Swift/XRT follow up observations of IGR J17269-4737/XTE J1727-476.
ATel #632; J. A. Kennea, D. N. Burrows, M. Chester, J. Nousek (PSU), D. Palmer (LANL), A. Wells (U. Leicester) and N. Gehrels (GSFC)
on 16 Oct 2005; 22:08 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Transient
On October 15th, 2005 at 21:42:02 Swift/XRT began a 1.1ks follow-up
observation of the Transient Source IGR J17269-4737, also known
as XTE J1727-476 (ATEL #624, ATEL #625). We report on the
analysis of these data, plus a refined analysis of the observation
previously reported on by Kennea et al. (ATEL #626). The new
observation was performed approximately 9 hours after the start
of the 50ks INTEGRAL TOO observation reported by Turler et al. (ATEL
#631).
The XRT data was taken in Window Timing (WT) mode due to the high
brightness
of the source, to avoid pile-up. Temporal analysis of this source shows
an steady XRT count rate of ~90 cts/s.
Spectral analysis of the WT data reveals that the source is fit by a
thermal bremsstrahlung model, with N_H = (4.8 +/- 0.1) x 1021
cm^-1
and kT = 0.91 +/- 0.01 keV. The model flux from this source is 1.9 x
10-9 erg/s/cm2 (0.2 - 10 keV). We note that the
spectral parameters
of this fit differ from those we previously reported in ATEL #626, this
is most likely due insufficient correction for the high degree of
pileup in the previous observation, which was taken in Photon
Counting (PC)
mode in order to ensure an accurate position for the transient, and the
location of the source in the corner of the CCD. We have
performed a more detailed analysis of the PC mode
data to better correct for these effects. Fitting the refined PC mode data and new
WT data simultaneously, with only the normalization independent, gives the
following fit with a thermal bremsstrahlung model:
N_H = (4.7 +/- 0.1) x 1021 cm^2
kT = 0.93 +/- 0.01 keV
Reduced Chi^2 = 1.87 (557 dof)
PC mode flux = 3.2 x 109 erg/s/cm^2 (0.2 - 10 keV)
WT mode flux = 1.9 x 109 erg/s/cm^2 (0.2 - 10 keV)
The flux from this source has therefore dropped to approximately 60% of
the flux measured on October 12th (these flux values are not
corrected for absorption). We note that this model has a higher N_H
than expected
from galactic value of 3x1021 cm^-2.
We have also fit a Disk Blackbody model to the combined PC/WT data.
This fit gives the follow parameters:
N_H = (3.2 +/- 0.1) x 1021 cm^-2
kT_inner = 0.53 +/- 0.01 keV
Reduced Chi^2 = 1.80 (557 dof)
The absorbed flux values remain unchanged from above. We note that this
model is a better fit, and that the N_H is consistent with the expected
Galactic value. The Disk Blackbody model fit suggests that
this transient is a black hole candidate.
We note the spectral fits given by Turler et al. (ATEL #631) differ
from these results. We find that although the thermal bremsstrahlung
model fit given by Turler et al adequately describe the Swift/XRT data
in
the 3-10 keV range, this model produces large residuals in the 0.2-3.0
keV range
not covered by INTEGRAL JEM-X. We find that a blackbody fit to the data
produces a fit consistent with the INTEGRAL value, kT = 0.36 +/- 0.01
keV, however the fit itself is poor (Reduced Chi^2 = 5.3 for 557 dof).
