38 research outputs found
VISUAL IMAGES AS HOOKS FOR PROBLEM-BASED LEARNING
ABSTRACT In problem-based learning students are presented with a problem as the motivation for group research. The presentation of the problem attempts to address the issue of ownership of content by both providing a context for knowledge acquisition and requiring a transformation of material to address the problem. The transformational aspect in principle provides the opportunity for creativity. We present two case studies, with different outcomes, and the results of a workshop, in which visual imagery was used as the hook for a PBL problem with a view to encouraging creativity in the responses
GRB 050717: A Long, Short-Lag Burst Observed by Swift and Konus
The long burst GRB 050717 was observed simultaneously by the Burst Alert Telescope (BAT) on Swift and the Konus instrument on Wind. Significant hard to soft spectral evolution was seen. Early gamma-ray and X-ray emission was detected by both BAT and the X-Ray Telescope (XRT) on Swift. The XRT continued to observe the burst for 7.1 days and detect it for 1.4 days. The X-ray light curve showed a classic decay pattern including evidence of the onset of the external shock emission at approx. 50 s after the trigger; the afterglow was too faint for a jet break to be detected. No optical, infrared or ultraviolet counterpart was discovered despite deep searches within 14 hours of the burst. The spectral lag for GRB 050717 was determined to be 2.5 +/- 2.6 ms, consistent, with zero and unusually short for a long burst. This lag measurement suggests that this burst has a high intrinsic luminosity and hence is at high redshift (z > 2.7). 050717 provides a good example of classic prompt and afterglow behavior for a gamma-ray burst
GRB 050505: A high redshift burst discovered by Swift
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.Comment: 9 pages, 6 figures. Accepted by MNRA
The prompt to late-time multiwavelength analysis of GRB 060210
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.Comment: 8 pages, 4 figures, accepted to Astronomy & Astrophysics after minor
change
The early X-ray afterglows of optically bright and dark Gamma-Ray Bursts
A systematical study on the early X-ray afterglows of both optically bright
and dark gamma-ray bursts (B-GRBs and D-GRBs) observed by Swift has been
presented. Our sample includes 25 GRBs. Among them 13 are B-GRBs and 12 are
D-GRBs. Our results show that the distributions of the X-ray afterglow fluxes
(), the gamma-ray fluxes (), and the ratio ()
for both the D-GRBs and B-GRBs are similar. The differences of these
distributions for the two kinds of GRBs should be statistical fluctuation.
These results indicate that the progenitors of the two kinds of GRBs are the
same population. Their total energy explosions are comparable. The suppression
of the optical emissions from D-GRBs should results from circumburst but not
their central engine.Comment: 10 pages, 3 figures, 1 table; accepted by ChJA
The exceptionally extended flaring activity in the X-ray afterglow of GRB 050730 observed with Swift and XMM-Newton
We present the results of a detailed spectral and temporal analysis of Swift
and XMM-Newton observations of the high redshift (z=3.969) GRB 050730. The
X-ray afterglow of GRB 050730 was found to decline with time with superimposed
intense flaring activity that extended over more than two orders of magnitude
in time. Seven distinct re-brightening events starting from 236 s up to 41.2 ks
after the burst were observed. The underlying decay of the afterglow was well
described by a double broken power-law model with breaks at t_1= 237 +/- 20 s
and t_2 = 10.1 (-2.2) (+4.6) ks. The temporal decay slopes before, between and
after these breaks were alpha_1 = 2.1 +/- 0.3, alpha_2 = 0.44 (-0.08) (+0.14)
and alpha_3 = 2.40 (+0.07) (-0.09), respectively. The spectrum of the X-ray
afterglow was well described by a photoelectrically absorbed power-law with an
absorbing column density N_H=(1.28 +/- 0.26) 10^22 cm^-2 in the host galaxy.
Strong X-ray spectral evolution during the flaring activity was present. In the
majority of the flares (6/7) the ratio Delta_t/t_p between the duration of the
event and the time when the flare peaks was nearly constant and about 0.6-0.7.
