A transient ultraluminous X-ray source in NGC 55

ROBBA, AlessandraAlessandraROBBAPINTO, CIROCIROPINTOPintore, FabioFabioPintoreRODRIGUEZ CASTILLO, Guillermo AndresGuillermo AndresRODRIGUEZ CASTILLOAMBROSI, ElenaElenaAMBROSIBARRA, FrancescoFrancescoBARRACUSUMANO, GiancarloGiancarloCUSUMANOD'AI', AntoninoAntoninoD'AI'DEL SANTO, MELANIAMELANIADEL SANTOKosec, PPKosecMARINO, ALESSIOALESSIOMARINOMiddleton, MMMiddletonRoberts, TTRobertsSALVAGGIO, ChiaraChiaraSALVAGGIOSORIA, RobertoRobertoSORIAWOLTER, Anna Luisa MariaAnna Luisa MariaWOLTERWalton, DDWalton2025-02-172025-02-1720220035-8711http://hdl.handle.net/20.500.12386/35984Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 1039 erg s-1 . The average number of ULXs per galaxy is still not well-constrained, especially given the uncertainty on the fraction of ULX transients. Here, we report the identification of a new transient ULX in the galaxy NGC 55 (which we label as ULX-2), thanks to recent XMM-Newton and the Neil Gehrels Swift Observatory observations. This object was previously classified as a transient X-ray source with a luminosity around a few 1038 erg s-1 in a 2010 XMM-Newton observation. Thanks to new and deeper observations (~130 ks each), we show that the source reaches a luminosity peak >1.6 × 1039 erg s-1. The X-ray spectrum of ULX-2 is much softer than in previous observations and fits in the class of soft ULXs. It can be well-described using a model with two thermal components, as often found in ULXs. The time-scales of the X-ray variability are of the order of a month and are likely driven by small changes in the accretion rate or due to super-orbital modulations, attributed to precession of the accretion disc, which is similar to other ULXs.STAMPAenA transient ultraluminous X-ray source in NGC 55Article10.1093/mnras/stac20182-s2.0-85136266963https://academic.oup.com/mnras/article/515/4/4669/66488422022MNRAS.515.4669RFIS/05 - ASTRONOMIA E ASTROFISICA