkening effect. We generate different realizations of multi-spots according to the model, and perform mock observations of the resulting light-curve modulations. We discuss to what extent one can recover the properties of the spots and the parameters for the differential rotation law from the periodogram analysis. Although our analytical model neglects the evolution of spots on the stellar surface (dynamical motion, creation, and annihilation), it provides a basic framework to interpret the photometric variation of stars, in particular from the existing Kepler data and the future space-born mission. It is also applicable to photometric modulations induced by rotation of various astronomical objects.

First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole

In this paper we provide a first physical interpretation for the Event Horizon Telescope's (EHT) 2017 observations of Sgr A*. Our main approach is to compare resolved EHT data at 230 GHz and unresolved non-EHT observations from radio to X-ray wavelengths to predictions from a library of models based on time-dependent general relativistic magnetohydrodynamics simulations, including aligned, tilted, and stellar-wind-fed simulations; radiative transfer is performed assuming both thermal and nonthermal electron distribution functions. We test the models against 11 constraints drawn from EHT 230 GHz data and observations at 86 GHz, 2.2 μm, and in the X-ray. All models fail at least one constraint. Light-curve variability provides a particularly severe constraint, failing nearly all strongly magnetized (magnetically arrested disk (MAD)) models and a large fraction of weakly magnetized models. A number of models fail only the variability constraints. We identify a promising cluster of these models, which are MAD and have inclination i ≤ 30°. They have accretion rate (5.2–9.5) × 10−9 M ⊙ yr−1, bolometric luminosity (6.8–9.2) × 1035 erg s−1, and outflow power (1.3–4.8) × 1038 erg s−1. We also find that all models with i ≥ 70° fail at least two constraints, as do all models with equal ion and electron temperature; exploratory, nonthermal model sets tend to have higher 2.2 μm flux density; and the population of cold electrons is limited by X-ray constraints due to the risk of bremsstrahlung overproduction. Finally, we discuss physical and numerical limitations of the models, highlighting the possible importance of kinetic effects and duration of the simulations.

X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

The X-ray satellite XMM-Newton has so far revealed coronal cycles in seven solar-like stars. Of these, the youngest stars ϵ Eridani (~400 Myr) and ι Horologii (~600 Myr) display the shortest X-ray cycles and the smallest amplitudes, defined as the variation of the X-ray luminosity between the maximum and minimum of the cycle. The X-ray cycle of ϵ Eridani was characterised by applying a novel technique that allowed us to model the corona of a solar-like star in terms of magnetic structures, such as those observed on the Sun (active regions, cores of active regions, and flares), at various filling factors. The high surface coverage of the magnetic structures on ϵ Eridani (65%–95%) that emerged from that study was suggested to be responsible for the low cycle amplitude in the X-ray band. It was also hypothesised that the basal surface coverage with magnetic structures may be higher on the corona of solar-like stars while they are young. To investigate this hypothesis, we started the X-ray monitoring campaign of Kepler-63 in 2019. The star had been observed once before, in 2014, with XMM-Newton. With an age of 210 ± 45 Myr and a photospheric cycle of 1.27 yr, Kepler-63 is the youngest star so far to be observed in X-rays in order to reveal its coronal cycle. Our campaign comprised four X-ray observations of Kepler-63 spanning 10 months (i.e. three-fifths of its photospheric cycle). The long-term X-ray light curve did not reveal a periodic variation of the X-ray luminosity, but a factor two change would be compatible with the considerable uncertainties in the low signal-to-noise data for this relatively distant star. In the case of ϵ Eridani, we describe the coronal emission measure distribution (EMD) of Kepler-63 with magnetic structures such as those observed on the Sun. The best match with the observations is found for an EMD composed of cores and flares of GOES Class C and M following the canonical flare frequency distribution. More energetic flares are occasionally present but they do not contribute significantly to the quasi-stationary high-energy component of the emission measure probed with our modelling. This model yields a coronal filling factor of 100%. This complete coverage of the corona with X-ray-emitting magnetic structures is consistent with the absence of an X-ray cycle, confirming the analogous results derived earlier for ϵ Eridani. Finally, combining our results with the literature on stellar X-ray cycles, we establish an empirical relation between the cycle amplitude LX,max/LX,min and the X-ray surface flux, FX,surf. From the absence of a coronal cycle in Kepler-63, we infer that stars with higher X-ray flux than Kepler-63 must host an EMD that comprises a significant fraction of higher energy flares than those necessary to model the corona of Kepler-63, that is, flares of Class X or higher. Our study opens a new path for studies of the solar-stellar analogy and the joint exploration of resolved and unresolved variability in stellar X-ray light curves.

