Evolution of the afterglow optical spectral shape of GRB 201015A in the first hour: evidence for dust destruction

Instruments such as the ROTSE, TORTORA, Pi of the Sky, MASTER-net, and others have recorded single-band optical flux measurements starting as early as $\thicksim$ 10 seconds after a gamma-ray burst trigger. The earliest measurements of optical spectral shape have been made only much later, typically on hour time scales, never starting less than a minute after trigger, until now. Beginning only 58 seconds after the Swift BAT triggerred on GRB201015A, we observed a sharp rise in flux to a peak, followed by an approximate power law decay light curve, $\propto t^{-0.81 \pm 0.03}$. Flux was measured simultaneously in three optical filter bands, g', r', and i', using our unique instrument mounted on the Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO). Our simultaneous multi-band observations of the early afterglow show strong colour evolution from red to blue, with a change in the optical log slope (after correction for Milky Way extinction) of $+0.72 \pm 0.14$; during this time the X-ray log slope remained constant. We did not find evidence for a two-component jet structure or a transition from reverse to forward shock that would explain this change in slope. We find that the majority of the optical spectral slope evolution is consistent with a monotonic decay of extinction, evidence of dust destruction. If we assume that the optical log slope is constant throughout this period, with the value given by the late-time slope, and we further assume an SMC-like extinction curve, we derive a change in the local extinction $A_\mathrm{v}^\mathrm{local}$ from $\thicksim$0.8 mag to 0.3 mag in $\thicksim$2500 s. This work shows that significant information about the early emission phase (and possibly prompt emission, if observed early enough) is being missed without such early observations with simultaneous multi-band instruments.Comment: 7 pages, 3 figures. Submitted to MNRA

Optical observations of NEA 3200 Phaethon (1983 TB) during the 2017 apparition

The near-Earth asteroid 3200 Phaethon (1983 TB) is an attractive object not only from a scientific viewpoint but also because of JAXA's DESTINY+ target. The rotational lightcurve and spin properties were investigated based on the data obtained in the ground-based observation campaign of Phaethon. We aim to refine the lightcurves and shape model of Phaethon using all available lightcurve datasets obtained via optical observation, as well as our time-series observation data from the 2017 apparition. Using eight 1-2-m telescopes and an optical imager, we acquired the optical lightcurves and derived the spin parameters of Phaethon. We applied the lightcurve inversion method and SAGE algorithm to deduce the convex and non-convex shape model and pole orientations. We analysed the optical lightcurve of Phaethon and derived a synodic and a sidereal rotational period of 3.6039 h, with an axis ratio of a/b = 1.07. The ecliptic longitude (lambda) and latitude (beta) of the pole orientation were determined as (308, -52) and (322, -40) via two independent methods. A non-convex model from the SAGE method, which exhibits a concavity feature, is also presented.Comment: 14 pages, 4 figures, 1 figure in Appendix A. Accepted for publication in Astronomy & Astrophysics (A&A

Evolution of the afterglow optical spectral shape of GRB 201015A in the first hour: evidence for dust destruction

Instruments such as the ROTSE, TORTORA, Pi of the Sky, MASTER-net, and others have recorded single-band optical flux measurements starting as early as $\thicksim$ 10 seconds after a gamma-ray burst trigger. The earliest measurements of optical spectral shape have been made only much later, typically on hour time scales, never starting less than a minute after trigger, until now. Beginning only 58 seconds after the Swift BAT triggerred on GRB201015A, we observed a sharp rise in flux to a peak, followed by an approximate power law decay light curve, $\propto t^{-0.81 \pm 0.03}$. Flux was measured simultaneously in three optical filter bands, g', r', and i', using our unique instrument mounted on the Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO). Our simultaneous multi-band observations of the early afterglow show strong colour evolution from red to blue, with a change in the optical log slope (after correction for Milky Way extinction) of $+0.72 \pm 0.14$; during this time the X-ray log slope remained constant. We did not find evidence for a two-component jet structure or a transition from reverse to forward shock that would explain this change in slope. We find that the majority of the optical spectral slope evolution is consistent with a monotonic decay of extinction, evidence of dust destruction. If we assume that the optical log slope is constant throughout this period, with the value given by the late-time slope, and we further assume an SMC-like extinction curve, we derive a change in the local extinction $A_\mathrm{v}^\mathrm{local}$ from $\thicksim$0.8 mag to 0.3 mag in $\thicksim$2500 s. This work shows that significant information about the early emission phase (and possibly prompt emission, if observed early enough) is being missed without such early observations with simultaneous multi-band instruments

