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Ultraluminous X-ray bursts in two ultracompact companions to nearby elliptical galaxies


A flaring X-ray source was found near the galaxy NGC 4697 (ref. 1). Two brief flares were seen, separated by four years. During each flare, the flux increased by a factor of 90 on a timescale of about one minute. There is no associated optical source at the position of the flares1, but if the source was at the distance of NGC 4697, then the luminosities of the flares were greater than 1039 erg per second. Here we report the results of a search of archival X-ray data for 70 nearby galaxies looking for similar flares. We found two ultraluminous flaring sources in globular clusters or ultracompact dwarf companions of parent elliptical galaxies. One source flared once to a peak luminosity of 9 × 1040 erg per second; the other flared five times to 1040 erg per second. The rise times of all of the flares were less than one minute, and the flares then decayed over about an hour. When not flaring, the sources appear to be normal accreting neutron-star or black-hole X-ray binaries, but they are located in old stellar populations, unlike the magnetars, anomalous X-ray pulsars or soft γ repeaters that have repetitive flares of similar luminosities.

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Figure 1: Chandra cumulative X-ray photon arrival time and light curve for Source 1 in the NGC 4636 globular cluster.
Figure 2: Individual and combined background-subtracted X-ray light curves for Source 2 in the NGC 5128 globular cluster or ultracompact dwarf.

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We thank T. Richtler for discussions. J.A.I. was supported by Chandra grant AR6-17010X and NASA ADAP grant NNX10AE15G. G.R.S. acknowledges the support of an NSERC Discovery Grant. A.J.R. was supported by the National Science Foundation grant AST-1515084. J.S. acknowledges support from NSF grants AST-1308124 and AST-1514763 and the Packard Foundation.

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Authors and Affiliations



J.A.I. led the Chandra data reduction and analysis, with contributions from W.P.M. for the XMM-Newton data reduction and analysis. T.S., I.P. and D.M. conducted the Chandra galaxy survey that yielded the two flare sources, with oversight from J.A.I. G.R.S., A.J.R., D.L., J.S., J.L. and J.M.M. contributed to the discussion and interpretation.

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Correspondence to Jimmy A. Irwin.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Plots of the cumulative X-ray photon arrival time for the five flares of Source 2 in NGC 5128.

The first four flares were observed by Chandra with the fifth flare by XMM-Newton. In ObsID 10723, the first photon of the observation was not received until 1,100 s after the observation began, and the observation ended mid-flare.

Extended Data Table 1 Summary of the Chandra and XMM-Newton Observations of Sources 1 and 2

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Irwin, J., Maksym, W., Sivakoff, G. et al. Ultraluminous X-ray bursts in two ultracompact companions to nearby elliptical galaxies. Nature 538, 356–358 (2016).

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