A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows

We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT.

Optical Characterization of Two Cataclysmic Variables: RBS 0490 and SDSS J075939.79+191417.3

We present optical photometric and spectroscopic observations of two cataclysmic variables (CVs), namely RBS 0490 and SDSS J075939.79+191417.3. The optical variations of RBS 0490 have been found to occur with a period of 1.689 ± 0.001 hr, which appears to be the probable orbital period of the system. Present photometric observations of SDSS J075939.79+191417.3 confirm and refine the previously determined orbital period as 3.14240928 ± 0.00000096 hr. The presence of long-duration eclipse features in the light curves of SDSS J075939.79+191417.3 indicates that eclipses might be due to an accretion disk and bright spot. The orbital inclination of SDSS J075939.79+191417.3 is estimated to be ∼78° using the eclipse morphology. The phased light-curve variations during the orbital cycle of RBS 0490 provide evidence of emission from an independent second accretion region or a second fainter pole. Optical spectra of RBS 0490 and SDSS J075939.79+191417.3 show the presence of strong Balmer and weak He ii (λ4686) emission lines, along with the detection of strong Hβ emission lines with a large equivalent width. The characteristic features of RBS 0490 seem to favor low-field polars, while SDSS J075939.79+191417.3 appears to be similar to nonmagnetic systems.

Variability in quasar light curves: using quasars as standard candles

A relation between the variational slope, sF , and the mean absolute magnitude, 〈M〉, in the light curves of 58 spectroscopically confirmed quasars is measured with a dispersion of 0.15dex.Assuming it holds for quasars in general, not only does this relation add to our working knowledge of quasar variability but it also shows great promise at accurately measuring luminosity distance to a quasar in a model independent way.An accurate, model independent measure of the luminosity distance would allow quasars to be added to the cosmic distance ladder, easily extending the ladder out far beyond the redshifts accessible to type Ia supernovae where cosmological parameters can be better constrained.

A Single-lined Spectroscopic Binary Companion to an Active and Deep Contact Binary in a Quintuple Stellar System

Abstract V410 Aur is a known deep and low-mass-ratio contact binary with a spectroscopically tertiary component and a visual companion. However, the physical and orbital properties of the tertiary are unknown. We constructed (O − C) curve with 117 new eclipse times and those collected from the literature, which shows a cyclical variation with a period of 25.44 (±1.17) yr and a projected semimajor axis of 0.0348 (±0.0021) days while it undergoes a long-term period decrease at a rate of dP/dt = −1.58 × 10−7daysyr−1. The cyclical variation is analyzed for the light-travel time effect. The minimum mass of the third body is determined as 1.39 (±0.13) M ⊙ that is much larger than the inferred value (0.97 M ⊙) of spectroscopic investigation, which indicates that the spectroscopically tertiary is a single-lined spectroscopic binary with an unseen component. The maximum orbital semimajor axis 6.19 (±0.67) au of the third body is determined. Gaia detected a visual companion to V410 Aur at practically the same distance from the Sun nicely confirming the physical bond. These results reveal that V410 Aur contains a single-lined spectroscopic binary with a visual companion in a quintuple stellar system. TESS photometric solutions confirmed that V410 Aur is a deep overcontact binary with a fill-out factor of 73.83(88)% where the additional light contribution is about 24.80(18)%. The continuous variations of light curves are explained by the evolution of a dark spot on the more massive component. The parabolic variation in the (O − C) curve may be caused by the mass transfer from the massive component to the less massive one in the deep overcontact binary.

