EP-WXT detects X-ray brightening of the blazar NVSS J004348+342626

ATel #16725; S. Q. Jiang (NAOC, CAS), Y. Wang (PMO, CAS), Q. C. Shui (IHEP, CAS), H. Q. Cheng, W. D. Zhang, W. Yuan, C. C. Jin, Z. X. Ling, Y. Liu, C. Zhang, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, S. X. Wen, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, S. N. Zhang (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team
on 20 Jul 2024; 11:09 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the X-ray brightening of the blazar NVSS J004348+342626 detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) during an observation starting at 2024-07-17T03:13:01(UTC) with an exposure of about 20 ks. During the observation, EP-WXT detected a point source with coordinates of R.A. = 10.955 deg, DEC = 34.428 deg (J2000) with an uncertainty of 2.2 arcmin (radius, 90% C.L. statistical and systematic). There are several known sources within the error circle, including a blazar (NVSS J004348+342626), a quasar (SDSS J004342.26+342631.2), and a few stars. We triggered a Swift target-of-opportunity observation which was taken ~1.5 days after the EP observation. An X-ray source was detected with the XRT at a position (R.A. = 10.9534, DEC = 34.4401 (J2000) with an uncertainty of 3.8 arcsec (radius, 90% C.L. statistical and systematic) that is spatially consistent with NVSS J004348+342626, confirming the association of the EP-WXT detection of the blazar. The blazar is also significantly detected in the UVW2 filter with the magnitude 19.84 +/- 0.1 (Vega).

The averaged EP-WXT spectrum in the 0.5-4 keV band can be fitted by an absorbed power-law (with the column density fixed at the Galactic value of 7.1 x 10^20 cm^-2) with a photon index of 1.4(-0.6/+0.7). The derived average 0.5-4 keV unabsorbed flux is 8.4(-2.7/+3.5) x 10^-12 erg/s/cm^2. The XRT spectrum can be fitted by an absorbed power-law model (with the same column density) with a photon index of 1.41(-0.15/+0.28). The unabsorbed flux in 0.5-4 keV is 5.85(-0.57/+0.62) x 10^-12 erg/s/cm^2, consistent with that of the EP-WXT source. The 90 percent uncertainties are given for the above parameters.

This is the first time that the blazar NVSS J004348+342626 is detected in the X-ray band. The 3-sigma 0.2-2 keV upper limit at this position is ~5.5e-13 erg/s/cm^2 set by the ROSAT All-Sky Survey (RASS), estimated by using the XMM-Newton upper limit server (Saxton et al. 2022, http://xmmuls.esac.esa.int/upperlimitserver/ , assuming an absorption column density of 10^21 cm^-2 and a photon index of 1.5). Thus the EP-WXT and Swift/XRT detections suggest that this blazar is brightening. NVSS J004348+342626 was reported to be a radio source (Condon et al. 1998), and also a gamma-ray source (4FGL J0043.8+3425) as included in the Fermi/LAT catalog (Abdollahi et al. 2020).

We thank the Neil Gehrels Swift Observatory team for making the ToO observation possible. Further follow-up observations are encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.

Reference:

[1] Saxton et al. 2022, Astronomy and Computing, Volume 38, article id. 100531

[2] Condon et al. 1998, The Astronomical Journal, 115, 1693-1716

[3] Abdollahi et al. 2020, The Astrophysical Journal Supplement Series, 247, 33