Chandra localization of the new accreting millisecond X-ray pulsar SRGA J144459.2-604207

ATel #16510; G. Illiano (INAF-OAR), F. Coti Zelati, A. Marino (ICE-CSIC), A. Papitto (INAF-OAR), A. Miraval Zanon (ASI), M. C. Baglio (INAF-OAB), M. Del Santo, T. D. Russell (INAF/IASF Palermo, Italy), F. Ambrosino (INAF-OAR)
on 7 Mar 2024; 15:00 UT
Credential Certification: Giulia Illiano (giulia.illiano@inaf.it)

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SRGA J144459.2-604207 is a new accreting millisecond X-ray pulsar discovered in outburst on 2024 February 21 by SRG/ART-XC (ATel #16464). Follow-up observations have been carried out in the X-ray band using MAXI (ATel #16469), Swift/XRT (ATel #16471), NICER (ATel #16474, ATel #16480), INTEGRAL (ATels #16485, #16507) and NinjaSat (ATel #16495). Despite optical/IR (ATels #16487, #16489, #16499) and radio (ATel #16475) observations having been performed, no firm counterpart has yet been reported in these bands.

To improve the accuracy of the X-ray position, we observed the source on 2024 March 6 (MJD 60375) for about 5 ks using the High Resolution Camera for the Imaging detector on board Chandra.

The source was detected at a net count rate of about 6 counts/s. Using CIAO/wavdetect, we estimate the source position to be:
RA (J2000) = 14h 44m 58.94s
Dec (J2000) = -60d 41' 55.3",
with a statistical uncertainty of 0.12" at the 95% confidence level (computed following Evans et al. 2010, ApJS, 189, 37). This uncertainty does not take into account the Chandra boresight uncertainty of 0.6" (90% confidence).

The estimated Chandra position is around 4.8 arcsec from the one derived using Swift/XRT (ATel #16471). Due to the high pile-up which affected the XRT pointing (ATel #16471), the error on the XRT position (i.e., 3.5" at 90% confidence) was possibly under-estimated. We also note that none of the proposed near-infrared and optical counterparts reported in ATel #16487, #16489, and #16499 are formally compatible with the Chandra localization.

Assuming the spectral shape reported in ATel #16474, the observed count rate converts to an unabsorbed flux of around 8E-10 erg/cm^2/s (2-10 keV). This indicates that the source has faded by roughly a third compared to the previously reported flux (ATel #16469, #16474).

During the observation, we also detected a type-I burst. The burst had a total duration of about 30 s with a typical Fast Rise Exponential Decay profile. We have obtained a preliminary e-folding decay time of 13.4 +/- 1.5 s.

We thank the Chandra team for promptly scheduling this observation.

REM (H-band) finding chart