Swift Satellite Research Articles

Classification of BATSE, Swift, and Fermi Gamma-Ray Bursts from Prompt Emission Alone

Although it is generally assumed that there are two dominant classes of gamma-ray bursts (GRBs) with different typical durations, it has been difficult to classify GRBs unambiguously as short or long from summary properties such as duration, spectral hardness, and spectral lag. Recent work used t-distributed stochastic neighborhood embedding (t-SNE), a machine-learning algorithm for dimensionality reduction, to classify all Swift GRBs as short or long. Here, the method is expanded, using two algorithms, t-SNE and UMAP, to produce embeddings that are used to provide a classification for 1911 BATSE bursts, 1321 Swift bursts, and 2294 Fermi bursts for which both spectra and metadata are available. Although the embeddings appear to produce a clear separation of each catalog into short and long bursts, a resampling-based approach is used to show that a small fraction of bursts cannot be robustly classified. Further, three of the 304 bursts observed by both Swift and Fermi have robust but conflicting classifications. A likely interpretation is that in addition to the two predominant classes of GRBs, there are additional, uncommon types of bursts which may require multiwavelength observations in order to separate them from more typical short and long GRBs.

Swift/UVOT: 18 Years of Long GRB Discoveries and Advances

The Neil Gehrels Swift Observatory (Swift) has been in operation for 18 years. The Ultra-Violet/Optical Telescope (UVOT) onboard Swift was designed to capture the earliest optical/UV emission from gamma-ray bursts (GRBs), spanning the first few minutes to days after the prompt gamma-ray emission. In this article, we provide an overview of the long GRBs (whose prompt gamma-ray duration is >2 s) observed by the Swift/UVOT, and review the major discoveries that have been achieved by the Swift/UVOT over the last 18 years. We discuss where improvements have been made to our knowledge and understanding of the optical/UV emission, particularly the early optical/UV afterglow.

Dust-scattering rings of GRB 221009A as seen by the Neil Gehrels Swift X-ray Observatory: can we count them all?

ABSTRACT We present the first results for the dust-scattering rings of GRB 221009A, coined as the gamma-ray bursts (GRBs) of the century, as observed by the Neil Gehrels Swift Observatory. We perform analysis of both time resolved observations and stacked data. The former approach enable us to study the expansion of the most prominent rings, associate their origin with the prompt X-ray emission of the GRB and determine the location of the dust layers. The stacked radial profiles increase the signal-to-noise ratio of the data and allows detection of fainter and overlapping peaks in the angular profile. We find a total of 16 dust concentrations (with hints of even more) that span about 15kpc in depth and could be responsible for the highly structured X-ray angular profiles. By comparing the relative scattered fluxes of the five most prominent rings we show that the layer with the largest amount of dust is located at about 0.44 kpc away from us. We finally compare the location of the dust layers with results from experiments that study the 3D structure of our Galaxy via extinction or CO radio observations, and highlight the complementarity of dust X-ray tomography to these approaches.

Investigating Time Variability of X-Ray Absorption in Swift GRBs

The existence of excess absorption in the X-ray spectra of GRBs is well known, but the primary location of the absorbing material is still uncertain. To gain more knowledge about this, we have performed a time-resolved analysis of the X-ray spectra of 199 GRBs observed by the Swift X-ray telescope, searching for evidence of a decreasing column density () that would indicate that the GRBs are ionizing matter in their surroundings. We structured the analysis as Bayesian inference and used an absorbed power law as our baseline model. We also explored alternative spectral models in cases where decreasing absorption was inferred. The analysis reveals seven GRBs that show signs of a decrease in , but we note that alternative models for the spectral evolution cannot be ruled out. We conclude that the excess absorption in the vast majority of GRBs must originate on large scales of the host galaxies and/or in the intergalactic medium. Our results also imply that an evolving column density is unlikely to affect the spectral analysis of the early X-ray spectra of GRBs. In line with this, we show that estimating the total from early Swift data in Window Timing mode reveals the same increasing trend with redshift as previous results based on data taken at later times, but with tighter constraints.

