ATCA radio observation of the region around IGR J18245-2452
ATel #4981; L. Pavan (ISDC Geneve), G. Wong (UWS Sydney), M. H. Wieringa (CSIRO Australia), N. F.H. Tothill, M. D. Filipovic (UWS Sydney), E. Bozzo, C. Ferrigno (ISDC Geneve), A. Papitto (IEEC-CSIC Barcelona), P. Romano (INAF-IASF Palermo)
on 16 Apr 2013; 09:57 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)
Subjects: Radio, X-ray, Binary, Neutron Star, Transient
The new INTEGRAL transient source IGR J18245-2452 (ATel #4925 #4927 #4929 #4934 #4959 #4960 #4961 #4964) was observed with the radio Australia Telescope Compact Array on 2013-04-05 for a total of 6 hours. The array was in H214 configuration. The observation was performed at two different frequencies: 9 and 5.5 GHz.
ATCA detected at both frequencies a single source in the vicinity of IGR J18245-2452, at R.A. 18h24m32.51s Dec. -24°52′07.9′′ (with a 90% confidence error of 0.5′′). This position is at 4.8′′ from the best fit position of IGR J18245-2452 (Heinke et al., ATel #4927), estimated by Swift/XRT with an uncertainty of 3.5′′ at a 90% confidence level (c.l.).
The mean flux density of this source was 0.62±0.03 (0.75±0.04) mJy at 9 (5.5) GHz respectively. This yields a mean spectral index of 0.4±0.2. The ATCA source was the only radio source detected within the inner core of the cluster. During the first 90 minutes of the observation it was strongly variable, reaching up to 2.5 times the mean flux density.
We note that, at the best of our knowledge, 12 radio pulsars were previously reported close to IGRJ18245-2452 (see Bogdanov et al. 2011 ApJ 730, 81 and references therein), but none of them has a position compatible with the source detected by ATCA.
The variability of the ATCA source in radio suggests a possible association with the INTEGRAL transient IGR J18245-2452, although puzzling given the onset of thermonuclear bursts (ATel #4959, #4960, #4961) subsequent to the radio observation.
A refined X-ray position (e.g., with Chandra) is needed to probe this association.
We report in the image linked at the end of this telegram the source estimated positions and the corresponding uncertainties at 90% c.l.. We plot the Swift best position of IGR J18245-2452 in red, the position of the ATCA source reported in this telegram in white, the optical source reported by Monard & Kuulkers ATel#4964 in magenta, and the previously known X-ray sources detected with Chandra in blue (Becker, et al. 2003 ApJ, 594, 798). We also show in green the approximate positions of the closest millisecond radio pulsars as inferred from Fig. 1 in Bogdanov et al. (2011). All regions are superimposed on the Swift/XRT image of the source reported in ATel #4927.
We thank the ATCA team for the rapid planning of the observation and the prompt assistance during the preliminary data analysis.