We showed that the observed spectral and temporal properties of the first three
flares are consistent with being due both to high-latitude emission, as
expected if the flares were produced by late internal shocks, or to refreshed
shocks, i.e. late time energy injections into the main afterglow shock by slow
moving shells ejected from the central engine during the prompt phase. The
event fully satisfies the E_p-E_iso Amati relation while is not consistent with
the E_p-E_jet Ghirlanda relation.Comment: 12 pages, 5 figures, accepted for publication in A&
An Intermediate-mass Black Hole of Over 500 Solar Masses in the Galaxy ESO 243-49
Ultra-luminous X-ray sources are extragalactic objects located outside the
nucleus of the host galaxy with bolometric luminosities >10^39 erg s^-1. These
extreme luminosities - if the emission is isotropic and below the theoretical
(i.e. Eddington) limit, where the radiation pressure is balanced by the
gravitational pressure - imply the presence of an accreting black hole with a
mass of ~10^2-10^5 times that of the Sun. The existence of such intermediate
mass black holes is in dispute, and though many candidates have been proposed,
none are widely accepted as definitive. Here we report the detection of a
variable X-ray source with a maximum 0.2-10 keV luminosity of up to 1.2 x 10^42
erg s^-1 in the edge-on spiral galaxy ESO 243-49, with an implied conservative
lower limit of the mass of the black hole of ~500 Msun. This finding presents
the strongest observational evidence to date for the existence of intermediate
mass black holes, providing the long sought after missing link between the
stellar mass and super-massive black hole populations.Comment: 5 pages, 2 figures, 1 table, published in Natur
Line Searches in Swift X-ray Spectra
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.Comment: Accepted by Ap
GRO J1008-57: an (almost) predictable transient X-ray binary
A study of archival RXTE, Swift, and Suzaku pointed observations of the transient high-mass X-ray binary GRO J1008â57 is presented. A new orbital ephemeris based on pulse arrival-timing shows the times of maximum luminosities during outbursts of GRO J1008â57 to be close to periastron at orbital phase â 0.03. This makes the source one of a few for which outburst dates can be predicted with very high precision. Spectra of the source in 2005, 2007, and 2011 can be well described by a simple power law with high-energy cutoff and an additional black body at lower energies. The photon index of the power law and the black-body flux only depend on the 15â50âkeV source flux. No apparent hysteresis effects are seen. These correlations allow us to predict the evolution of the pulsarâs X-ray spectral shape over all outbursts as a function of just one parameter, the sourceâs flux. If modified by an additional soft component, this prediction even holds during GRO J1008â57âs 2012 type II outburst
The early X-ray emission from GRBs
We present observations of the early X-ray emission for a sample of 40
gamma-ray bursts (GRBs) obtained using the Swift satellite for which the
narrow-field instruments were pointed at the burst within 10 minutes of the
trigger. Using data from the Burst Alert and X-Ray Telescopes, we show that the
X-ray light curve can be well described by an exponential that relaxes into a
power law, often with flares superimposed. The transition time between the
exponential and the power law provides a physically defined timescale for the
burst duration. In most bursts the power law breaks to a shallower decay within
the first hour, and a late emission "hump" is observed which can last for many
hours. In other GRBs the hump is weak or absent. The observed variety in the
shape of the early X-ray light curve can be explained as a combination of three
components: prompt emission from the central engine; afterglow; and the late
hump. In this scenario, afterglow emission begins during or soon after the
burst and the observed shape of the X-ray light curve depends on the relative
strengths of the emission due to the central engine and that of the afterglow.
There is a strong correlation such that those GRBs with stronger afterglow
components have brighter early optical emission. The late emission hump can
have a total fluence equivalent to that of the prompt phase. GRBs with the
strongest late humps have weak or no X-ray flares.Comment: Submitted to ApJ on December 16. Revised following referee's
comments. Accepted for publication in ApJ. Minor correction to Fig. 14
caption and updated reference