Mangano V., Sbarufatti B. (INAF-IASFPA), J. K. Cannizzo (NASA/UMBC)
report on behalf of the Swift XRT Team:
We have analyzed the first five orbits of Swift-XRT data of
GRB 090530 (trigger 353567; Cannizzo, et al., GCN Circ. 9438),
consisting of 22 s in Windowed Timing (WT) mode from T+81 to T+103 s and
9 ks in Photon Counting (PC) mode from T+105 s to T+24.4 ks.
The best position for the X-ray afterglow is the XRT UVOT-enhanced
position: RA, Dec = 179.41846, +26.59341 which is equivalent to:
RA (J2000): 11h 57m 40.43s
Dec (J2000): +26d 35' 36.3"
with an uncertainty of 1.5 arcsec (radius, 90% confidence), as given
by Beardmore, et al., GCN Circ 9445.
The 0.3-10 keV X-ray light curve is best fitted by a broken powerlaw
with early decay index of about -5.7, late decay index -0.60+/-0.04
and a break at about 140 s after the trigger. In the first orbit
a small amplitude flare is detected at T+300 s. Flaring activity
is also visible along the later decay.
If decaying at the present rate, the predicted rate after 24h from the
trigger is 2.0e-2 counts/s. However, we remark that a second break
followed by a steepening of the lightcurve is possibly expected.
The issue will be clarified with following Swift observations that
will resume on Sunday May 31st.
The average initial WT spectrum (covering the initial steep decay)
can be modeled as an absorbed power-law with index 2.3 (+0.5-0.4),
absorbing column NH < 11E20 cm-2 (three sigma upper limit) and
observed(unabsorbed) average flux in the 0.3-10 keV energy range of
2.9(3.6)E-10 ergs cm-2 s-1.
The average PC spectrum (roughly covering the flatter part of the
is best fitted by an absorbed power-law with index 2.00+/-0.15
The absorbing column is NH = (8.2 -2.9+3.6)E20 cm-2 in excess with
respect to the Galactic value of 1.78E+20 cm-2 (Kalberla et al. 2005).
The average observed(unabsorbed) flux is 4.0(4.9)E-12 ergs cm-2 s-1.
The count-rate to flux conversion factor is 5.4E-11.
All quoted errors are at 90% confidence level.
The results of the XRT-team automatic analysis are available at
This circular is an official product of the Swift-XRT team.