A Review of Early-Time Optical Follow-ups with 2-m Robotic Telescopes

We present the first optical observations of a Gamma Ray Burst (GRB) afterglow using the 2-m robotic Liverpool Telescope (LT), which is owned and operated by Liverpool John Moores University and situated on La Palma. We briefly discuss the capabilities of LT and its suitability for rapid follow-up observations of early optical and infrared GRB light curves. In particular, the combination of aperture, site, instrumentation and rapid response (robotic over-ride mode aided by telescope's rapid slew and fully-opening enclosure) makes the LT ideal for investigating the nature of short bursts, optically-dark bursts, and GRB blast-wave physics in general. We briefly describe the LT's key position in the RoboNet-1.0 network of robotic telescopes. We present the LT observations of GRB041006 and use its gamma-ray properties to predict the time of the break in optical light curve, a prediction consistent with the observations.

X-ray and optical orbital modulation of EXO 0748-676: A co-variability study using XMM-Newton

Investigation of the progenitors and outbursts of classical and recurrent novae

The Galactic neutron star X-ray binary Her X-1 displays a well-known 35-day superorbital modulation in its X-ray and optical light curves. Detected across a broad energy range, the modulation is prevalent in X-rays, cycling between low and high states. The 35-day modulation is believed to be the result of the periodic occultation of the neutron star by a warped precessing accretion disc. Using optical observations of Her X-1 during both the anomalous low state (ALS) and the normal high state, it is shown that the orbital light curve of Her X-1 varies systematically over the 35-day precession cycle. The 35-day precessional profile is remarkably consistent between the ALS and normal high state of Her X-1, suggesting only a very slight change in the form of the disc warp between the two states. Comparison of optical and X-ray light curves suggests that a significant component of the X-ray flux during the ALS originates from the companion star.

We investigate the multi-band properties of the GRB 161017A with [Formula: see text], which was detected by Swift and Fermi satellites, and other instruments. The optical and X-ray afterglows were all detected at early times after the prompt emission. The optical light curve shows a very bright onset peak at about 100 s for 13 mag of [Formula: see text]-band, while the X-ray light curve occurs several flares at the beginning. We investigate the origin of X-ray and optical afterglows by analyzing the optical and X-ray data. Considering the smooth onset bump in the early time of the optical band and the erratic pulses for the X-ray lightcurve, we suppose that the early optical afterglow may be produced by the external shock, while the early time of X-ray light curve is dominated by flares. Therefore, GRB optical afterglows with smooth onset bump features at early time are possibly produced by external — forward shock (FS). According to the fireball external-model, the temporal slopes of the onset bumps are determined by both the medium density profile and the electron spectral index. Therefore, the afterglow onset bumps would be an ideal probe to study the properties of the fireball and the circumburst medium. The density profile has a slope of [Formula: see text], which suggests that the circumburst environment of the GRB 161017A would be an intermediate regime that are between the homogeneous interstellar medium (ISM) and wind-type medium. In addition, the optical light curve of the GRB 161017A exhibits a plateau feature and rebrightening after the onset bump, which may be related to the long-acting central engine of GRBs.

The Swift mission has discovered an intriguing feature of gamma-ray burst (GRBs) afterglows, a phase of shallow decline of the flux in the X-ray and optical light curves. This behaviour is typically attributed to energy injection into the burst ejecta. At some point this phase ends, resulting in a break in the light curve, which is commonly interpreted as the cessation of the energy injection. In a few cases, however, while breaks in the X-ray light curve are observed, optical emission continues its slow flux decline. This behaviour suggests a more complex scenario. In this paper, we present a model that invokes a double component outflow, in which narrowly collimated ejecta are responsible for the X-ray emission while a broad outflow is responsible for the optical emission. The narrow component can produce a jet break in the X-ray light curve at relatively early times, while the optical emission does not break due to its lower degree of collimation. In our model both components are subject to energy injection for the whole duration of the follow-up observations. We apply this model to GRBs with chromatic breaks, and we show how it might change the interpretation of the GRBs canonical light curve. We also study our model from a theoretical point of view, investigating the possible configurations of frequencies and the values of GRB physical parameters allowed in our model.

