MAXI J0911-655/Swift J0911.9-6452: Swift XRT localization of the new transient in NGC 2808

ATel #8884; J. A. Kennea (PSU), P. A. Evans, A. P. Beardmore, K. L. Page (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano (INAF-IASFPA), K. Yamaoka (Nagoya U.), M. Serino (RIKEN) and H. Negoro (Nihon U.)
on 30 Mar 2016; 15:40 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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We report on Swift target-of-opportunity observations of the new transient source MAXI J0911-655 AKA Swift J0911.9-6452, reported by Serino et al (ATEL #8872) to be near the globular cluster NGC 2808. Swift took 1.6ks of Photon Counting (PC) mode data, and a bright uncatalogued point source was localized at the following coordinates: RA/Dec(J2000) = 138.0118, -64.8673, which is equivalent to:

RA(J2000) = 09h 12m 02.82s,
Dec(J2000) = -64d 52m 02.2s,

with an estimated uncertainty of 3.5 arc-seconds radius (90% containment). This new transient lies 13.8 arc-seconds from the center of NGC 2808, 74.8 arc-seconds from the center of the BAT error circle, and 43 arc-minutes from the center of the MAXI error circle (~6 arc-minutes outside the combined 90% and systematic MAXI error circle). We therefore conclude that this XRT source is counterpart of MAXI J0911-655/Swift J0911.9-6452, which is indeed a newly discovered transient inside NGC 2808.

A search of the Simbad database finds no known X-ray point sources inside the XRT error circle, although two Chandra point sources do lie within 5 arcseconds of the XRT error region (Chandra sources 13 and 15 from Sevillat et al., 2008, A&A, 490, 641). One of these Chandra sources is identified as being the counterpart of XMM-Newton source “C4” (Servillat et al., 2008, A&A, 480, 397), which is identified by that paper as being a CV, so is likely unrelated. Higher spatial resolution observations will be required in order to accurately determine whether these objects are related.

The XRT source is piled up in PC mode, with a pile-up corrected count rate of 3.7 +/- 0.2 count s^-1. The pile-up corrected spectrum can be well described by a power-law with photon index 1.55 +/- 0.16, and absorption N_H = 4 +/- 1 x 10²¹ cm^-2. The flux (corrected for absorption) is 2.6 +/- 0.2 x 10^-10 erg cm^-2 s^-1 (0.5 - 10 keV), which assuming a 9.6 kpc distance (for NGC 2808), converts to a luminosity of 2.9 x 10³⁶ erg s^-1, consistent with the MAXI calculated value (Serino et al., ATEL #8872).

In addition to catalog searches, we also examined archival observations of NGC 2808 taken by Swift/XRT in 2014, with a total exposure time of 7ks. In the combined observations, no point source was detected at the location of the new transient.

Further observations in order to determine the nature of this object are encouraged.