eRASSt J040515.6-745202: A new X-ray transient discovered by eROSITA
ATel #14646; A. Rau (MPE), I. Grotova (MPE), A. Gokus (ECAP), F. Haberl (MPE), Z. Liu (MPE), M. Schramm (Saitama University), D. Homan (AIP), O. Koenig (ECAP), I. Kreykenbohm (ECAP), M. Krumpe (AIP), P. Maggi (Universite de Strasbourg), G. Vasilopoulos (Yale), P. Weber (ECAP), J. Wilms (ECAP)
on 20 May 2021; 07:25 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)
Subjects: X-ray, Transient
During the third all-sky survey (eRASS3), the eROSITA instrument (Predehl et al. 2021) aboard the Russian/German Spektrum-Roentgen-Gamma (SRG) mission (Sunyaev et al. 2021) discovered a new X-ray transient, designated as eRASSt J040515.6-745202, and located in the Magellanic Bridge at:
RA(J2000)= 04:05:15.30 (61.31456 deg)
DEC(J2000)= -74:51:58.1 (-74.86683 deg)
with an estimated positional uncertainty of ~5" radius (incl. systematics).
The source was discovered following an alert generated by the eROSITA Near-Real Time Analysis. Detailed analysis of the eROSITA all-sky survey data showed that the object was detected when it first entered the instrument field of view on 2021 April 28th and that it remained at approximately constant brightness (~1.5-2.5 cnt/s in the 0.2-5keV band) until at least 2021 May 2nd.
The eROSITA X-ray spectrum with a combined exposure of 347s is well fit with an absorbed black body plus power law model with kT=1.7(+2.28/-0.6)keV and slope of 2.4(+0.7/-0.6). Here, N_H was fixed to the expected Galactic foreground value along the line of sight of N_H=1e21 cm^-2 (HI4PI Collaboration, Ben Bekhti et al. 2016). The corresponding 0.2-5keV flux is ~6.4e-12 erg/cm^2/s, more than 10x times brighter than in the 3-sigma upper limits derived from the previous eROSITA scans over the position taken in April and September 2020.
Follow-up ToO observations with the Neil Gehrels Swift Observatory were performed on May 12th. A total of 1682s were obtained with the XRT in photon counting mode. The source was detected with a net count rate of (0.094+/-0.008) cnt/s. The spectrum can be fit with an absorbed powerlaw with a slope of 1.6+/-0.2. The N_H was fixed again to the Galactic foreground value. The resulting 0.2-5keV flux is (3.2+/-0.6)E-12 erg/cm^2/s. UVOT/UVW1 observations with an exposure of 1754s revealed two sources consistent with the X-ray position:
Source 1:
RA(J2000)= 04:05:16.54 (61.31894 deg)
Dec(J2000= -74:51:59.5 (-74.86652 deg)
UVW1= 22.60+/-0.32 mag
Source 2:
RA(J2000)= 04:05:15.00 (61.31251 deg)
Dec(J2000)= -74:52:02.5 (-74.86736 deg)
UVW1= 21.87+/-0.21 mag
Optical follow-up with the Skynet P6 telescope on May 4th also detected Source 1 at a brightness of V~19.2mag. This object is also seen in archival VISTA near-IR bands and by WISE (WISEA J040516.50-745159.3). Its W1-W2 color (~0.0mag) suggests it not to be an AGN. We did not find any previous optical or near-IR detection of Source 2. However, we note that there is an archival GALEX source (6385095153005105290) detected at a position roughly in between the two UVOT sources, but formally not consistent with either. Its magnitudes are NUV~21.3mag & FUV~21.5mag.
Additional follow-up observations with XMM-Newton have been scheduled.
We are grateful to the Swift team for rapidly approving and scheduling the ToO observations.
