Prior to the launch of the Swift mission several X-ray line detections were reported in Gamma Ray Burst afterglow spectra. To date, these pre-Swift era results have not been conclusively confirmed. ...The most contentious issue in this area is the choice of statistical method used to evaluate the significance of these features. In this paper we compare three different methods already extant in the literature for assessing the significance of possible line features and discuss their relative advantages and disadvantages. The methods are demonstrated by application to observations of 40 bursts from the archive of Swift XRT at early times (< few ks post trigger in the rest frame of the burst). Based on this thorough analysis we found no strong evidence for emission lines. For each of the three methods we have determined detection limits for emission line strengths in bursts with spectral parameters typical of the Swift-era sample. We also discuss the effects of the current calibration status on emission line detection.
Mon.Not.Roy.Astron.Soc.373:1416-1422,2006 Optical time series photometry of the short period magnetic white dwarf +
probable brown dwarf binary SDSS 121209.31+013627.7 reveals pulse-like
variability ...in all bands from i' to u', peaking at u'. These modulations are
most likely due to a self-eclipsing accretion hot spot on the white dwarf,
rotating into view every 88.43 minutes. This period is commensurate with the
radial velocity period determined by Schmidt et al. 2005 of ~90 minutes, and
consistent with the rotation period of the accretor being equal to the binary
orbital period. We combine our observations with those recently published by
Koen and Maxted 2006 to provide an accurate ephemeris. We also detect the
system in X-rays with Swift, and estimate the accretion rate at ~1x10^-13Msun
per year. We suggest that SDSS1212 is most likely a magnetic cataclysmic
variable in an extended state of very low accretion, similar to the
well-studied Polar EF Eri. Alternatively, the putative brown dwarf is not
filling its Roche Lobe and the system is a detached binary in which the white
dwarf is efficiently accreting from the wind of the secondary. Six such
post-common envelope, ``pre-Polar'' systems - termed ``low accretion rate
Polars (LARPs)'' by Schwope et al. 2002 - have previously been identified
through optical cyclotron emission lines. Cyclotron emission from SDSS1212 has
recently been detected in the near-IR Debes et al. 2006 but, if detached, it
would be the first ``LARP'' with a probably sub-stellar secondary. It is
unclear whether an L-dwarf wind is strong enough to provide the measured
accretion rate. We suggest further observations to distinguish between the
Roche Lobe over-flow and wind accretion scenarios.
We present our analysis of the multiwavelength photometric & spectroscopic
observations of GRB 060210 and discuss the results in the overall context of
current GRB models. All available optical data ...underwent a simultaneous
temporal fit, while X-ray and gamma-ray observations were analysed temporally &
spectrally. The results were compared to each other and to possible GRB models.
The X-ray afterglow is best described by a smoothly broken power-law with a
break at 7.4 hours. The late optical afterglow has a well constrained single
power-law index which has a value between the two X-ray indices, though it does
agree with a single power-law fit to the X-ray. An evolution of the hardness of
the high-energy emission is demonstrated and we imply a minimum host extinction
from a comparison of the extrapolated X-ray flux to that measured in the
optical. We find that the flaring gamma-ray and X-ray emission is likely due to
internal shocks while the flat optical light curve at that time is due to the
external shock. The late afterglow is best explained by a cooling break between
the optical and X-rays and continued central engine activity up to the time of
the break. The required collimation corrected energy of ~ 2x10^52 erg, while at
the high end of the known energy distribution, is not unprecedented.
Optical time series photometry of the short period magnetic white dwarf + probable brown dwarf binary SDSS 121209.31+013627.7 reveals pulse-like variability in all bands from i' to u', peaking at u'. ...These modulations are most likely due to a self-eclipsing accretion hot spot on the white dwarf, rotating into view every 88.43 minutes. This period is commensurate with the radial velocity period determined by Schmidt et al. 2005 of ~90 minutes, and consistent with the rotation period of the accretor being equal to the binary orbital period. We combine our observations with those recently published by Koen and Maxted 2006 to provide an accurate ephemeris. We also detect the system in X-rays with Swift, and estimate the accretion rate at ~1x10^-13Msun per year. We suggest that SDSS1212 is most likely a magnetic cataclysmic variable in an extended state of very low accretion, similar to the well-studied Polar EF Eri. Alternatively, the putative brown dwarf is not filling its Roche Lobe and the system is a detached binary in which the white dwarf is efficiently accreting from the wind of the secondary. Six such post-common envelope, ``pre-Polar'' systems - termed ``low accretion rate Polars (LARPs)'' by Schwope et al. 2002 - have previously been identified through optical cyclotron emission lines. Cyclotron emission from SDSS1212 has recently been detected in the near-IR Debes et al. 2006 but, if detached, it would be the first ``LARP'' with a probably sub-stellar secondary. It is unclear whether an L-dwarf wind is strong enough to provide the measured accretion rate. We suggest further observations to distinguish between the Roche Lobe over-flow and wind accretion scenarios.
