Abstract
The X-ray spectra of gamma-ray bursts (GRBs) can generally be described by an absorbed power law. The landmark discovery of thermal X-ray emission in addition to the power law in the unusual ...GRB 060218, followed by a similar discovery in GRB 100316D, showed that during the first thousand seconds after trigger the soft X-ray spectra can be complex. Both the origin and prevalence of such spectral components still evade understanding, particularly after the discovery of thermal X-ray emission in the classical GRB 090618. Possibly most importantly, these three objects are all associated with optical supernovae (SNe), begging the question of whether the thermal X-ray components could be a result of the GRB-SN connection, possibly in the shock breakout. We therefore performed a search for blackbody components in the early Swift X-ray spectra of 11 GRBs that have or may have associated optical SNe, accurately recovering the thermal components reported in the literature for GRBs 060218, 090618 and 100316D. We present the discovery of a cooling blackbody in GRB 101219B/SN2010ma, and in four further GRB-SNe we find an improvement in the fit with a blackbody which we deem possible blackbody candidates due to case-specific caveats. All the possible new blackbody components we report lie at the high end of the luminosity and radius distribution. GRB 101219B appears to bridge the gap between the low-luminosity and the classical GRB-SNe with thermal emission, and following the blackbody evolution we derive an expansion velocity for this source of the order of 0.4c. We discuss potential origins for the thermal X-ray emission in our sample, including a cocoon model which we find can accommodate the more extreme physical parameters implied by many of our model fits.
GRB 080913 at Redshift 6.7 Greiner, J; Krühler, T; Fynbo, J. P. U ...
Astrophysical journal/The Astrophysical journal,
03/2009, Volume:
693, Issue:
2
Journal Article, Web Resource
Peer reviewed
Open access
We report on the detection by Swift of GRB 080913, and subsequent optical/near-infrared follow-up observations by GROND, which led to the discovery of its optical/NIR afterglow and the recognition of ...its high-z nature via the detection of a spectral break between the i' and z' bands. Spectroscopy obtained at the ESO-VLT revealed a continuum extending down to l = 9400 A, and zero flux for 7500 A<l < 9400 A, which we interpret as the onset of a Gunn-Peterson trough at z = 6.695± 0.025 (95.5% confidence level), making GRB 080913 the highest-redshift gamma-ray burst (GRB) to date, and more distant than the highest-redshift QSO. We note that many redshift indicators that are based on promptly available burst or afterglow properties have failed for GRB 080913. We report on our follow-up campaign and compare the properties of GRB 080913 with bursts at lower redshift. In particular, since the afterglow of this burst is fainter than typical for GRBs, we show that 2 m class telescopes can identify most high-redshift GRBs.
ABSTRACT 4U 0142+61 is one of a small class of persistently bright magnetars. Here, we report on a monitoring campaign of 4U 0142+61 from 2011 July 26 to 2016 June 12 using the Swift X-ray Telescope, ...continuing a 16-year timing campaign with the Rossi X-ray Timing Explorer. We show that 4U 0142+61 had two radiatively loud timing events, on 2011 July 29 and 2015 February 28, both with short soft γ-ray bursts, and a long-lived flux decay associated with each case. We show that the 2015 timing event resulted in a net spin-down of the pulsar that is due to overrecovery of a glitch. We compare this timing event to previous such events in other pulsars with high magnetic fields and discuss net spin-down glitches now seen in several young, high-B pulsars.
The Neil Gehrels Swift Observatory carried out prompt searches for gravitational-wave (GW) events detected by the LIGO/Virgo Collaboration (LVC) during the second observing run ("O2"). Swift ...performed extensive tiling of eight LVC triggers, two of which had very low false-alarm rates (GW170814 and the epochal GW170817), indicating a high confidence of being astrophysical in origin; the latter was the first GW event to have an electromagnetic counterpart detected. In this paper we describe the follow-up performed during O2 and the results of our searches. No GW electromagnetic counterparts were detected; this result is expected, as GW170817 remained the only astrophysical event containing at least one neutron star after LVC's later retraction of some events. A number of X-ray sources were detected, with the majority of identified sources being active galactic nuclei. We discuss the detection rate of transient X-ray sources and their implications in the O2 tiling searches. Finally, we describe the lessons learned during O2 and how these are being used to improve the Swift follow-up of GW events. In particular, we simulate a population of gamma-ray burst afterglows to evaluate our source ranking system's ability to differentiate them from unrelated and uncataloged X-ray sources. We find that 60%-70% of afterglows whose jets are oriented toward Earth will be given high rank (i.e., "interesting" designation) by the completion of our second follow-up phase (assuming that their location in the sky was observed), but that this fraction can be increased to nearly 100% by performing a third follow-up observation of sources exhibiting fading behavior.
