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Register a new userDiscovery of 3 s pulsations from the Brightest Hard X-ray Source in M31
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Guillermo A. Rodriguez Castillo
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We report the discovery with XMM-Newton of 3-s X-ray pulsations from 3XMM J004232.1+411314, a dipping source that dominates the hard X-ray emission of M31. This finding unambiguously assesses the neutron star (NS) nature of the compact object. We also measured an orbital modulation of 4.15 h and a projected semi-axis at $a_{mathrm{X}} sin i= 0.6$ lt-s, which implies a low-mass companion of about 0.2$-$0.3$M_{odot}$ assuming a NS of 1.5 $M_{odot}$ and an orbital inclination $i=70^{circ}-80^{circ}$. The barycentric orbit-corrected pulse period decreased by $sim$28 ms in about 16 yr, corresponding to an average spin-up rate of $dot{P} sim -6 times 10^{-11}$ s s$^{-1}$ ; pulse period variations, probably caused to by X-ray luminosity changes, were observed on shorter time scales. We identify two possible scenarios for the source: a mildly magnetic NS with $B_{mathrm{p}}simeq$ few $times10^{10}$ G if the pulsar is far from its equilibrium period $P_{mathrm{eq}}$, and a relatively young highly magnetic NS with $B_{mathrm{eq}}simeq 10^{13}$ G if spinning close to $P_{mathrm{eq}}$.
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We performed a search for eclipsing and dipping sources in the archive of the EXTraS project - a systematic characterization of the temporal behaviour of XMM-Newton point sources. We discovered dips in the X-ray light curve of 3XMM J004232.1+411314, which has been recently associated with the hard X-ray source dominating the emission of M31. A systematic analysis of XMM-Newton observations revealed 13 dips in 40 observations (total exposure time $sim$0.8 Ms). Among them, four observations show two dips, separated by $sim$4.01 hr. Dip depths and durations are variable. The dips occur only during low-luminosity states (L$_{0.2-12}<1times10^{38}$ erg s$^{-1}$), while the source reaches L$_{0.2-12}sim2.8times10^{38}$ erg s$^{-1}$. We propose this system to be a new dipping Low-Mass X-ray Binary in M31 seen at high inclination (60$^{circ}$-80$^{circ}$), the observed dipping periodicity is the orbital period of the system. A blue HST source within the Chandra error circle is the most likely optical counterpart of the accretion disk. The high luminosity of the system makes it the most luminous dipper known to date.
We report on the discovery of a periodic modulation in the bright supersoft X-ray source XMMU J004252.5+411540 detected in the 2000-2004 XMM-Newton observations of M31. The source exhibits X-ray pulsations with a period P~217.7 s and a quasi-sinusoidal pulse shape and pulsed fraction ~7-11%. We did not detect statistically significant changes in the pulsation period on the time scale of 4 years. The X-ray spectra of XMMU J004252.5+411540 are extremely soft and can be approximated with an absorbed blackbody of temperature 62-77 eV and a weak power law tail of photon index ~1.7-3.1 in the 0.2-3.0 keV energy band. The X-ray properties of the source and the absence of an optical/UV counterpart brighter than 19 mag suggest that it belongs to M31. The estimated bolometric luminosity of the source varies between ~2e38 and ~8e38 ergs/s at 760 kpc, depending on the choice of spectral model. The X-ray pulsations and supersoft spectrum of XMMU J004252.5+411540 imply that it is almost certainly an accreting white dwarf, steadily burning hydrogen-rich material on its surface. We interpret X-ray pulsations as a signature of the strong magnetic field of the rotating white dwarf. Assuming that the X-ray source is powered by disk accretion, we estimate its surface field strength to be in the range 4e5 G <B_{0}<8e6 G. XMMU J004252.5+411540 is the second supersoft X-ray source in M31 showing coherent pulsations, after the transient supersoft source XMMU J004319.4+411758 with 865.5 s pulsation period.
We report on the discovery of pulsations at a period of ~47 s in the persistent X-ray source 1RXS J225352.8+624354 (1RXS J2253) using five Chandra observations performed in 2009. The signal was also detected in Swift and ROSAT data, allowing us to infer over a 16-yr baseline an average, long-term period increasing rate of ~17 ms per year and therefore to confirm the signal as the spin period of an accreting, spinning-down neutron star. The pulse profile of 1RXS J2253 (~50-60% pulsed fraction) is complex and energy independent (within the statistical uncertainties). The 1-10 keV Chandra spectra are well fit by an absorbed power-law model with photon index ~1.4 and observed flux of (2-5)e-12 erg cm^-2 s^-1. The source was also detected by INTEGRAL in the 17-60 keV band at a persistent flux of ~6e-12 erg cm^-2 s^-1, implying a spectral cut off around 15 keV. We also carried out optical spectroscopic follow-up observations of the 2MASS counterpart at the Nordic Optical Telescope. This made it possible to first classify the companion of 1RXS J2253 as a B0-1III-Ve (most likely a B1Ve) star at a distance of about 4-5 kpc (favouring an association with the Perseus arm of the Galaxy). The latter finding implies an X-ray luminosity of ~3e34 erg s^-1, suggesting that 1RXS J2253 is a new member of the sub-class of low-luminosity long-orbital-period persistent Be/X-ray pulsars in a wide and circular orbit (such as X Persei).
We report the detection of coherent pulsations from the ultraluminous X-ray source NGC 7793 P13. The ~0.42s nearly sinusoidal pulsations were initially discovered in broadband X-ray observations using XMM-Newton and NuSTAR taken in 2016. We subsequently also found pulsations in archival XMM-Newton data taken in 2013 and 2014. The significant (>>5 sigma) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star with an observed peak luminosity of ~1e40 erg/s (assuming isotropy), well above the Eddington limit for a 1.4 M_sun accretor. This makes P13 the second ultraluminous X-ray source known to be powered by an accreting neutron star. The pulse period varies between epochs, with a slow but persistent spin up over the 2013-2016 period. This spin-up indicates a magnetic field of B ~ 1.5e12 G, typical of many accreting pulsars. The most likely explanation for the extreme luminosity is a high degree of beaming, however this is difficult to reconcile with the sinusoidal pulse profile.
We report the detection of weak pulsations from the archetypal ultraluminous X-ray source (ULX) NGC 1313 X-2. Acceleration searches reveal sinusoidal pulsations in segments of two out of six new deep observations of this object, with a period of $sim$ 1.5 s and a pulsed fraction of $sim$ 5%. We use Monte Carlo simulations to demonstrate that the individual detections are unlikely to originate in false Poisson noise detections given their very close frequencies; their strong similarity to other pulsations detected from ULXs also argues they are real. The presence of a large bubble nebula surrounding NGC 1313 X-2 implies an age of order 1 Myr for the accreting phase of the ULX, which implies that the neutron stars magnetic field has not been suppressed over time by accreted material, nor has the neutron star collapsed into a black hole, despite an average energy output into the nebula two orders of magnitude above Eddington. This argues that most of the accreted material has been expelled over the lifetime of the ULX, favouring physical models including strong winds and/or jets for neutron star ULXs.
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