1. Swift observations of X-Ray naked GRBs
Loredana Veterea,
Phil Evansb, Judith L. Racusinc, David N. Burrowsa, Neil Gehrelsc
aDepartment of Astronomy & Astrophysics, Pennsylvania State University, USA, bDepartment of Physics and Astronomy, University of Leicester, UK,
cNASA/Goddard Space Flight Center, USA
“Deciphering the Ancient Universe with Gamma-Ray Bursts”
Kyoto, April
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2. Some data: Swift detected 502 GRBs through GRB 100420A
XRT observed 450 Swift bursts (~90% of BAT detections)
420 afterglows detected (>93% of observations)
XRT detected 95% of long Swift GRBs observed
>98% in less than 300 s

3. GRBs with “fast” decay:
black BAT – red XRT
050421
XRT LC showed two early flares on top of a very rapid decay (=3) and fell below the detection threshold after ~1000s.
Fluence keV= (1.8±07) 10-7 erg cm-2
T keV = 10.3 ± 2.0 s
Godet et al. (2006) --> prompt emission.
first Swift GRB with no forward shock observed.
Five cases (050421, , A, B, A) with very rapid decays (~3)
Non-detected after ~1000s
2.6 s < T90 < 44.5 s
1.0 × 10-7erg cm-2 < F10-150keV < 11 × 10-7 erg cm-2
The first event that showed such behaviour is GRB and Godet et al. (2006) argued that, due to its spectral and temporal properties, this X-ray emission was the tail of the prompt emission. If this is correct then GRB would be the first Swift GRB with no forward shock observed.

4. α~3.1
α~ 2.8
α~ 2.7
α~ 2.5

5. GRBs with NO XRT detection:
No afterglow detected for 7 long GRBs
5 cases (060728, , , and ) Swift slewed in less than 300s
due to engineering work on the XRT, observation started ~14 h
due to the Earth limb observing constraint, XRT observation started ~ 3.6 h
Name
BAT Fluence
keV
(10-7 erg cm-2 )
Photon index
T90
(s)
080315
1.6  0.5
2.5  0.6
65  5
070126
2.0  0.5
51  5
061218
0.4  0.1
2.7  0.6
4.1  0.5
061027
4.7  1.5
1.9  0.5
150  20
060728
2.3  0.7
1.4  0.4
60  10
050911
3.1  0.6
1.8  0.3
16  2
050528
4.5  0.7
2.3  0.2
10.8  2
Name
Time to Observation since BAT trigger
Total exposure
(103 s)
3 Upper Limit
(10-14 erg cm-2 s-1 )
080315
129 sec
1.4
070126
116 sec 80
6.2
061218
106 sec 11
5.8
061027
124 sec
1.5 20
060728 26
8.4
050911
3.6 hours 45
1.7
050528
14 hours 39
2.7

6. BAT extrapolated LC
050528
050911
060728
061027
061218
070126
080315
We analysed the gamma-ray data from BAT and extrapolated the fluxes values in XRT energy range for all 7 events. For XRT instead we evaluated the 3  upper limit for the entire exposure assuming a Crab-like spectrum with the Galactic absorbing column in the direction of the source.
The BAT light curves extrapolated into the keV band compared to the XRT upper limits show a huge step. Moreover if we compare this plot to the canonical grb afterglow it is evident that our events must have been an order of magnitude or even fainter than all other long GRB.

7. There seems to be no evident similarity between the light curves of this two “classes” of peculiar objects. In particular the huge step in the light curve that characterizes the GRBs with no-detection is absent for the “fast decaying” GRBs. The BAT and XRT data in fact smoothly join as expected.
Note also that the most energetic one is GRB for which we don’t have any X-ray emission detected even if XRT started observing only 106 s after the BAT trigger.

8. All XRT Long GRB
It’s also important to point out that more than 2% of all Long GRB detected by XRT have BAT fluence (F10-150keV) lower than the one measured for our 7 GRBs and their X-ray emission is perfectly detected up to 1×106 s after the BAT trigger.

9. High redshift ? GRB 060801 Short T90 15-350 keV = 0.5 ± 0.1 s z=1.1
F( keV) (erg cm-2 s-1)
F( keV) (erg cm-2 s-1)
GRB
Short T keV = 0.5 ± 0.1 s
z=1.1
Fluence keV= (8.1±1.0)10-7 erg cm-2
GRB
Long T keV = 48 ± 2 s
z=0.97
Fluence keV= (4.3±1.0) 10-7 erg cm-2

10. Conclusions:
The combined analysis of the BAT and XRT has shown that prompt X-ray emission smoothly transitions into the decaying afterglow (Barthelmy et al. 2005, O'Brien et al. 2006)
The non-detection of X-ray afterglows is therefore very atypical.
These 7 events are long GRBs with fluences varying from 4×10-8 erg cm-2 to 4.7×10-7erg cm-2 but unusually X-ray faint.
Respect fast decaying GRBs, they have comparable fluences but show a strong discrepancy in the combined BAT-XRT light curves.
A possible explanation would be that the local surrounding density is so low that the blastwave did not sweep up enough matter to produce a sufficiently bright afterglow, the so-called “naked” GRB model.
This behavior could be due for example to a BH-NS merger (with a mass ratio of 10:1 as in Davies et al. 2005) or a collapsar occurring in a region of low density, thus forming a naked GRB.
Evidently more work is needed to model in detail the progenitors of these unusual X-ray–dark GRBs.
As the BAT light curves and the T90 range suggest, these events are long GRBs …….
If a GRB forms in fact with very little surrounding interstellar material, only a weak forward shock will occur and its emission would be very faint and below the XRT sensitivity threshold.

11. Thank you!