Evolution of the afterglow optical spectral shape of GRB 201015A in the first hour: evidence for dust destruction

Instruments such as the ROTSE, TORTORA, Pi of the Sky, MASTER-net, and others have recorded single-band optical flux measurements starting as early as $\thicksim$ 10 seconds after a gamma-ray burst trigger. The earliest measurements of optical spectral shape have been made only much later, typically on hour time scales, never starting less than a minute after trigger, until now. Beginning only 58 seconds after the Swift BAT triggerred on GRB201015A, we observed a sharp rise in flux to a peak, followed by an approximate power law decay light curve, $\propto t^{-0.81 \pm 0.03}$. Flux was measured simultaneously in three optical filter bands, g', r', and i', using our unique instrument mounted on the Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO). Our simultaneous multi-band observations of the early afterglow show strong colour evolution from red to blue, with a change in the optical log slope (after correction for Milky Way extinction) of $+0.72 \pm 0.14$; during this time the X-ray log slope remained constant. We did not find evidence for a two-component jet structure or a transition from reverse to forward shock that would explain this change in slope. We find that the majority of the optical spectral slope evolution is consistent with a monotonic decay of extinction, evidence of dust destruction. If we assume that the optical log slope is constant throughout this period, with the value given by the late-time slope, and we further assume an SMC-like extinction curve, we derive a change in the local extinction $A_\mathrm{v}^\mathrm{local}$ from $\thicksim$0.8 mag to 0.3 mag in $\thicksim$2500 s. This work shows that significant information about the early emission phase (and possibly prompt emission, if observed early enough) is being missed without such early observations with simultaneous multi-band instruments

Evolution of the afterglow optical spectral shape of GRB 201015A in the first hour: evidence for dust destruction

Instruments such as the ROTSE, TORTORA, Pi of the Sky, MASTER-net, and others have recorded single-band optical flux measurements starting as early as $\thicksim$ 10 seconds after a gamma-ray burst trigger. The earliest measurements of optical spectral shape have been made only much later, typically on hour time scales, never starting less than a minute after trigger, until now. Beginning only 58 seconds after the Swift BAT triggerred on GRB201015A, we observed a sharp rise in flux to a peak, followed by an approximate power law decay light curve, $\propto t^{-0.81 \pm 0.03}$. Flux was measured simultaneously in three optical filter bands, g', r', and i', using our unique instrument mounted on the Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO). Our simultaneous multi-band observations of the early afterglow show strong colour evolution from red to blue, with a change in the optical log slope (after correction for Milky Way extinction) of $+0.72 \pm 0.14$; during this time the X-ray log slope remained constant. We did not find evidence for a two-component jet structure or a transition from reverse to forward shock that would explain this change in slope. We find that the majority of the optical spectral slope evolution is consistent with a monotonic decay of extinction, evidence of dust destruction. If we assume that the optical log slope is constant throughout this period, with the value given by the late-time slope, and we further assume an SMC-like extinction curve, we derive a change in the local extinction $A_\mathrm{v}^\mathrm{local}$ from $\thicksim$0.8 mag to 0.3 mag in $\thicksim$2500 s. This work shows that significant information about the early emission phase (and possibly prompt emission, if observed early enough) is being missed without such early observations with simultaneous multi-band instruments