Search for gravitational waves associated with Gamma Ray Bursts during the LIGO-Virgo run O3b

We present the search for gravitational waves associated with Gamma Ray Bursts (GRBs) detected by the Fermi and Swift satellites during the second part of the third observing run (O3b) of Advanced LIGO and Advanced Virgo, from 2019 November 1 to 2020 March 27. This search is carried out with two different methods, a modelled search targeting compact binary mergers with at least one neutron star, which is used for 17 short GRBs, and a search for generic transients, used for all the 86 GRBs. We find no statistically significant gravitational wave signal associated with any of these GRBs. Considering several source types and signal morphologies, we set lower bounds on the estimated distance to each GRB.

A Strong X-Ray Polarization Signal from the Magnetar 1RXS J170849.0-400910

Magnetars are the most strongly magnetized neutron stars, and one of the most promising targets for X-ray polarimetric measurements. We present here the first Imaging X-ray Polarimetry Explorer observation of the magnetar 1RXS J170849.0-400910, jointly analyzed with a new Swift observation and archival NICER data. The total (energy- and phase-integrated) emission in the 2–8 keV energy range is linerarly polarized, at a ∼35% level. The phase-averaged polarization signal shows a marked increase with energy, ranging from ∼20% at 2–3 keV up to ∼80% at 6–8 keV, while the polarization angle remains constant. This indicates that radiation is mostly polarized in a single direction. The spectrum is well reproduced by a combination of either two thermal (blackbody) components or a blackbody and a power law. Both the polarization degree and angle also show a variation with the spin phase, and the former is almost anticorrelated with the source counts in the 2–8 and 2–4 keV bands. We discuss the possible implications and interpretations, based on a joint analysis of the spectral, polarization, and pulsation properties of the source. A scenario in which the surface temperature is not homogeneous, with a hotter cap covered by a gaseous atmosphere and a warmer region in a condensed state, provides a satisfactory description of both the phase- and energy-dependent spectro-polarimetric data. The (comparatively) small size of the two emitting regions, required to explain the observed pulsations, does not allow to reach a robust conclusion about the presence of vacuum birefringence effects.

The outburst of the changing-look AGN IRAS 23226-3843 in 2019

Aims. IRAS 23226-3843 has previously been classified as a changing-look active galactic nucleus (AGN) based on observations taken in the 1990s in comparison to X-ray data (Swift, XMM-Newton, and NuSTAR) and optical spectra taken after a very strong X-ray decline in 2017. In 2019, Swift observations revealed a strong rebrightening in X-ray and UV fluxes. We aimed to study this outburst in greater detail. Methods. We took follow-up Swift, XMM-Newton, and NuSTAR observations of IRAS 23226-3843 together with optical spectra (SALT and SAAO 1.9 m telescope) from 2019 until 2021. Results. IRAS 23226-3843 showed a strong X-ray and optical outburst in 2019. It varied in the X-ray continuum by a factor of 5 and in the optical continuum by a factor of 1.6 within two months. This corresponds to a factor of 3 after correction for the host galaxy contribution. The Balmer and Fe II emission-line intensities showed comparable variability amplitudes during the outburst in 2019. The Hα emission-line profiles of IRAS 23226-3843 changed from a blue-peaked profile in the years 1997 and 1999 to a broad double-peaked profile in 2017 and 2019. However, there were no major profile variations in the extremely broad double-peaked profiles despite the strong intensity variations in 2019. One year after the outburst, IRAS 23226-3843 changed its optical spectral type and became a Seyfert type 2 object in 2020. Blue outflow components are present in the optical Balmer lines and in the Fe band in the X-rays. A deep broadband XMM-Newton/NuSTAR spectrum was taken during IRAS 23226-3843’s maximum state in 2019. This spectrum is qualitatively very similar to a spectrum taken in 2017, but by a factor of 10 higher. The soft X-ray band appears featureless. The soft excess is well modeled with a Comptonization model. A broadband fit with a power-law continuum, Comptonized soft excess, and Galactic absorption gives a good fit to the combined EPIC-pn and NuSTAR spectrum. In addition, we see a complex and broadened Fe K emission-line profile in the X-rays. The changing-look character in IRAS 23226-3843 is most probably caused by changes in the accretion rate – based on the short-term variations on timescales of weeks to months.

