Identification of AGN in the XMM-Newton X-ray survey of the SMC

Abstract

Context. Finding Active Galactic Nuclei (AGN) behind the Magellanic Clouds (MCs) is difficult because of the high stellar density in these fields. Although the first AGN behind the Small Magellanic Cloud (SMC) were reported in the 1980s, it is only recently that the number of AGN known behind the SMC has increased by several orders of magnitude. Aims. The mid-infrared colour selection technique has been proven to be an efficient means of identifying AGN, especially obscured sources. The X-ray regime is complementary in this regard and we use XMM-Newton observations to support the identification of AGN behind the SMC. Methods. We present a catalogue of AGN behind the SMC by correlating an updated X-ray point source catalogue from our XMM-Newton survey of the SMC with already known AGN from the literature as well as a list of candidates obtained from the ALLWISE mid-infrared colour selection criterion. We studied the properties of the sample with respect to their redshifts, luminosities and X-ray spectral characteristics. We also identified the near-infrared counterpart of the sources from the VISTA observations. Results. The redshift and luminosity distributions of the sample (where known) indicate that we detect sources from nearby Seyfert galaxies to distant and obscured quasars. The X-ray hardness ratios are compatible with those typically expected for AGN. The VISTA colours and variability are also consistent in this regard. A positive correlation was observed between the integrated X-ray flux (0.2-12 keV) and the ALLWISE and VISTA magnitudes. We further present a sample of new candidate AGN and candidates for obscured AGN. All of these make an interesting subset for further follow-up studies. An initial spectroscopic follow-up of 6 out of the 81 new candidates showed all six sources are active galaxies, albeit two with narrow emission lines.

VVV-WIT-04: an extragalactic variable source caught by the VVV Survey

“…In fact, the VVV colors of VVV-WIT-04 are in full agreement with a very reddened quasar, similar to the ones found behind the Magellanic Clouds by Ivanov et al (2016) using VISTA data, when applied the extinction of A V = 6.52 mag towards the position of VVV-WIT-04 (see Section 2). Its WISE colors are also consistent with an AGN (e.g., Mateos et al 2012;Maitra et al 2019).…”

“…Catalogued as the quasar LQAC 228-055 001 (Souchay et al 2015;Gattano et al 2018), the archive radio data of PMN J1515-5559 are consistent with non-thermal radiation from a compact radio source as expected for an AGN. As discussed in Section 3 (see also Figures 2 and 3) the colour of VVV-WIT-04 is not a typical star colour, but rather it is in agreement with a very reddened quasar (e.g., Mateos et al 2012;Ivanov et al 2016;Maitra et al 2019), leading us to suggest that VVV-WIT-04 has an extragalactic origin as the near-IR counterpart of PMN J1515-5559.…”

Neutron Stars and Black Holes in the Small Magellanic Cloud: The SMC NuSTAR Legacy Survey

“…However, this source does not appear in the Sturm et al (2013b) catalog of background AGN in the SMC, which was based on identifications with radio sources. This source also does not appear in the catalog of newly identified AGN behind the SMC (Maitra et al 2019), which on the other hand was Xray/NIR selected.…”

“…XMM-Newton X-ray mosaic image (left) and UV-optical mosaic image (right) of the SMC with the three fields observed by NuSTAR presented in this work marked. Left: The image was created by combining XMM-Newton observations in the direction of the nearby star-forming galaxy, available until April 2017 (Maitra et al 2019) and following the procedure described by Haberl et al (2012). The RGB color is composed of three energy bands 0.2-1.0 keV (red), 1.0-2.0 keV (green) and 2.0-4.5 keV (blue).…”

Discovery of a very young high-mass X-ray binary associated with the supernova remnant MCSNR J0513-6724 in the LMC

“…As the X-ray and the infrared flux in AGN are expected to be correlated, as shown in Fig. 11 in Maitra et al (2019), we estimated the expected magnitudes of the counterpart in the ALLWISE bands (W1, W2, W3) for the given X-ray flux of the source. An AGN counterpart, if it exists, is expected to have W1, W2 and W3 in the range of 14.6-17.5, 13.4-16.4 and 10.6-12.7 mag respectively, and should therefore be visible.…”