Swift follow-up observation of ASASSN-15tx
ATel #8422; F. Coti Zelati (U. Insubria, U. Amsterdam, INAF-OAB), A. Papitto (CSIC-IEEC), N. Rea (U. Amsterdam; CSIC-IEEC), E. Bozzo, C. Ferrigno (ISDC, Switzerland)
on 16 Dec 2015; 22:46 UT
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)
Subjects: X-ray, Transient
On 2015 December 11 Swift/XRT observed the field around the newly discovered
transient source ASASSN-15tx (Atel #8400), for about 2 ks.
The XRT was operated in photon counting mode. Only two photons were
recorded over the energy range 0.3-10 keV within a circular region
centered on the position of the optical counterpart (Atel #8400) and within
a radius of 20 pixels (47.2 arcsec).
Applying the prescription for low number statistics given by Gehrels (1986), we derive a conservative upper limit of 5.5e-3 count/sec (3 σ c.l.) for the count rate within the entire region mentioned above (0.3-10 keV).
Assuming a blackbody spectral model with kT = 0.2-0.3 KeV, this corresponds
to an unabsorbed flux of 1.8-2.2E-13 erg/cm2/s. In case a power-law spectral model with
a photon index of 1-3 is assumed, the corresponding flux is 3.2-4.2E-13 erg/cm2/s.
We fixed in both cases the value of the absorption column density to the
estimated Galactic value in the direction of the source (1.4E21
cm-2; Willingale et al. 2013).
The UVOT observed ASASSN-15tx with the UVW1 filter yielding a magnitude
of UVW1=15.58±0.04 in the Vega system and not corrected for absorption (effective exposure time 1956 s).