X-ray intraday variability and power spectral density profiles of the blazar 3C 273 with XMM–Newton during 2000–2021

ABSTRACT We present X-ray intraday variability and power spectral density (PSD) analyses of the longest 23 pointed XMM–Newton observations of the blazar 3C 273 that were taken during 2000–2021. These good time intervals contain between 5 and 24.6 h of data. Variability has been estimated in three energy bands: 0.2–2 keV (soft), 2–10 keV (hard), and 0.2–10 keV (total). 9 of the 23 observations exhibited some variability, though no major variations exceeding 5 per cent were detected. Typical time-scales for variability were ∼1 ks. For those variable light curves, we find that a power-law model provides good fits to each PSD, with most of the slopes between −1.7 and −2.8. Although no variations of hardness ratio (HR) could be measured in any individual observation, an anticorrelation in flux and HR is found in long term data that indicates a harder when brighter trend. Our flux and spectral analyses indicate that both particle acceleration and synchrotron cooling processes make an important contribution to the emission from this blazar.

Young Exoplanet Transit Initiative follow-up observations of the T Tauri star CVSO 30 with transit-like dips

ABSTRACT The T Tauri star CVSO 30, also known as PTFO 8-8695, was studied intensively with ground-based telescopes as well as with satellites over the last decade. It showed a variable light curve with additional repeating planetary transit-like dips every ∼10.8 h. However, these dimming events changed in depth and duration since their discovery and from autumn 2018 on, they were not even present or near the predicted observing times. As reason for the detected dips and their changes within the complex light curve, e.g. a disintegrating planet, a circumstellar dust clump, stellar spots, possible multiplicity, and orbiting clouds at a Keplerian co-rotating radius were discussed and are still under debate. In this paper, we present additional optical monitoring of CVSO 30 with the meter class telescopes of the Young Exoplanet Transit Initiative in Asia and Europe over the last 7 yr and characterize CVSO 30 with the new Early Data Release 3 of the European Space Agency-Gaia mission. As a result, we describe the evolution of the dimming events in the optical wavelength range since 2014 and present explanatory approaches for the observed variabilities. We conclude that orbiting clouds of gas at a Keplerian co-rotating radius are the most promising scenario to explain most changes in CVSO 30’s light curve.

Identifications of RR Lyrae Stars and Quasars from the Simulated Data of Mephisto-W Survey

We have investigated the feasibilities and accuracies of the identifications of RR Lyrae stars and quasars from the simulated data of the Multi-channel Photometric Survey Telescope (Mephisto) W Survey. Based on the variable sources light curve libraries from the Sloan Digital Sky Survey (SDSS) Stripe 82 data and the observation history simulation from the Mephisto-W Survey Scheduler, we have simulated the uvgriz multi-band light curves of RR Lyrae stars, quasars and other variable sources for the first year observation of Mephisto W Survey. We have applied the ensemble machine learning algorithm Random Forest Classifier (RFC) to identify RR Lyrae stars and quasars, respectively. We build training and test samples and extract ∼150 features from the simulated light curves and train two RFCs respectively for the RR Lyrae star and quasar classification. We find that, our RFCs are able to select the RR Lyrae stars and quasars with remarkably high precision and completeness, with purity = 95.4% and completeness = 96.9% for the RR Lyrae RFC and purity = 91.4% and completeness = 90.2% for the quasar RFC. We have also derived relative importances of the extracted features utilized to classify RR Lyrae stars and quasars.

Eclipsing white dwarf binaries in Gaia and the Zwicky Transient Faaccility

ABSTRACT Gaia provided the largest ever catalogue of white dwarf stars. We use this catalogue, along with the third public data release of the Zwicky Transient Facility (ZTF), to identify new eclipsing white dwarf binaries. Our method exploits light-curve statistics and the box least-squares algorithm to detect periodic light-curve variability. The search revealed 18 new binaries, of which 17 are eclipsing. We use the position in the Gaia H-R diagram to classify these binaries and find that the majority of these white dwarfs have MS companions. We identify one system as a candidate eclipsing white dwarf–brown dwarf binary and a further two as extremely low-mass white dwarf binaries. We also provide identification spectroscopy for 17 of our 18 binaries. Running our search method on mock light curves with real ZTF sampling, we estimate our efficiency of detecting objects with light curves similar to the ones of the newly discovered binaries. Many more binaries are to be found in the ZTF footprint as the data releases grow, so our survey is ongoing.

