A Transient Sub-Eddington Black Hole X-Ray Binary Candidate in the Dust Lanes of Centaurus A

Abstract

We report the discovery of a bright X-ray transient CXOU J132527.6−430023 in the nearby early-type galaxy NGC 5128. The source was first detected over the course of five Chandra observations in 2007, reaching an unabsorbed outburst luminosity of (1-2) × 10 38 erg s −1 in the 0.5-7.0 keV band before returning to quiescence. Such luminosities are possible for both stellar-mass black hole and neutron star (NS) X-ray binary transients. Here, we attempt to characterize the nature of the compact object. No counterpart has been detected in the optical or radio sky, but the proximity of the source to the dust lanes allows for the possibility of an obscured companion. The brightness of the source after a >100-fold increase in X-ray flux makes it either the first confirmed transient non-ultraluminous X-ray black hole system in outburst to be subject to detailed spectral modeling outside the Local Group, or a bright (>10 38 erg s −1 ) transient NS X-ray binary, which are very rare. Such a large increase in flux would appear to lend weight to the view that this is a black hole transient. X-ray spectral fitting of an absorbed power law yielded unphysical photon indices, while the parameters of the best-fit absorbed disk blackbody model are typical of an accreting ∼10 M black hole in the thermally dominant state.

Existence of Chandrasekhar’s limit in generalized uncertainty white dwarfs

“…This implies that the GUP-enhanced equation of state prevents gravitational collapse and halts the formation of compact astrophysical objects denser than white dwarfs. This prediction contradicts astronomical observations that confirm the existence of pulsars [7–9] and black holes [10–12]. Moreover, it has been observed that the masses of white dwarfs fall well within the Chandrasekhar limit [13–15],MnormalCh=2.01823π2(ℏcG)3/21(μemu)2≈5.76μe2M⊙,apart from the super-Chandrasekhar white dwarfs, that may well be double-degenerate mergers [16–18].…”

Spectral Properties of X-Ray Binaries in Centaurus A

“…Circular extraction regions were produced, centered on the chip position of each source, whether it is detected in an observation or not. As was the case for the work of Burke et al (2012), these regions had radius r p7 , equal to 90% of the 7 keV extraction radius at that chip position. We then excluded regions in the jet, radio lobe and nucleus from our source list.…”

“…There is a large fall in flux from this source, the spectral counts falling to 29 in the final observation from over 1100 in the first; however, it is still above our detection limit. The low inner-disk temperature in the thermal state, combined with the high inter-observation variability, strongly suggests that this is a BH LMXB transient candidate, similar to S14 (Burke et al 2012). NS LMXBs at this luminosity, L x > 10 38 erg s −1 , typically have a much flatter spectral shape over the Chandra band.…”

“…This spectral state phenomenology allows, with high quality spectra, a source to be classified as a candidate BH XB on the basis of its having a cool accretion disk at 10 38 erg s −1 , as discussed by White & Marshall (1984). Burke et al (2012) used the Cen A VLP data to show that such distinctions are now possible for sources outside the Local Group, presenting evidence that CXOU J132527.6-430023 (S14 , Table 1) is a BH LMXB.…”

Existence of Chandrasekhar's limit in GUP white dwarfs

“…This implies that GUP-enhanced equation of state prevents gravitational collapse and halts the formation of compact astrophysical objects denser than white dwarfs. This prediction contradicts astronomical observations that confirm the existence of pulsars [6][7][8] and black holes [9][10][11]. Moreover, it has been observed that the masses of (non-magnetic) white dwarfs fall well within the Chandrasekhar limit [12][13][14],…”