ABSTRACT
V3890 Sgr is a recurrent nova that has been seen in outburst three times so far, with the most recent eruption occurring on 2019 August 27 ut. This latest outburst was followed in detail by ...the Neil Gehrels Swift Observatory, from less than a day after the eruption until the nova entered the Sun observing constraint, with a small number of additional observations after the constraint ended. The X-ray light curve shows initial hard shock emission, followed by an early start of the supersoft source phase around day 8.5, with the soft emission ceasing by day 26. Together with the peak blackbody temperature of the supersoft spectrum being ∼100 eV, these timings suggest the white dwarf mass to be high, $\sim 1.3\, {\rm M_{\odot }}$. The UV photometric light curve decays monotonically, with the decay rate changing a number of times, approximately simultaneously with variations in the X-ray emission. The UV grism spectra show both line and continuum emission, with emission lines of N, C, Mg, and O being notable. These UV spectra are best dereddened using a Small Magellanic Cloud extinction law. Optical spectra from SMARTS show evidence of interaction between the nova ejecta and wind from the donor star, as well as the extended atmosphere of the red giant being flash-ionized by the supersoft X-ray photons. Data from NICER reveal a transient 83 s quasi-periodic oscillation, with a modulation amplitude of 5 per cent, adding to the sample of novae that show such short variabilities during their supersoft phase.
Abstract
We report the Fermi LAT
γ
-ray detection of the 2021 outburst of the symbiotic recurrent nova RS Ophiuchi. In this system, unlike classical novae from cataclysmic binaries, the ejecta from ...the white dwarf form shocks when interacting with the dense circumstellar wind environment of the red giant companion. We find the LAT spectra from 50 MeV to ∼20–23 GeV, the highest-energy photons detected in some subintervals, are consistent with
π
0
-decay emission from shocks in the ejecta as proposed by Tatischeff & Hernanz for its previous 2006 outburst. The LAT light curve displayed a fast rise to its peak >0.1 GeV flux of ≃6 × 10
−6
ph cm
−2
s
−1
beginning on day 0.745 after its optically constrained eruption epoch of 2021 August 8.50. The peak lasted for ∼1 day and exhibited a power-law decline up to the final LAT detection on day 45. We analyze the data on shorter timescales at early times and found evidence of an approximate doubling of emission over ∼200 minutes at day 2.2, possibly indicating a localized shock-acceleration event. Comparing the data collected by the American Association of Variable Star Observers, we measured a constant ratio of ∼ 2.8 × 10
−3
between the
γ
-ray and optical luminosities except for a ∼5×smaller ratio within the first day of the eruption likely indicating attenuation of
γ
rays by ejecta material and lower high-energy proton fluxes at the earliest stages of the shock development. The hard X-ray emission due to bremsstrahlung from shock-heated gas traced by the Swift-XRT 2–10 keV light curve peaked at day ∼6, later than at GeV and optical energies. Using X-ray derived temperatures to constrain the velocity profile, we find the hadronic model reproduces the observed >0.1 GeV light curve.
This paper discusses Swift observations of the g-ray burst GRB 050315 (z = 1.949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t super(-5)) ...for 6200 s and several breaks. However, both the prompt hard X-ray/g-ray emission (observed by the BAT) and the first 6300 s of X-ray emission (observed by the XRT) can be explained by exponential decays, with similar decay constants. Extrapolating the BAT light curve into the XRT band suggests that the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt emission; this strong similarity between the prompt g-ray and early X-ray emission may be related to the simple temporal and spectral character of this X-ray-rich GRB. The prompt (BAT) spectrum was steep down to 615 keV and appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline, the X-ray afterglow did not fade for 61.2x 10 super(4) s, after which time it decayed with a temporal index of a- 0.7, followed by a second break at 62.5 x 10 super(5) s to a slope of a 6 2. The apparent "plateau" in the X-ray light curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break, then the jet opening angle was f sub(0) 65, implying E sub(g) 10 super(50) ergs.
Context.
The 30-yr recurrent symbiotic nova V3890 Sgr exploded on 2019 August 28 and was observed with multiple X-ray telescopes.
Swift
and AstroSat monitoring revealed slowly declining hard X-ray ...emission from shocks between the nova ejecta and the stellar wind of the companion. Later, highly variable super-soft-source (SSS) emission was seen. An
XMM-Newton
observation during the SSS phase captured the high degree of X-ray variability in terms of a deep dip in the middle of the observation.
Aims.
This observation adds to the growing sample of diverse SSS spectra and allows spectral comparison of low- and high-state emission to identify the origin of variations and subsequent effects of such dips, all leading to new insights into how the nova ejecta evolve.
Methods.