Investigation of Orbital Decay and Global Modeling of the Planet WASP-43 b

Abstract WASP-43 b is one of the most important candidates for detecting an orbital decay. We investigate pieces of evidence for this expectation as variations in its transit timings, based on the ground and space observations. The data set includes the transit observations at the TÜBİTAK National Observatory of Turkey and Transiting Exoplanet Survey Satellite (TESS). We present a global model of the system, based on the most precise photometry from space, ground, and archival radial velocity data. Using the homogenized data set and modeled light curves, we measure the mid-transit times for WASP-43 b. Our analysis agrees with a linear ephemeris for which we refine the light elements for future observations of the system. However, there is a negative difference between the transit timings derived from TESS data in two sectors (9 and 35) and a hint of an orbital period decrease in the entire data set. Both findings are statistically insignificant due to the short baseline of observations, which prevents us from drawing firm conclusions about the orbital decay of this ultra-short-period planet. However, assuming the effect of this decrease of the period in the planet's orbit, we derive a lower limit for the reduced tidal quality factor as Q * ′ > ( 4.01 ± 1.15 ) × 10 5 from the best-fitting quadratic function. Finally, we calculate a probable rotational period for this system as 7.52 days from the out-of-transit flux variation in the TESS light curves due to spot modulation.

Enhanced Localization of Transients Based on a Novel Cross-correlation Method

Abstract Locating gamma-ray transients including gamma-ray bursts (GRBs) and soft gamma repeater (SGR) bursts is of great importance in multi-messenger and multi-wavelength astronomy, such as guiding optical and radio follow-up observations and evaluating the association of a GRB with a gravitational wave or an SGR with a galaxy or fast radio burst (FRB). A time delay (or triangulation) method with multiple missions has been utilized to localize GRBs since their discovery in the 1970s. In this paper, we propose to use the Modified Cross-correlation Function (called Li-CCF hereafter) to derive the accurate time delay from the high time resolution light curves recorded by GRB detectors. This method has been extensively verified with simulation and a sample of GRBs and SGRs with precise positions, using event-by-event data (TTE or Evt data) of Insight-HXMT/HE, Fermi/GBM, Swift/BAT, and GECAM. We find that the 1σ uncertainty (annulus half-width) can be less than 0.3 degrees despite the relatively short distance between these satellites. We note that the localization error given by Li-CCF can be reduced by several times compared with the traditional CCF method or the Fermi/GBM-alone localization, depending on the timing, spectral lag and direction of the GRB, and the distance and number of satellites used. Finally, systematic errors of this time delay method, including the effect of GRB spectral lags and background variation in light curves, have been studied in detail and several improvements have been proposed to deal with those effects.

Deep transfer learning for the classification of variable sources

Ongoing or upcoming surveys such as Gaia, ZTF, or LSST will observe the light curves of billions or more astronomical sources. This presents new challenges for identifying interesting and important types of variability. Collecting a sufficient amount of labeled data for training is difficult, especially in the early stages of a new survey. Here we develop a single-band light-curve classifier based on deep neural networks and use transfer learning to address the training data paucity problem by conveying knowledge from one data set to another. First we train a neural network on 16 variability features extracted from the light curves of OGLE and EROS-2 variables. We then optimize this model using a small set (e.g., 5%) of periodic variable light curves from the ASAS data set in order to transfer knowledge inferred from OGLE and EROS-2 to a new ASAS classifier. With this we achieve good classification results on ASAS, thereby showing that knowledge can be successfully transferred between data sets. We demonstrate similar transfer learning using HIPPARCOS and ASAS-SN data. We therefore find that it is not necessary to train a neural network from scratch for every new survey; rather, transfer learning can be used, even when only a small set of labeled data is available in the new survey.

