Transcript A Highly Polarised Jet in XTE J1748-288

UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICS
MULLARD SPACE SCIENCE LABORATORY
The Canonical Swift/UVOT Lightcurve
Sam Oates
(UCL-MSSL)
On behalf of the Swift/UVOT team
The Sample Criteria
To investigate the nature of optical/UV afterglows, we required a large
number of well sampled, good quality UVOT lightcurves.
• Peak magnitude in v <18th mag
• GRBs without significant colour evolution
• Observations must have commenced within the first 400s
after the trigger
– Most GRBs in the sample were observed within the first 100s
• Observed until at least 105s after the trigger
26 GRBs contained in the sample.
All GRBs in the sample were reduced systematically
The count rates in each filter were normalized to the v
The normalized lightcurves were binned with a ∆t/t = 0.2
The Lightcurves
– Ordered by peak magnitude
12th Magnitude
General behaviour:
•10 lightcurves peak or
shallow at the start of
observations
•16 decay from the start
of observations
Brightest GRBs decay the quickest
17.8 Magnitude
Mean peak time:
400s
Temporal Index vs. Peak Magnitude
Temporal Index Before 500s
Temporal Index After 500s
No correlation
Significant correlation
Spearman rank correlation: -0.23 at 73%
Spearman rank correlation: 0.59 at 99.9%
Temporal Index:
Before 500s vs. after 500s
Behaviour before 500s:
•Wide range of values
α<500s= 0
Best fit mean:
α<500s = -0.48 dispersion 0.68
Behaviour after 500s:
•All lightcurves are decaying
Best fit mean:
α>500s = -0.87 dispersion 0.31
Behaviour after 500s is not affected by the
behaviour before 500s
Spearman rank correlation -0.17 with a poor
statistical significance at 60%
α<500s= α>500s
What could be the cause of the early rise?
X
After the peak a decay α=(3p=1)/4 is expected
• Reverse shock?
–
• α=-1.75 for p=2 and α=-2.5 for p=3
– GRB 050726, GRB 061007 and GRB 070529 are the only GRBs
consistent with α<-1.75 (at 95% confidence level)
• Passage of νm?
X
– Expected to produce a rise α=0.5 followed by a decay α=3(1-p)/4 ~ -1
– Implies colour evolution at early times
– Would expect to see a step change in the UVOT lightcurve as the
observing filter changes from white to v.
• Dust Destruction?
X
– Initially reddened and dim
– Would expect to see the afterglow brighten and become less red as the
dust is destroyed
– GRB 060607a is the only GRB with a red excess
GRB 060607a
GRB 070420
What could be the cause of the early rise?
• Peak of the forward shock?
Expected to produce a rise α= 2 or 3 followed by a decay
α=3(1-p)/4 ~ -1
Dependant on the Lorentz factor of the shell
GRBs with rise
GRBs without
rise
Tpeak=400s
Tupper <130s
Г0~475
Г0>650
Rdec,peak~8.8x1016cm
Rdec,upper<6x1016cm
(Equation 1., Molinari et al. 2006)
The shells of the GRBs
with observed rises have
lower Lorentz factors
and are decelerated at a
larger distance
Comparison with XRT canonical model
Optical/UV afterglows do not follow the same behaviour as the X-ray afterglows,
at least in the early afterglow.
What could be the reason for the correlation
between magnitude and decay?
(Assuming all GRBs have a similar energy budget)
• A steeper value for p for the brighter GRBs (?)
• Energy is released over a longer period for faint GRBs
• The observers viewing angle of a jet will affect the
optical afterglow observed.
– The larger the observers viewing angle the shallower the
temporal decay and the lower the peak magnitude.
(e.g Panaitescu & Vestrand 2008)
Luminosity at restframe wavelength 1600Å
Luminosity at restframe wavelength 1600Å
Mean = 11.38
Mean = 10.16
Mean = 8.91
Standard deviation = 0.66
Standard deviation = 0.70
Standard deviation = 0.82
The log luminosities at each epoch indicate a single distribution
Conclusions
• The optical afterglow before 500s may rise or decay.
• All optical afterglows after 500s are decaying.
• The brightest GRBs decay the quickest.
• The peak in the optical afterglows is most likely
due to the start of the forward shock
• The typical optical afterglow behaves very
differently to the XRT canonical model especially
before 500s.