The changing rotation period of 3200 Phaethon

International audience3200 Phaethon is one of the largest potentially hazardous asteroids. It is also the first asteroid to be discovered in spacecraft images, the parent body of the Geminid meteor stream, and the target of JAXA's upcoming DESTINY+ mission. We have developed a physical model of Phaethon's shape and rotation state, using radar data from Arecibo and Goldstone in 2007 and 2017, stellar occultations from 2019 through 2021, and lightcurves from many apparitions (from 1989 through 2021). Phaethon has a volume-equivalent diameter of about 5.3 kilometers. It is approximately spheroidal with an equatorial ridge, resembling the shapes of 101955 Bennu and 162173 Ryugu. The earliest available lightcurve of Phaethon is from 1989 October 9 (published in Wisniewski et al. 1997). Hanuš et al. (2016) noticed that their shape model's rotation phase differed from that 1989 observation by about 20 minutes. They noted that this may be from Phaethon's rotation period changing due to activity, but they could not say whether this was real or just a timing error in that one lightcurve, since the next lightcurves are from 1994. A model with a constant rotation period provided a good fit to all of the data from 1994 through 2020. However, this model could not adequately fit the lightcurves that were acquired from four different observatories in 2021. There was a difference of about 15 minutes between the predicted and observed rotation phases — clearly greater than what the observations' uncertainties could permit. We initially considered that Phaethon's rotation period may have changed before the 2021 observations, perhaps due to activity when it was near perihelion in December 2020. However, we found that a constant rotational acceleration provides a good fit to all of the data from 1989 through 2021, clearly accounting for the discrepancies seen in 1989 and 2021 while also slightly improving the model's agreement with the data during other apparitions. We find that Phaethon's sidereal rotation period in December 2017 was 3.603944 hours. Its spin rate is increasing at a rate of 2.1×10-6 deg/day2, which corresponds to its rotation period decreasing by about 4 milliseconds per year

The changing rotation period of 3200 Phaethon

International audience3200 Phaethon is one of the largest potentially hazardous asteroids. It is also the first asteroid to be discovered in spacecraft images, the parent body of the Geminid meteor stream, and the target of JAXA's upcoming DESTINY+ mission. We have developed a physical model of Phaethon's shape and rotation state, using radar data from Arecibo and Goldstone in 2007 and 2017, stellar occultations from 2019 through 2021, and lightcurves from many apparitions (from 1989 through 2021). Phaethon has a volume-equivalent diameter of about 5.3 kilometers. It is approximately spheroidal with an equatorial ridge, resembling the shapes of 101955 Bennu and 162173 Ryugu. The earliest available lightcurve of Phaethon is from 1989 October 9 (published in Wisniewski et al. 1997). Hanuš et al. (2016) noticed that their shape model's rotation phase differed from that 1989 observation by about 20 minutes. They noted that this may be from Phaethon's rotation period changing due to activity, but they could not say whether this was real or just a timing error in that one lightcurve, since the next lightcurves are from 1994. A model with a constant rotation period provided a good fit to all of the data from 1994 through 2020. However, this model could not adequately fit the lightcurves that were acquired from four different observatories in 2021. There was a difference of about 15 minutes between the predicted and observed rotation phases — clearly greater than what the observations' uncertainties could permit. We initially considered that Phaethon's rotation period may have changed before the 2021 observations, perhaps due to activity when it was near perihelion in December 2020. However, we found that a constant rotational acceleration provides a good fit to all of the data from 1989 through 2021, clearly accounting for the discrepancies seen in 1989 and 2021 while also slightly improving the model's agreement with the data during other apparitions. We find that Phaethon's sidereal rotation period in December 2017 was 3.603944 hours. Its spin rate is increasing at a rate of 2.1×10-6 deg/day2, which corresponds to its rotation period decreasing by about 4 milliseconds per year