Empirically revealed properties of Rieger-type cycles of stellar activity

Context. The Rieger cycles were discovered in the Sun as a specific 154-day periodicity of flare occurrence; they strongly influence terrestrial space weather. This phenomenon is far from being understood. Various proposed mechanisms for this periodicity need further verification in stars with stellar parameters different from those of the Sun. Aims. In this work, we study the Rieger-type cycle (RTC) periods PRTC of stellar activity surveyed in the photometric data of the Kepler space telescope. Methods. The processing of 1726 stellar light curves reveals statistics of PRTC values for different main-sequence stars with different effective temperatures Teff and periods of rotation P. This study uses as an index of stellar activity the squared amplitude of the first rotational harmonic A12 of the stellar light curve variability. Results. The obtained information on PRTC of the considered stars confirms the phenomenological analogy between stellar RTCs and the solar Rieger cycles. Two types of RTCs were found: (1) activity cycles with PRTC independent on the stellar rotation, which are typical for the stars with Teff ≲ 5500 K, and (2) activity cycles with PRTC proportional to the stellar rotation period P, which take place on stars with Teff ≳ 6300 K. These two types of RTCs can be driven by the Kelvin and Rossby waves, respectively. The Rossby wave-driven RTCs show a relation with the location of tachocline at shallow depths in the hot stars. This confirms the theoretical predictions of the connection of the RTC with the tachocline. At the same time, the Kelvin wave-driven RTCs do not show this connection. Apparently, both types of wave drivers of RTCs can coexist, resulting in the joint modulation of the magnetic flux tubes emergence by Kelvin and Rossby waves, and the corresponding behavior of PRTC. Conclusions. The signatures of two types of wave drivers discovered for RTCs and their different relations with the tachocline call for a revision and further elaboration of the theory of RTCs.

8.9 hr Rotation in the Partly Burnt Runaway Stellar Remnant LP 40-365 (GD 492)

Abstract We report the detection of 8.914 hr variability in both optical and ultraviolet light curves of LP 40−365 (also known as GD 492), the prototype for a class of partly burnt runaway stars that have been ejected from a binary due to a thermonuclear supernova event. We first detected this 1.0% amplitude variation in optical photometry collected by the Transiting Exoplanet Survey Satellite (TESS). Reanalysis of observations from the Hubble Space Telescope at the TESS period and ephemeris reveal a 5.8% variation in the ultraviolet of this 9800 K stellar remnant. We propose that this 8.914 hr photometric variation reveals the current surface rotation rate of LP 40−365, and is caused by some kind of surface inhomogeneity rotating in and out of view, though a lack of observed Zeeman splitting puts an upper limit on the magnetic field of <20 kG. We explore ways in which the present rotation period can constrain progenitor scenarios if angular momentum was mostly conserved, which suggests that the survivor LP 40−365 was not the donor star but was most likely the bound remnant of a mostly disrupted white dwarf that underwent advanced burning from an underluminous (Type Iax) supernova.

Bayesian Time-resolved Spectroscopy of Multipulse GRBs: Variations of Emission Properties among Pulses

Abstract Gamma-ray bursts (GRBs) are highly variable and exhibit strong spectral evolution. In particular, the emission properties vary from pulse to pulse in multipulse bursts. Here we present a time-resolved Bayesian spectral analysis of a compilation of GRB pulses observed by the Fermi/Gamma-ray Burst Monitor. The pulses are selected to have at least four time bins with a high statistical significance, which ensures that the spectral fits are well determined and spectral correlations can be established. The sample consists of 39 bursts, 117 pulses, and 1228 spectra. We confirm the general trend that pulses become softer over time, with mainly the low-energy power-law index α becoming smaller. A few exceptions to this trend exist, with the hardest pulse occurring at late times. The first pulse in a burst is clearly different from the later pulses; three-fourths of them violate the synchrotron line of death, while around half of them significantly prefer photospheric emission. These fractions decrease for subsequent pulses. We also find that in two-thirds of the pulses, the spectral parameters (α and peak energy) track the light-curve variations. This is a larger fraction compared to what is found in previous samples. In conclusion, emission compatible with the GRB photosphere is typically found close to the trigger time, while the chance of detecting synchrotron emission is greatest at late times. This allows for the coexistence of emission mechanisms at late times.