Mon.Not.Roy.Astron.Soc.368:1101-1109,2006 We report the discovery and subsequent multi-wavelength afterglow behaviour
of the high redshift (z = 4.27) Gamma Ray Burst GRB 050505. This burst is the
...third most distant burst, measured by spectroscopic redshift, discovered after
GRB 000131 (z = 4.50) and GRB 050904 (z = 6.29). GRB 050505 is a long GRB with
a multipeaked gamma-ray light curve, with a duration of T_90 = 63+/-2 s and an
inferred isotropic release in gamma-rays of ~4.44 x 10^53 ergs in the 1-10^4
keV rest frame energy range. The Swift X-Ray Telescope followed the afterglow
for 14 days, detecting two breaks in the light curve at 7.4(+/-1.5) ks and 58.0
(+9.9/-15.4) ks after the burst trigger. The power law decay slopes before,
between and after these breaks were 0.25 (+0.16/-0.17), 1.17 (+0.08/-0.09) and
1.97 (+0.27/-0.28) respectively. The light curve can also be fit with a
`smoothly broken' power law model with a break observed at ~ T+18.5 ks, with
decay slopes of ~0.4 and ~1.8 before and after the break respectively. The
X-ray afterglow shows no spectral variation over the course of the Swift
observations, being well fit with a single power law of photon index ~1.90.
This behaviour is expected for the cessation of continued energisation of the
ISM shock followed by a break caused by a jet, either uniform or structured.
Neither break is consistent with a cooling break. The spectral energy
distribution indeed shows the cooling frequency to be below the X-ray but above
optical frequencies. The optical -- X-ray spectrum also shows that there is
significant X-ray absorption in excess of that due to our Galaxy but very
little optical/UV extinction, with E(B-V) ~0.10 for a SMC-like extinction
curve.
We present our analysis of the multiwavelength photometric & spectroscopic observations of GRB 060210 and discuss the results in the overall context of current GRB models. All available optical data ...underwent a simultaneous temporal fit, while X-ray and gamma-ray observations were analysed temporally & spectrally. The results were compared to each other and to possible GRB models. The X-ray afterglow is best described by a smoothly broken power-law with a break at 7.4 hours. The late optical afterglow has a well constrained single power-law index which has a value between the two X-ray indices, though it does agree with a single power-law fit to the X-ray. An evolution of the hardness of the high-energy emission is demonstrated and we imply a minimum host extinction from a comparison of the extrapolated X-ray flux to that measured in the optical. We find that the flaring gamma-ray and X-ray emission is likely due to internal shocks while the flat optical light curve at that time is due to the external shock. The late afterglow is best explained by a cooling break between the optical and X-rays and continued central engine activity up to the time of the break. The required collimation corrected energy of ~ 2x10^52 erg, while at the high end of the known energy distribution, is not unprecedented.
We report the discovery and subsequent multi-wavelength afterglow behaviour of the high redshift (z = 4.27) Gamma Ray Burst GRB 050505. This burst is the third most distant burst, measured by ...spectroscopic redshift, discovered after GRB 000131 (z = 4.50) and GRB 050904 (z = 6.29). GRB 050505 is a long GRB with a multipeaked gamma-ray light curve, with a duration of T_90 = 63+/-2 s and an inferred isotropic release in gamma-rays of ~4.44 x 10^53 ergs in the 1-10^4 keV rest frame energy range. The Swift X-Ray Telescope followed the afterglow for 14 days, detecting two breaks in the light curve at 7.4(+/-1.5) ks and 58.0 (+9.9/-15.4) ks after the burst trigger. The power law decay slopes before, between and after these breaks were 0.25 (+0.16/-0.17), 1.17 (+0.08/-0.09) and 1.97 (+0.27/-0.28) respectively. The light curve can also be fit with a `smoothly broken' power law model with a break observed at ~ T+18.5 ks, with decay slopes of ~0.4 and ~1.8 before and after the break respectively. The X-ray afterglow shows no spectral variation over the course of the Swift observations, being well fit with a single power law of photon index ~1.90. This behaviour is expected for the cessation of continued energisation of the ISM shock followed by a break caused by a jet, either uniform or structured. Neither break is consistent with a cooling break. The spectral energy distribution indeed shows the cooling frequency to be below the X-ray but above optical frequencies. The optical -- X-ray spectrum also shows that there is significant X-ray absorption in excess of that due to our Galaxy but very little optical/UV extinction, with E(B-V) ~0.10 for a SMC-like extinction curve.
We investigate the origin of the GRB 060912A, which has observational properties that make its classification as either a long or short burst ambiguous. Short duration GRBs (SGRBs) are thought to ...have typically lower energies than long duration bursts, can be found in galaxies with populations of all ages and are likely to originate from different progenitors to the long duration bursts. However, it has become clear that duration alone is insufficient to make a distinction between the two populations in many cases, leading to a desire to find additional discriminators of burst type. GRB 060912A had a duration of 6 s and occurred only ~10 arcsec from a bright, low redshift (\(z=0.0936\)) elliptical galaxy, suggesting that this may have been the host, which would favour it being a short-burst. However, our deep optical imaging and spectroscopy of the location of GRB 060912A using the VLT shows that GRB 060912A more likely originates in a distant star forming galaxy at z=0.937, and is most likely a long burst. This demonstrates the risk in identifying bright, nearby galaxies as the hosts of given GRBs without further supporting evidence. Further, it implies that, in the absence of secure identifications, "host" type, or more broadly discriminators which rely on galaxy redshifts, may not be good indicators of the true nature of any given GRB.
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