TITLE: GCN CIRCULAR NUMBER: 41131 SUBJECT: The SVOM alert sb25072103 is not due to a GRB, but to a new galactic transient SVOM J195836+32283. DATE: 25/07/21 17:00:16 GMT FROM: Diego Gotz at CEA diego.gotz@cea.fr

D. Götz, N. Dagoneau, A. Saccardi (CEA), D. Zhao, Y. Ma, L. Xin (NAOC), J.-L. Atteia (IRAP) on behalf of the SVOM collaboration report:

The SVOM alert sb25072103 (08:43:10 UTC) is not due to a GRB, but most probably to a new galactic transient source. The ECLAIRs alert was initially identified as a trigger from the known galactic High Mass X-ray Binary, 4U 1954+31. SVOM slewed to the source location, and, due to Earth occultation, MXT and VT could not observe the field right away.

As soon as MXT started the observation, about one hour later, at 09:32:37 UTC, its on board software detected an uncatalogued source in its field of view, not consistent with the position of 4U 1954+31. The MXT observation continued until 11:47:43 UTC.

Subsequent analysis of the entire MXT X-band data showed that the source was not fading and, on the contrary, increased its flux by a factor about two over the entire duration of the observation. In addition the source presents a thermal spectrum, compatible with a black body of temperature ~0.2 keV. Based on these properties and on its position in the galactic plane (l=69.03, b= 1.6) we can exclude that this alert is due to a GRB, and we propose to classify this source as a new galactic transient, SVOM J195836+32283.

The J2000 X-ray coordinates are:

R.A. = 19h 58m 36s Dec. = +32° 28’ 20”

with a 90% c.l. error radius of 30 arc sec (18 arc sec of statistical error).

The MXT error region was in the VT field of view during the SVOM second orbit. A faint uncatalogued optical source was detected within the MXT error region in VT_R band with a brightness of 22.2+/-0.4 mag, compared to the PanSTARRS images at the following J2000 coordinates

R.A. = 19h 58m 36.29s Dec. = +32° 28’ 25.7”

with an error of 0.5 arc sec.

The VT images were obtained between 11:11:09 and 11:42:49 UTC with an effective exposure time of 20x100 seconds. The VT magnitude is not corrected for the Galactic extinction on the line of sight.

This source lies at 6 arc seconds from the MXT position, and we suggest that it might be the counterpart of the X-ray source.

Follow-up observations at other wavelengths are encouraged.

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE. SVOM/VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

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