Using Neural Networks to Differentiate Newly Discovered BL Lacertae Objects and FSRQs among the 4FGL Unassociated Sources Employing Gamma-Ray, X-Ray, UV/Optical, and IR Data

Among the ∼2157 unassociated sources in the third data release (DR3) of the fourth Fermi catalog, ∼1200 were observed with the Neil Gehrels Swift Observatory pointed instruments. These observations yielded 238 high signal-to-noise ratio X-ray sources within the 95% Fermi uncertainty regions. Recently, Kerby et al. employed neural networks to find blazar candidates among these 238 X-ray counterparts to the 4FGL unassociated sources and found 112 likely blazar counterpart sources. A complete sample of blazars, along with their subclassification, is a necessary step to help understand the puzzle of the blazar sequence and for the overall completeness of the gamma-ray emitting blazar class in the Fermi catalog. We employed a multi-perceptron neural network classifier to identify flat spectrum radio quasars (FSRQs) and BL Lac objects among these 112 blazar candidates using the gamma-ray, X-ray, UV/optical, and IR properties. This classifier provided probability estimates for each source to be associated with one or the other category, such that P fsrq represents the probability for a source to be associated with the FSRQ subclass. Using this approach, four FSRQs and 50 BL Lac objects are classified as such with >99% confidence, while the remaining 58 blazars could not be unambiguously classified as either BL Lac objects or FSRQs.

Neil Gehrels–Swift Observatory’s Ultraviolet/Optical Telescope Observations of Small Bodies in the Solar System

The Neil–Gehrels Swift Observatory has added extensively to our understanding of small bodies in our solar system through its capabilities to rapidly respond to short-live events such as outbursts and collisions, through its near-ultraviolet coverage, and by its ability to track time-dependent changes through monitoring campaigns. These capabilities have enabled many significant studies, including the onset and evolution of different sources of water in comet C/2009 P1 (Garradd), the unprecedented changes in the rotation period of comet 41P/Tuttle–Giacobini–Kresák, near-UV spectroscopic observations of asteroids that can help us understand how their properties evolve over time, and the first observations of the aftermath of a collision between a 100 m sized asteroid and the large primitive asteroid 596 (Scheila). In this review paper, we will highlight some of the observational results of Swift-UVOT in the field of small-body research.

SN 2020qlb: A hydrogen-poor superluminous supernova with well-characterized light curve undulations

Context. SN 2020qlb (ZTF20abobpcb) is a hydrogen-poor superluminous supernova (SLSN-I) that is among the most luminous (maximum Mg = −22.25 mag) and that has one of the longest rise times (77 days from explosion to maximum). We estimate the total radiated energy to be > 2.1 × 1051 erg. SN 2020qlb has a well-sampled light curve that exhibits clear near and post peak undulations, a phenomenon seen in other SLSNe, whose physical origin is still unknown. Aims. We discuss the potential power source of this immense explosion as well as the mechanisms behind its observed light curve undulations. Methods. We analyze photospheric spectra and compare them to other SLSNe-I. We constructed the bolometric light curve using photometry from a large data set of observations from the Zwicky Transient Facility (ZTF), Liverpool Telescope (LT), and Neil Gehrels Swift Observatory and compare it with radioactive, circumstellar interaction and magnetar models. Model residuals and light curve polynomial fit residuals are analyzed to estimate the undulation timescale and amplitude. We also determine host galaxy properties based on imaging and spectroscopy data, including a detection of the [O III]λ4363, auroral line, allowing for a direct metallicity measurement. Results. We rule out the Arnett 56Ni decay model for SN 2020qlb’s light curve due to unphysical parameter results. Our most favored power source is the magnetic dipole spin-down energy deposition of a magnetar. Two to three near peak oscillations, intriguingly similar to those of SN 2015bn, were found in the magnetar model residuals with a timescale of 32 ± 6 days and an amplitude of 6% of peak luminosity. We rule out centrally located undulation sources due to timescale considerations; and we favor the result of ejecta interactions with circumstellar material (CSM) density fluctuations as the source of the undulations.

AT2021acak: a Candidate Tidal Disruption Event Found in the Zwicky Transient Facility Survey

We report a candidate tidal disruption event (TDE) found in the Zwicky Transient Facility survey data. This candidate, with its transient name AT2021acak, showed brightness increases of ∼1 mag around MJD 59500 and subsequent power-law–like brightness declines. We have conducted multiple optical spectroscopic observations with the 2.4 m Lijiang telescope and one observation at X-ray and ultraviolet (UV) bands with the Neil Gehrels Swift Observatory (Swift). The optical spectra of the source show broad H and He emission lines and Fe emission features. Possible 0.3–2 keV X-ray and bright UV emission of the source was detected. We analyze the declines of the optical light curves, the emission features of the optical spectra, and the constructed broad-band UV and optical spectra. The properties derived from the analyses are consistent with those of reported (candidate) TDEs, and in particular very similar to those of ASASSN-18jd. The identification is complicated by the host being likely an AGN, and thus further observations of the event and quiescent host are required in order to have a clear understanding of the nature of this transient event.

