National Aeronautics and Space Administration

Last Update: March 26th, 2018

What are the (apparently) brightest X-ray sources in the sky as seen from the Earth?

In the soft X-ray band (0.2 - 5 keV) the Sun is by far the brightest persistent X-ray source (by a factor of about a million) due to its proximity to the Earth. The flaring Sun is also arguably the brightest 'transient' or variable X-ray source, but a giant gamma-ray flare of the magnetar SGR 1806-20 on 27 Dec 2004 had a peak flux (in a hard band from 45 keV to 10 MeV) which rivalled a very large solar flare in its observed flux at the Earth (and covered a much broader energy range, to boot). A few other giant flares have been observed from the handful of other known magnetars.

Next after these sources, come the class of so-called soft X-ray transients (SXTs), sometimes also called X-ray novae, which are actually outbursts of X-ray binary systems containing accreting neutron stars or black holes; SXTs can stay bright for durations of days to months, and in some cases the ratio of maximum to minimum X-ray emission can be huge (many orders of magnitude).

Only one source (excluding the Sun) in the `top ten' brightest sources is a persistently bright X-ray source, namely the low-mass X-ray binary system Scorpius X-1. For completeness, after these 'top ten' sources, the brightest members of some of the other various classes of X-ray sources are listed, first for Galactic objects, then for extragalactic classes of objects.

A name in bold-face indicates that it is a steady or slowly varying X-ray source. A name in italics indicates that it is an X-ray transient source, e.g., an X-ray nova or a gamma-ray burst). An `@" indicates that this is the level seen during a flare (e.g., Algol and HR 1099) or an outburst (e.g., SS Cyg).

The energy range quoted for the X-ray fluxes and luminosities of the non-solar sources is 2-10 keV. The fluxes are given both in terms of cgs units and in multiples of the Crab flux, where 1 Crab = 2.4 x 10^-8 erg/s/cm².

Note: The Galactic Center source Sgr A^* currently has a weak X-ray flux only a millionth of the quoted level, but, based on the observed fading X-ray fluorescence and hard X-ray emission from giant molecular clouds in the Galactic Center, Terrier, Sunyaev and others have argued that, a hundred or more years ago, Sgr A^* must have been emitting at this extremely high level.

How do the sensitivity and angular resolution of current X-ray telescopes compare with those of earlier instruments?

The currently operating Chandra and XMM-Newton X-ray observatories have detected X-ray sources that are one ten-billionth (1 x 10^-10) of the brightness of the first cosmic X-ray source ever detected, Scorpius X-1, which is also the brightest persistent (non-solar) X-ray source in the sky : i.e., sources as faint as 3 x 10^-17 erg cm^-2 s^-1 are now known e.g., in the Chandra Deep Field South 7-Megasecond source catalog (Luo et al. 2017, ApJS, 228, 2), compared to the flux of 3 x 10^-7 erg cm^-2 s^-1 for Sco X-1 (both fluxes quoted are in the 2-7 keV X-ray band). This improvement in sensitivity, achieved in the 5+ decades from 1962 to the present, of X-ray telescopes is very similar to the improvement achieved over the four centuries of telescopic observations of the sky in the visible (optical) band since Galileo Galilei's first observations. In fact, the faintest objects in the Hubble Extreme Deep Field (cf. Illingworth et al. 2013, ApJS, 209, 6) are of 31st magnitude and have a brightness, which corresponds to 1 x 10^-10 (one ten-billionth) of that of the faintest naked-eye stars (which are about 6th magnitude).

The earliest X-ray detectors were wide-angle instruments that had either no or limited (degrees scale) angular resolution. The first fully imaging X-ray telescope put into space was the Einstein (HEAO-2) Observatory, launched in 1978, which had instruments with angular resolution as good as 2 arcseconds (the High Resolution Imager) up to 60 arcseconds (the Imaging Proportional Counter): the ROSAT Observatory which operated during the 1990s had instruments with similar capabilities. The currently operating Chandra X-ray Observatory (launched in 1999) has a mirror which is figured to such an exquisite accuracy that its resolution is about 0.5 arcseconds (3600 arcseconds = 1 degree). Thus, in the 40 years of cosmic X-ray astronomy, the angular resolution has improved by a factor of more than 10,000 in linear scale, equivalent to an improvement of 10⁸ in area on the sky. The latter measure is crucially important when looking for counterparts to X-ray sources in other wavelength regions, since the smaller the sky area to be searched, the smaller the chance of a misidentification with an unrelated object.

Web page author and maintainer: Stephen A. Drake