We present hard X-ray and optical observations of the eclipsing AM Her system V2301 Oph. The X-ray data were obtained using the PCA detector of the Rossi X-Ray Timing Explorer satellite during 1997 May, and the optical data were obtained using the 1 and 1.5 m telescopes of the Cerro Tololo Inter-American Observatory during 1996 May and 1997 June. V2301 Oph was bright in both the optical and hard X-rays during our observations. This, when coupled with its eclipsing nature, makes V2301 Oph an ideal testbed for theories of the large-scale topology of AM Her flows and the radiative shocks in AM Her systems. The X-ray emission from V2301 Oph was modulated strongly on the orbital period. During the bright orbital phases, the X-ray flux was Fx≈3.6 × 10−11 ergs cm-2 s-1 over the energy range E=2-10 keV. The X-ray emission did not go to zero during the faint orbital phases; it was ~10% of the bright phase level. The X-ray spectrum could be fitted by (1) optically thin thermal bremsstrahlung (temperature kTx≈9-19 keV) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV, and (2) power-law continuum (index ≈2) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV. The absorption columns were large for all fits, nH~(3-10) × 1022 cm-2. The nH are model dependent, but their large sizes are secure because they are set by the rollover in the X-ray spectrum at 3-4 keV. The hardness of the X-ray spectrum was roughly constant during the bright orbital phases. During the faint orbital phases, the X-ray spectral properties were not well determined, but it did appear that the spectrum hardened. There were total eclipses in both the X-ray and optical light curves. The X-ray light curves and eclipses were consistent with a dominant hot spot and a secondary hot spot. The dominant hot spot was not a point source; it had to cover about 50° in longitude on the surface of the white dwarf. We argue that the X-ray light curve and eclipse shape also suggest that the accretion occurs in a sheetlike geometry rather than in a columnar geometry. The optical light curves and eclipses were consistent with emission from the white dwarf photosphere, an extended emission region that sat above the surface of the white dwarf, and the X-ray-heated face of the companion star.

We present a comprehensive analysis of the optical and X-ray light curves\n(LCs) and spectral energy distributions (SEDs) of a large sample of gamma-ray\nburst (GRB) afterglows to investigate the relationship between the optical and\nX-ray emission after the prompt phase. We collected the optical data from the\nliterature and determined the shapes of the optical LCs. Then, using previously\npresented X-ray data we modeled the optical/X-ray SEDs. We studied the SED\nparameter distributions and compared the optical and X-ray LC slopes and\nshapes. The optical and X-ray spectra become softer as a function of time while\nthe gas-to-dust ratios of GRBs are higher than the values calculated for the\nMilky Way and the Large and Magellanic Clouds. For 20% of the GRBs the\ndifference between the optical and X-ray slopes is consistent with 0 or 1=4\nwithin the uncertainties (we did it not consider the steep decay phase), while\nin the remaining 80% the optical and X-ray afterglows show significantly\ndifferent temporal behaviors. Interestingly, we find an indication that the\nonset of the forward shock in the optical LCs (initial peaks or shallow phases)\ncould be linked to the presence of the X-ray flares. Indeed, when X-ray flares\nare present during the steep decay, the optical LC initial peak or end plateau\noccurs during the steep decay; if instead the X-ray flares are absent or occur\nduring the plateau, the optical initial peak or end plateau takes place during\nthe X-ray plateau. The forward-shock model cannot explain all features of the\noptical (e.g. bumps, late re-brightenings) and X-ray (e.g. flares, plateaus)\nLCs. However, the synchrotron model is a viable mechanism for GRBs at late\ntimes. In particular, we found a relationship between the presence of the X-ray\nflares and the shape of the optical LC that indicates a link between the prompt\nemission and the optical afterglow.\n

The flat decay phase in the first 1e2-1e4 seconds of the X-ray light curve of Gamma Ray Bursts (GRBs) has not yet found a convincing explanation. The fact that the optical and X-ray lightcurves are often different, with breaks at different times, makes contrived any explanation based on the same origin for both the X-ray and optical fluxes. We here assume that the central engine can be active for a long time, producing shells of decreasing bulk Lorentz factors Gamma. We also assume that the internal dissipation of these late shells produces a continuous and smooth emission (power-law in time), usually dominant in X-rays and sometimes in the optical. When Gamma of the late shells is larger than 1/theta_j, where theta_j is the jet opening angle, we see only a portion of the emitting surface. Eventually, Gamma becomes smaller than 1/theta_j, and the entire emitting surface is visible. Thus there is a break in the light curve when Gamma=1/theta_j, which we associate to the time at which the plateau ends. After the steeply decaying phase which follows the early prompt, we see the sum of two emission components: the "late-prompt" emission (due to late internal dissipation), and the "real afterglow" emission (due to external shocks). A variety of different optical and X-ray light curves are then possible, explaining why the X-ray and the optical light curves often do not track each other (but sometimes do), and often they do not have simultaneous breaks.