The Extragalactic Serendipitous Swift Survey (ExSeSS) – I. Survey definition and measurements of the X-ray number counts

ABSTRACT We present the Extragalactic Serendipitous Swift Survey (ExSeSS), providing a new well-defined sample constructed from the observations performed using the Swift X-ray Telescope. The ExSeSS sample consists of 79 342 sources detected in the medium (1–2 keV), hard (2–10 keV), or total (0.3–10 keV) energy bands, covering 2086.6 deg2 of sky across a flux range of f0.3–10 keV ∼ 10−15 − 10−10 erg s−1 cm−2. Using the new ExSeSS sample we present measurements of the differential number counts of X-ray sources as a function of 2–10 keV flux that trace the population of Active Galactic Nuclei (AGNs) in a previously unexplored regime. We find that taking the line-of-sight absorption column density into account has an effect on the differential number count measurements and is vital to obtain agreement with previous results. In the hard band, we obtain a good agreement between the ExSeSS measurements and previous, higher energy data from NuSTAR and Swift/BAT when taking into account the varying column density of the ExSeSS sample as well as the X-ray spectral parameters of each of the samples we are comparing to. We also find discrepancies between the ExSeSS measurements and AGN population synthesis models, indicating a change in the properties of the AGN population over this flux range that is not fully described by current models at these energies, hinting at a larger, moderately obscured population at low redshifts (z ≲ 0.2) that the models are not currently taking into account.

GRB 160410A: The first chemical study of the interstellar medium of a short GRB

ABSTRACT Short gamma-ray bursts (SGRBs) are produced by the coalescence of compact binary systems which are remnants of massive stars. GRB 160410A is classified as a short-duration GRB with extended emission and is currently the farthest SGRB with a redshift determined from an afterglow spectrum and also one of the brightest SGRBs to date. The fast reaction to the Neil Gehrels Swift Observatory alert allowed us to obtain a spectrum of the afterglow using the X-shooter spectrograph at the Very Large Telescope (VLT). The spectrum shows several absorption features at a redshift of z = 1.7177, in addition, we detect two intervening systems at z = 1.581 and z = 1.444. The spectrum shows Ly α in absorption with a column density of log (N(H i)/cm2) = 21.2 ± 0.2 which, together with Fe ii, C ii, Si ii, Al ii, and O i, allow us to perform the first study of chemical abundances in a SGRB host galaxy. We determine a metallicity of [X/H] = −2.3 ± 0.2 for Fe ii and −2.5 ± 0.2 for Si ii and no dust depletion. We also find no evidence for extinction in the afterglow spectral energy distribution modelling. The environment has a low degree of ionization and the C iv and Si iv lines are completely absent. We do not detect an underlying host galaxy down to deep limits. Additionally, we compare GRB 160410A to GRB 201221D, another high-z short GRB that shows absorption lines at z = 1.045 and an underlying massive host galaxy.

A search for fast radio bursts associated with gamma-ray bursts using the YNAO 40-m radio telescope

We report on the search results of Fast Radio Bursts (FRBs) from three Gamma- ray Bursts (GRBs) at 2256 MHz using the 40-m radio telescope located at the YunNan Astronomical Observatory (YNAO). The search for signals was triggered by the Burst Alert Telescope (BAT) on- board the Swift satellite. Radio single pulses are searched in the data over a large range of dispersion measure from 0 to 5000 pc/cm3 in step of 50 pc/cm3. No FRB-like emission from the prompt phase of GRBs are detected with significance > 5?. If there are FRBs related to the GRBs, we set an upper limit on the ux density of radio pulses of 2.5 Jy for GRB140512A and 8.0 Jy for GRBs 140629A and 140703A with the sensitivity of the telescope. A statistical analysis of the GRB data reveals that the events detected above 5? are consistent with the thermal noise uctuations.