Based on an initial visual inspection, quantitative modelling approaches were conceptualised to test hypotheses of interpretation. The light curve was analysed with a power spectrum analysis before and after the dip and with an eclipse model to test the hypothesis of occulting clumps as in U Sco. A phenomenological spectral model (SPEX) was used to fit the complex Reflection Grating Spectrometer (RGS) spectrum accounting for all known atomic physics. A blackbody source function was assumed, as in all atmosphere radiation transport models, while the complex radiation transport processes were not modelled. Instead, one or multiple absorbing layers were used to model the absorption lines and edges, taking into account all state-of-the-art knowledge of atomic physics.
Results.
In addition to the central deep dip, there is an initial rise of similar depth and shape, and, after the deep dip, there are smaller dips of ~10% amplitude, which might be periodic over 18.1-min. Our eclipse model of the dips yields clump sizes and orbital radii of 0.5–8 and 5–150 white dwarf radii, respectively. The simultaneous
XMM-Newton
UV light curve shows no significant variations beyond slow fading. The RGS spectrum contains both residual shock emission at short wavelengths and the SSS emission at longer wavelengths. The shock temperature has clearly decreased compared to an earlier
Chandra
observation (day 6). The dip spectrum is dominated by emission lines as in U Sco. The intensity of underlying blackbody-like emission is much lower with the blackbody normalisation yielding a similar radius to that of the brighter phases, while the lower bolometric luminosity is ascribed to lower
T
eff
. This would be inconsistent with clump occultations unless Compton scattering of the continuum emission reduces the photon energies to mimic a lower effective temperature. However, systematic uncertainties are high. The absorption lines in the bright SSS spectrum are blueshifted by 870 ± 10 km s
−1
before the dip and are slightly faster, 900 ± 10 km s
−1
, after the dip. The reproduction of the observed spectrum is astonishing, especially that only a single absorbing layer is necessary while three such layers are needed to reproduce the RGS spectrum of V2491 Cyg. The ejecta of V3890 Sgr are thus more homogeneous than many other SSS spectra indicate. Abundance determination is in principle possible but highly uncertain. Generally, solar abundances are found, except for N and possibly O, which are higher by an order of magnitude.
Conclusions.
High-amplitude variability of SSS emission can be explained in several ways without having to give up the concept of constant bolometric luminosity. Variations in the photospheric radius can expose deeper lying plasma that could pulse with 18.1 min and that would yield a higher outflow velocity. Also, clump occultations are consistent with the observations.
Aims. We present a new measurement of the cosmic X-ray background (CXRB) in the 1.5-7 keV energy band, performed by exploiting the Swift X-ray telescope (XRT) data archive. We also present a ...CXRB spectral model in a wider energy band (1.5-200 keV), obtained by combining these data with the recently published Swift-BAT measurement. Methods. From the XRT archive we collect a complete sample of 126 high Galactic latitude gamma-ray burst (GRB) follow-up observations. This provides a total exposure of 7.5 Ms and a sky-coverage of ~7 square degrees which represents a serendipitous survey, well suited for a direct measurement of the CXRB in the 1.5-10 keV interval. Our work is based on a complete characterization of the instrumental background and an accurate measurement of the stray-light contamination and vignetting calibration. Results. We find that the CXRB spectrum in the 1.5-7 keV energy band can be equally well fitted by a single power-law with photon index $\Gamma=1.47\pm0.07$ or a single power-law with photon index $\Gamma=1.41\pm0.06$ and an exponential roll-off at 41 keV. The measured flux in the 2-10 keV energy band is $2.18 \pm0.13 \times10^{-11}$ erg cm-2 s-1 deg-2 in the 2-10 keV band. Combining Swift-XRT with Swift-BAT (15-200 keV) we find that, in the 1.5-200 keV band, the CXRB spectrum can be well described by two smoothly-joined power laws with the energy break at $29.0\pm0.5$ keV corresponding to a $\nu F_{\nu}$ peak located at $22.4\pm0.4$ keV. Conclusions. Taking advantage of both the Swift high energy instruments (XRT and BAT), we produce an analytical description of the CXRB spectrum over a wide (1.5-200 keV) energy band. This model is marginally consistent with the HEAO1 measurement (~10% higher) at energies higher than 20 keV, while it is significantly (30%) higher at low energies (2-10 keV).
ABSTRACT PSR J1846-0258 is an object that straddles the boundary between magnetars and rotation powered pulsars. Though behaving for many years as a rotation-powered pulsar, in 2006, it exhibited ...distinctly magnetar-like behavior-emitting several short hard X-ray bursts, and a flux increase. Here we report on 7 years of post-outburst timing observations of PSR J1846-0258 using the Rossi X-ray Timing Explorer and the Swift X-ray Telescope. We measure the braking index over the post-magnetar outburst period to be n = 2.19 0.03. This represents a change of Δn = −0.46 0.03 or a 14.5 difference from the pre-outburst braking index of n = 2.65 0.01, which itself was measured over a span of 6.5 years. A change to a pulsar braking index so large and long-lived is unprecedented and poses a significant challenge to models of pulsar spin-down.
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