INTEGRAL and RXTE spectral analysis of IGR J17480-2446, the new transient in Terzan 5

During the ongoing INTEGRAL ToO observation of MAXI J1659-152, 
IBIS/ISGRI detected again the recently 
discovered X-ray transient located in the direction of Terzan 5 
(ATels #2919, #2920, #2922 ,#2924, #2929, #2932, #2933, #2937, #2939). An association of the source with the transient LMXB EXO 1745-248 
was initially suggested (ATels #2919, #2920), but this was considered less 
likely after the reanalysis of archival Chandra and RXTE observations 
(ATels #2933, #2937).

Since the transient was first reported using INTEGRAL data (ATel #2919), we suggest to name it IGR J17480-2446.  The IBIS/ISGRI spectrum of IGR J17480-2446 obtained from the latest 
available INTEGRAL data (from 2010 October 13 at 20:18 to 2010 October 15 at 00:05) can be reasonably well described in the 20-80 keV energy range by a power-law with photon index 5.2+/-0.3 (at 90% c.l., chi^2/d.o.f.=1.1/5).

The estimated 20-40 keV flux is 6.3e-10 erg/cm^2/s (effective exposure time of 43 ks at large off-axis angles of 12-14 degrees). These results can be compared with the previous INTEGRAL observations performed on 2010 Oct 10 and 2010 Oct 11 (ATels #2919, #2924), when the 20-40 keV fluxes were respectively 3.4e-10 erg/s/cm^2 and 4.8e-10 erg/s/cm^2, and the power-law index 2.6+/-0.2 and 2.4+/-0.2.

We also analyzed the RXTE/PCA observations performed on 2010-10-13 at 
00:13 UTC and on 2010-10-14 between 04:23 and 08:42 UT, excluding the eclipse and the Type I X-ray burst (ATels #2929, #2932, #2939). The broad band spectrum can be fitted using a multi-component model: photoelectric absorption fixed at the value 0.5e22 cm^-2 (found using 
Swift/XRT, ATel #2922), a black-body, a cut-off power-law and a broad Gaussian line centered at 6.6 keV due to the contamination from the 
Galactic ridge emission.

The evolution of the spectral parameters evidences 1) a similar black-body temperature in the two observations, respectively kT=(1.3+/-0.2) keV and kT=(1.1+/-0.1) keV 2) an increased contribution of the black-body emission with an effective emitting radius passing from (3.2+/-0.8) km to (8.8+/-1.3) km for a 10 kpc distance, 3) a lowering of the cut-off energy from (12.2+/-1.4) keV to (5.0 +/-0.2) keV, 4) a hardening of the spectral index from 0.6+/-0.2 to 0.20+/-0.12, 5) an increase of the 3-20 keV flux from (2.8e-9+/-0.2e-9) erg/s/cm^2 to (8.4e-9+/-0.3e-9) erg/s/cm^2.

The INTEGRAL observation reported here is contemporary to the second RXTE observation and the joint spectral fitting evidences a good agreement of the two data-set, confirming that IGR J17480-2446 is probably passing from a hard to a soft state, while still in the raising part of the outburst. A more in-depth analysis of the type-I X-ray burst observed with the JEM-X 
monitor on board INTEGRAL (ATel #2924) revealed the presence at 3.2 sigma c.l. of burst 
oscillations during the ~20 s following the burst on-set at the barycentric corrected frequency of 11.04+/-0.01 Hz, in agreement with the result reported in ATel #2932.

Further INTEGRAL observation of the FOV around the source will be performed on 2010 October 16 at 19:21:40.