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 19-23 2010 As is known, geography can have a great influence on the way of people’s life, the way people think and the way they behave. This lesson is to give the students an idea how to understand and appreciate a different culture from the angle of geography by introducing Japan. As a result, the students can see and communicate with the people from a different culture in a more interactive and positive way.

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

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. Fluence15-150 keV= (1.8±07) 10-7 erg cm-2 T90 15-350 keV = 10.3 ± 2.0 s Godet et al. (2006) --> prompt emission. first Swift GRB with no forward shock observed. Five cases (050421, 070531, 080727A, 081016B, 100205A) 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 GRB050421 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 050421 would be the first Swift GRB with no forward shock observed.

α~3.1 α~ 2.8 α~ 2.7 α~ 2.5

GRBs with NO XRT detection: No afterglow detected for 7 long GRBs 5 cases (060728, 061027, 061218, 070126 and 080315) Swift slewed in less than 300s 050528 due to engineering work on the XRT, observation started ~14 h 050911 due to the Earth limb observing constraint, XRT observation started ~ 3.6 h Name BAT Fluence 15-150 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 25 + 24 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

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 0.3-10 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.

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 061218 for which we don’t have any X-ray emission detected even if XRT started observing only 106 s after the BAT trigger.

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.

High redshift ? GRB 060801 Short T90 15-350 keV = 0.5 ± 0.1 s z=1.1 F(0.3-10 keV) (erg cm-2 s-1) F(0.3-10 keV) (erg cm-2 s-1) GRB 060801 Short T90 15-350 keV = 0.5 ± 0.1 s z=1.1 Fluence15-150 keV= (8.1±1.0)10-7 erg cm-2 GRB 070208 Long T90 15-350 keV = 48 ± 2 s z=0.97 Fluence15-150 keV= (4.3±1.0) 10-7 erg cm-2

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.

Thank you!