Constraining the X-ray reflection in low accretion-rate active galactic nuclei using XMM-Newton, NuSTAR, and Swift

Context. An interesting feature of active galactic nuclei (AGN) accreting at low rates is the weakness of the reflection features in their X-ray spectra, which may result from the gradual disappearance of the torus with decreasing accretion rates. It has been suggested that low-luminosity AGN (LLAGN) would exhibit a different reflector configuration than high-luminosity AGN, covering a smaller fraction of the sky or simply having less material. Additionally, we note that the determination of the spectral index (Γ) and the cut-off energy of the primary power-law emission is affected by the inclusion of reflection models, showing their importance in studying accretion mechanisms. This is especially valid in the case of the LLAGN which has previously shown a high dispersion in the relation between Γ and the accretion rate. Aims. Our purpose is to constrain the geometry and column density of the reflector in a sample of LLAGN covering a broad X-ray range of energy by combining data from XMM-Newton+ NuSTAR + Swift. The spectral analysis also allows us to investigate the accretion mechanism in LLAGN. Methods. We used XMM-Newton+ NuSTAR + Swift observations of a hard X-ray flux-limited sample of 17 LLAGN from BASS/DR2 with accretion rates of λEdd = LBol/LEdd < 10−3. We fit all spectra using the reflection model for torus (BORUS) and accretion disk (XILLVER) reflectors. Results. We found a tentative correlation between the torus column density and the accretion rate, with LLAGN showing a lower column density than the high-luminosity objects. We also confirm the relation between Γ and λEdd, with a smaller scatter than previously reported, thanks to the inclusion of high-energy data and the reflection models. Our results are consistent with a break at λEdd ∼ 10−3, which is suggestive of a different accretion mechanism compared with higher accretion AGN.

X-ray and UV radiation in the planet-forming T-Tauri system PDS 70. Signs of accretion and coronal activity

ABSTRACT Planet formation takes place in protoplanetary discs around young T-Tauri stars. PDS 70 is one of the first confirmed examples of a system where the planets are currently forming in gaps in the disc, and can be directly imaged. One of the main early influences on planet formation is the lifetime of the protoplanetary disc, which is limited by the intense stellar X-ray and UV radiation. Stellar coronal activity and accretion of material onto the star are both potential sources of XUV radiation. Previous Swift observations detected UV emission, which were consistent with a low rate of accretion. We present follow up observations with the XMM-Newton observatory, which observed PDS 70 simultaneously in X-ray and UV in order to determine intensity of XUV radiation in the system, and identify if the source is coronal, accretion, or both. We detect a strong source in both X-ray and UV, with an average X-ray 0.2–12 keV luminosity of 1.37 × 1030 erg s−1, and a possible flare which increased the luminosity to 2.8 × 1030 erg s−1. The UV flux density is in excess of what would be expected from chromospheric emission, and supports the interpretation that PDS 70 has continuing weak accretion less than ∼10−10 M⊙ yr−1. The implications of the detected X-ray and UV radiation are that the disc is likely to be in the final stages of dispersal, and will be completely evaporated in the next million years, bringing an end to the primary planet formation process.

Swift J1728.9−3613 is a black hole X-ray binary: a spectral and timing study usingNICER

ABSTRACTWe have studied different timing and spectral properties of the new Galactic X-ray transient Swift J1728.9−3613 using NICER and Swift, discovered by the Burst Alert Telescope on the Neil Gehrels Swift Observatory. The source went through multiple transitions to different spectral states during the outburst, and the complete evolution created a ‘q’-shaped track in the hardness-intensity diagram. A partial hysteresis is also observed in the RMS-intensity diagram, which is another well-defined feature of black hole transients. In soft intermediate states, power-density spectra were dominated by broad-band noise components, and two type-B quasi-periodic oscillations were detected. We have fitted 1–10 keV energy spectra obtained from NICER observations that were performed during the outburst, and the temporal evolution of spectral parameters is studied. A small-scale reflare happened near MJD 58584.69, resulting in finite changes in spectral parameters, and the 1–10 keV averaged flux also increased. We observed that the innermost radius of the accretion disc was almost constant during the soft state and we have measured the mass of the compact object to be ∼4.6 M⊙, considering a non-spinning black hole binary system. The soft-to-hard transition occurred when the bolometric luminosity was 0.01 times the Eddington luminosity. Based on our combined study of the evolution of the timing and spectral properties, we conclude that the new source Swift J1728.9−3613 is a black hole X-ray binary.