We present a multiwavelength analysis of 63 Gamma-Ray Bursts observed with the world's three largest robotic optical telescopes, the Liverpool and Faulkes Telescopes (North and South). Optical emission was detected for 24 GRBs with brightnesses ranging from R = 10 to 22 mag in the first 10 minutes after the burst. By comparing optical and X-ray light curves from t = 100 to about 10^6 seconds, we introduce four main classes, defined by the presence or absence of temporal breaks at optical and/or X-ray wavelengths. While 15/24 GRBs can be modelled with the forward-shock model, explanation of the remaining nine is very challenging in the standard framework even with the introduction of energy injection or an ambient density gradient. Early X-ray afterglows, even segments of light curves described by a power-law, may be due to additional emission from the central engine. 39 GRBs in our sample were not detected and have deep upper limits (R < 22 mag) at early time. Of these, only ten were identified by other facilities, primarily at near infrared wavelengths, resulting in a dark burst fraction of about 50%. Additional emission in the early time X-ray afterglow due to late-time central engine activity may also explain some dark bursts by making the bursts brighter than expected in the X-ray band compared to the optical band.

In the era of rapid and accurate localisation of Gamma Ray Bursts by the Swift satellite, high quality early time multi‐wavelength light curves, obtained by space and ground‐based robotic telescopes, have shown that the standard ‘smooth temporal power law decays’ typical of late‐time afterglow emission can be substantially modified at early times by e.g. energy injection from long‐lived central engines, and/or interactions between the ejecta and clumps in the surrounding circumburst medium. Well‐sampled optical light curves (covering a wide range in time, brightness and redshift) together with early‐time polarimetry provide a powerful probe of the physics of GRBs, their ejecta and their environments. Here we summarise the GRB followup programme being conducted on a network of the world’s three largest robotic telescopes that aims to obtain early‐time multicolour photometric and polarimetric measurements crucial for the understanding of GRB physics.

We present a detailed study of the prompt and afterglow emission from Swift GRB 061126 using BAT, XRT, UVOT data and multicolor optical imaging from 10 ground-based telescopes. GRB 061126 was a long burst (T90 = 191 s) with four overlapping peaks in its γ-ray light curve. The X-ray afterglow, observed from 26 minutes to 20 days after the burst, shows a simple power-law decay with αX = 1.290 ± 0.008. Optical observations presented here cover the time range from 258 s (Faulkes Telescope North) to 15 days (Gemini North) after the burst; the decay rate of the optical afterglow shows a steep-to-shallow transition (from α1 = 1.48 ± 0.06 to α2 = 0.88 ± 0.03) approximately 13 minutes after the burst. We suggest the early, steep component is due to a reverse shock and show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartition value, is a few 10 times larger than in the forward shock in the early afterglow phase. The ejecta might be endowed with primordial magnetic fields at the central engine. The optical light curve implies a late-time break at about 1.5 days after the burst, while there is no evidence of the simultaneous break in the X-ray light curve. We model the broadband emission and show that some afterglow characteristics (the steeper decay in X-ray and the shallow spectral index from optical to X-ray) are difficult to explain in the framework of the standard fireball model. This might imply that the X-ray afterglow is due to an additional emission process, such as late-time central engine activity rather than blast-wave shock emission. The possible chromatic break at 1.5 days after the burst would give support to the additional emission scenario.

RS Cae is the third target in our series of XMM-Newton observations of soft X-ray-dominated polars. Our observational campaign aims to better understand and describe the multiwavelength data, the physical properties of the system components, and the short- and long-term behavior of the component fluxes in RS Cae. We employ stellar atmosphere, stratified accretion-column, and widely used X-ray spectral models. We fit the XMM-Newton spectra, model the multiband light curves, and opt for a mostly consistent description of the spectral energy distribution. Results. Our XMM-Newton data of RS Cae are clearly dominated by soft X-ray emission. The X-ray light curves are shaped by emission from the main accretion region, which is visible over the whole orbital cycle, interrupted only by a stream eclipse. The optical light curves are formed by cyclotron and stream emission. The XMM-Newton X-ray spectra comprise a black-body-like and a plasma component at mean temperatures of 36eV and 7keV. The spectral fits give evidence of a partially absorbing and a reflection component. Multitemperature models, covering a broader temperature range in the X-ray emitting accretion regions, reproduce the spectra appropriately well. Including archival data, we describe the spectral energy distribution with a combination of models based on a consistent set of parameters and derive a lower limit estimate of the distance d > 750pc. Conclusions. The high bolometric soft-to-hard flux ratios and short-term variability of the (X-ray) light curves are characteristic of inhomogeneous accretion. RS Cae clearly belongs in the group of polars that show a very strong soft X-ray flux compared to their hard X-ray flux. The different black-body fluxes and similar hard X-ray and optical fluxes during the XMM-Newton and ROSAT observations show that soft and hard X-ray emission are not directly correlated.

