Swift and Chandra X-ray detections in the nucleus of SDSS J133519.91+072807.4

ATel #16576; Lorena Hernandez-Garcia (MAS/UV), Paula Sanchez Saez (ESO), Claudio Ricci (UDP), Patricia Arevalo (UV), Franz Bauer (PUC), Amelia Bayo (ESO), Santiago Bernal (UV), Murray Brightman (Caltech), Mirko Krumpe (AIP), Paulina Lira (UChile), Mary Loli Martinez-Aldama (UdeC), Alejandra Muñoz-Arancibia (CMM/MAS/UChile)
on 8 Apr 2024; 01:36 UT
Credential Certification: Lorena Hernandez-Garcia (lorena.hernandez@uv.cl)

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We report the detection of an X-ray source in a Swift/XRT observation from February 4th 2024, at a distance of 6'' +/- 3''.6 from the nucleus of the galaxy SDSS J133519.91+072807.4 (redshift 0.024, classified spectroscopically as Starforming by the SDSS pipeline). A Chandra observation taken on April 3rd 2024 confirms a nuclear origin for the X-ray detection.

SDSS J133519.91+072807.4 began showing significant nuclear variability in the Zwicky Transient Facility (ZTF; Bellm et al., 2019, PASP, 131, 018002) alert stream in December 2019 (as ZTF19acnskyy, RA = 13:35:19.936, DEC = +7:28:7.66). The ALeRCE broker (Forster et al., 2021, AJ, 161, 242) light curve classifier (Sanchez-Saez et al., 2021, AJ, 161, 141) classified it as an active galactic nucleus (AGN).

Our group has been monitoring the source with Swift since 2021. No X-ray detections were obtained in the XRT observations of August 2021 (MJD 59381), July 2022 (MJD 59775), and July 2023 (MJD 60140). The Swift observation taken on February 4th 2024, showed X-ray emission in the 0.3-2 keV energy band at a distance of 6'' +/- 3''.6 from the nucleus of the galaxy. We modelled the spectrum with a black body model with kT =104 +/- 9 eV temperature and obtained a flux of 7.4x10^-12 erg/cm^2/s. Assuming the redshift of SDSS J133519.91+072807.4, a luminosity of 9.62x10^42 erg/s is estimated.

After the first XRT detection, we requested new ToO observations with Swift. Five additional observations have been carried out, obtaining the following fluxes in the 0.3-2 keV energy band for the six available observations, assuming a black body model. Errors are given at a 68.00% confidence:

2024-02-04 : 7.4[6.7 - 7.6]e-12 erg/cm^2/s
2024-02-13 : 1.8[1.3 - 2.1]e-12 erg/cm^2/s
2024-02-18 : 2.2[1.1 - 2.3]e-12 erg/cm^2/s
2024-03-05 : 4.4[3.1 - 5.5]e-13 erg/cm^2/s
2024-03-07 : 6.5[5.7 - 6.8]e-12 erg/cm^2/s
2024-04-04 : 2.8[2.5 - 3.0]e-12 erg/cm^2/s

To confirm the position of the X-ray emission, we requested a DDT observation with Chandra. The source was observed on April 3rd 2024, during 2 ksec. The image shows a source located at RA = 13:35:19.93 DEC = +7:28:07.48 , i.e., coincident with the nucleus of SDSS J133519.91+072807.4.

We thank the Swift and Chandra teams for the rapid scheduling of ToO and DDT observations.