Ultraviolet and X-ray Light-Curves of Novae Observed by the Neil Gehrels Swift Observatory

With rapid response capabilities, and a daily planning of its observing schedule, the Neil Gehrels Swift Observatory is ideal for monitoring transient and variable sources. Here we present a sample of the 12 novae with the most detailed ultraviolet (UV) follow-up by Swift—the first uniform analysis of such UV light-curves. The fading of these specific light-curves can be modelled as power-law decays (plotting magnitude against log time), showing that the same physical processes dominate the UV emission for extended time intervals in individual objects. After the end of the nuclear burning interval, the X-ray emission drops significantly, fading by a factor of around 10–100. The UV changes, however, are of a lower amplitude, declining by 1–2 mag over the same time period. The UV light-curves typically show a break from flatter to steeper around the time at which the X-ray light-curve starts a steady decline from maximum, ∼0.7–1.3 TSSSend. Considering populations of both classical and recurrent novae, and those with main sequence or giant companions, we do not find any strong differences in the UV light-curves or their evolution, although the long-period recurrent novae are more luminous than the majority of the classical novae.

Flares in the changing look AGN Mrk 590 – I. The UV response to X-ray outbursts suggests a more complex reprocessing geometry than a standard disc

ABSTRACT Mrk 590 is a known changing-look active galactic nuclei (AGNs) which almost turned off in 2012, and then in 2017 partially re-ignited into a repeat flaring state, unusual for an AGN. Our Swift observations since 2013 allow us to characterize the accretion-generated emission and its reprocessing in the central engine of a changing-look AGN. The X-ray and UV variability amplitudes are higher than those typically observed in ‘steady-state’ AGN at similar moderate accretion rates; instead, the variability is similar to that of highly accreting AGN. The unusually strong X-ray to UV correlation suggests that the UV-emitting region is directly illuminated by X-ray outbursts. We find evidence that the X-rays are reprocessed by two UV components, with the dominant one at ∼3 d and a faint additional reprocessor at near-zero lag. However, we exclude a significant contribution from diffuse broad line region continuum, known to contribute for bonafide AGN. A near-zero lag is expected for a standard ‘lamp-post’ disc reprocessing model with a driving continuum source near the black hole. That the overall UV response is dominated by the ∼3-d lagged component suggests a complicated reprocessing geometry, with most of the UV continuum not produced in a compact disc, as also found in recent studies of NGC 5548 and NGC 4151. None the less, the observed flares display characteristic time-scales of ∼100 rest-frame days, consistent with the expected thermal time-scale in an accretion disc.

The Spectral Evolution Patterns and Implications of Gamma-Ray Burst X-Ray Flares

We present a time-resolved spectral analysis of bright X-ray flares observed by Swift X-ray telescope from 2004 December to 2021 December. We selected 58 bright flares from 56 gamma-ray burst X-ray afterglows to perform time-resolved spectral analysis. We find that there is a strong spectral evolution in X-ray flares. Spectral evolution exists in all the X-ray flares. We have not found flares without spectral evolutions in our samples. The spectrum of an X-ray afterglow is dominated by the flare when it exists. Four spectral evolution patterns were found, i.e., hard-to-soft, intensity tracking, soft-to-hard, and anti-intensity tracking. Most of the flares (46 flares, 79.3%) show a spectral evolution from hard to soft. Five (8.6%) flares are intensity tracking. Two (3.5%) flares show soft-to-hard. Five (8.6%) flares show anti-intensity tracking. Therefore, hard-to-soft spectral evolution patterns dominate the spectral evolutions of X-ray flares. In other words, the hard-to-soft spectral evolution pattern is the main evolution pattern of X-ray flares. Hard-to-soft, intensity tracking, and soft-to-hard spectral evolution patterns are the same as prompt emission spectral evolutions, indicating that the origin of flares should be the same as prompt emissions and the flares are the lower energy band emissions of the prompt emissions. The spectral evolution intensities are independent of the peak time and FWHM of the flares. In other words, the spectral evolution intensity is irrelevant to the flare occurrence time and its duration but related to the peak fluxes of flares. This means that the more luminous the flares, the stronger the spectral evolutions.