We present the study on GRB 071112C X-ray and optical light curves. In these\ntwo wavelength ranges, we have found different temporal properties. The R-band\nlight curve showed an initial rise followed by a single power-law decay, while\nthe X-ray light curve was described by a single power-law decay plus a\nflare-like feature. Our analysis shows that the observed temporal evolution\ncannot be described by the external shock model in which the X-ray and optical\nemission are produced by the same emission mechanism. No significant color\nchanges in multi-band light curves and a reasonable value of the initial\nLorentz factor ({\\Gamma}0 = 275 \\pm 20) in a uniform ISM support the afterglow\nonset scenario as the correct interpretation for the early R-band rise. The\nresult suggests the optical flux is dominated by afterglow. Our further\ninvestigations show that the X-ray flux could be created by an additional\nfeature related to energy injection and X-ray afterglow. Different theoretical\ninterpretations indicate the additional feature in X-ray can be explained by\neither late internal dissipation or local inverse-Compton scattering in the\nexternal shock.\n

Context. M-dwarf stars are the most numerous stars in the Galaxy, and are highly magnetically active. They exhibit bursts of radiation and matter, called flares and coronal mass ejections which have the potential to strongly affect the habitability of their planets. Aims. We investigate variability through simultaneous optical and X-ray observations, forming the largest statistical sample of M dwarfs observed in this way so far. Such simultaneous observations at different wavelengths, which correspond to emissions from different layers of the stellar atmosphere, are required to constrain the flare frequency and energetics and to understand the physics of flares. Methods. We used light curves from the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission (SRG) and the Transiting Exoplanet Survey Satellite (TESS) for a sample of M dwarfs observed simultaneously with both instruments. Specifically, we identified 256 M dwarfs in the TESS Southern Continuous Viewing Zone (SCVZ), which corresponds to a sky area of 452.39 (deg2), with simultaneous TESS and eROSITA detection. For this work, we selected the 25 X-ray brightest or most X-ray variable stars. We used photometric data from Gaìa and 2MASS to obtain stellar parameters such as distances, colours, masses, radii, and bolometric luminosities. X-ray fluxes and luminosities were determined from observed eROSITA count rates using appropriate rate-to-flux conversion factors. We defined and examined various variability diagnostics in both wavebands and how these parameters are related to each other. Results. Our stars are nearby (mostly within ~100 pc), rotating fast (Prot &lt; 9 d), and display a high optical flare frequency, as expected from the selection of particularly X-ray-active objects. The optical duty cycle – defined as the fraction of observing time in which the stars were in a high activity state – is well correlated with the optical flare rate and was therefore used as proxy for optical variability. The X-ray and optical duty cycles are positively correlated, and there is a trend of faster rotators tending to have higher X-ray and optical variability. For stars with many X-ray flaring events, the chances of these events being found together with optical flares are high. A quantitative variability study of individual flares in the X-ray light curves is severely affected by data gaps due to the low (4h) cadence during the eROSITA all-sky survey. To mitigate this, we made use of the optical flares observed with TESS combined with knowledge accumulated from solar flares to put additional constraints on the peak flux and timing of X-ray events. With this method we could perform an exponential fit to 17 X-ray light curves in the aftermath of an optical flare, and we find that the energies for these X-ray flares are well correlated with the corresponding optical flare energy. We also found two peculiar flaring events with uncharacteristically long duration and high energies observed in both their X-ray and optical light curves. Conclusions. Despite the substantial uncertainties associated with our analysis, which are mostly related to the poor sampling of the eROSITA light curves, our results showcase in an exemplary way the relevance of simultaneous all-sky surveys in different wavebands for obtaining unprecedented quantitative information on stellar variability.