N. Degenaar

DISCOVERY, PROGENITOR AND EARLY EVOLUTION OF A STRIPPED ENVELOPE SUPERNOVA iPTF13bvn

The intermediate Palomar Transient Factory reports our discovery of a young supernova, iPTF13bvn, in the nearby galaxy, NGC 5806 (22.5 Mpc). Our spectral sequence in the optical and infrared suggests a Type Ib classification. We identify a blue progenitor candidate in deep pre-explosion imaging within a 2σ error circle of 80 mas (8.7 pc). The candidate has an M_B luminosity of −5.52 ± 0.39 mag and a B − I color of 0.25 ± 0.25 mag. If confirmed by future observations, this would be the first direct detection for a progenitor of a Type Ib. Fitting a power law to the early light curve, we find an extrapolated explosion date around 0.6 days before our first detection. We see no evidence of shock cooling. The pre-explosion detection limits constrain the radius of the progenitor to be smaller than a few solar radii. iPTF13bvn is also detected in centimeter and millimeter wavelengths. Fitting a synchrotron self-absorption model to our radio data, we find a mass-loading parameter of 1.3×10^(12) g cm^(−1). Assuming a wind velocity of 10^3 km s^(−1), we derive a progenitor mass-loss rate of 3 × 10^(−5) M☉ yr^(−1). Our observations, taken as a whole, are consistent with a Wolf–Rayet progenitor of the supernova iPTF13bvn.

A CHANDRA/HETGS CENSUS OF X-RAY VARIABILITY FROM Sgr A* DURING 2012

We present the first systematic analysis of the X-ray variability of Sgr A ∗ during the Chandra X-ray Observatory’s 2012 Sgr A ∗ X-ray Visionary Project. With 38 High Energy Transmission Grating Spectrometer observations spaced an average of 7 days apart, this unprecedented campaign enables detailed study of the X-ray emission from this supermassive black hole at high spatial, spectral and timing resolution. In 3 Ms of observations, we detect 39 X-ray flares from Sgr A ∗ , lasting from a few hundred seconds to approximately 8 ks, and ranging in 2–10 keV luminosity from ∼10 34 erg s −1 to 2 × 10 35 erg s −1 . Despite tentative evidence for a gap in the distribution of flare peak count rates, there is no evidence for X-ray color differences between faint and bright flares. Our preliminary X-ray flare luminosity distribution dN/dL is consistent with a power law with index −1.9 +0.3 −0.4 ; this is similar to some estimates of Sgr A ∗ ’s near-IR flux distribution. The observed flares contribute one-third of the total X-ray output of Sgr A ∗ during the campaign, and as much as 10% of the quiescent X-ray emission could be comprised of weak, undetected flares, which may also contribute high-frequency variability. We argue that flares may be the only source of X-ray emission from the inner accretion flow.

The behavior of subluminous X-ray transients near the Galactic center as observed using the X-ray telescope aboard Swift

In this paper we report on the spectral analysis of seven X-ray transients, which were found to be active during a monitoring campaign of the Galactic center carried out in 2006 and 2007 using the X-ray telescope aboard the Swift satellite. This campaign detected new outbursts of five known X-ray transients and discovered two new systems. Their 2-10 keV peak luminosities range from similar to 10(34) to 6 x 10(36) erg s(-1), which implies that all seven X-ray transients are subluminous compared to the bright X-ray transients that have peak luminosities of 10(37-39) erg s(-1). Two of the sources discussed in this paper are confirmed neutron star systems (AX J1745.6-2901 and GRS 1741-2853), while the five others have an unknown nature. We discuss the characteristics of the observed outbursts and the duty cycles of the various systems. Several of the detected transients seem to undergo enhanced X-ray activity with levels intermediate between quiescence and full outburst. We discuss the possibility that the subluminous appearance of the eclipsing X-ray burster AX J1745.6-2901 is due to line-of-sight effects. We detected two type-I X-ray bursts with a duration of 50-60 s from AX J1745.6-2901, which we discuss in view of the bursting behavior of low-luminosity X-ray transients. Assuming that we are dealing with accreting neutron stars and black holes, we estimate the time-average accretion rate, , of the transients, which is an important input parameter for binary evolution models that attempt to explain the nature of subluminous X-ray transients. Our estimates lie in the range of 3 x 10(-13) M-circle dot yr(-1) less than or similar to less than or similar to 1 x 10(-10) M-circle dot yr(-1), if the systems are neutron star X-ray binaries and between 4 x 10(-14) M-circle dot yr(-1) less than or similar to less than or similar to 2 x 10(-11) M-circle dot yr(-1) for a scenario where the accreting object is a black hole. Some of the systems have such low estimated mass-accretion rates that they possibly pose a challenge for binary evolution models.

The Variable Quiescent X-Ray Emission of the Transient Neutron Star XTE J1701–462

We present the results of continued monitoring of the quiescent neutron star low-mass X-ray binary XTE J1701-462 with Chandra and Swift. A new Chandra observation from 2010 October extends our tracking of the neutron star surface temperature from ~800 days to ~1160 days since the end of an exceptionally luminous 19 month outburst. This observation indicates that the neutron star crust may still be slowly cooling toward thermal equilibrium with the core; another observation further into quiescence is needed to verify this. The shape of the overall cooling curve is consistent with that of a broken power law, although an exponential decay to a constant level cannot be excluded with the present data. To investigate possible low-level activity, we conducted a monitoring campaign of XTE J1701-462 with Swift during 2010 April-October. Short-term flares - presumably arising from episodic low-level accretion - were observed up to a luminosity of ~1e35 erg/s, ~20 times higher than the normal quiescent level. We conclude that flares of this magnitude are not likely to have significantly affected the equilibrium temperature of the neutron star and are probably not able to have a measurable impact on the cooling curve. However, it is possible that brighter and longer periods of low-level activity have had an appreciable effect on the equilibrium temperature.

Cooling of the crust in the neutron star low-mass X-ray binary MXB 1659-29

In quasi-persistent neutron star transients, long outbursts cause the neutron star crust to be heated out of thermal equilibrium with the rest of the star. During quiescence, the crust then cools back down. Such crustal cooling has been observed in two quasi-persistent sources: KS 1731-260 and MXB 1659-29. Here we present an additional Chandra observation of MXB 1659-29 in quiescence, which extends the baseline of monitoring to 6.6 yr after the end of the outburst. This new observation strongly suggests that the crust has thermally relaxed, with the temperature remaining consistent over 1000 days. Fitting the temperature cooling curve with an exponential plus constant model we determine an e-folding timescale of 465 +/- 25 days, with the crust cooling to a constant surface temperature of kT(eff)(infinity) = 54 +/- 2 eV (assuming D = 10 kpc). From this, we infer a core temperature in the range (3.5-8.3) x 10(7) K (assuming D = 10 kpc), with the uncertainty due to the surface composition. Importantly, we tested two neutron star atmosphere models as well as a blackbody model, and found that the thermal relaxation time of the crust is independent of the chosen model and the assumed distance.

Constraining the physics of the r-mode instability in neutron stars with X-ray and ultraviolet observations

Rapidly rotating neutron stars in low-mass X-ray binaries may be an interesting source of gravitational waves (GWs). In particular, several modes of stellar oscillation may be driven unstable by GW emission, and this can lead to a detectable signal. Here we illustrate how current X-ray and ultraviolet observations can constrain the physics of the r-mode instability. We show that the core temperatures inferred from the data would place many systems well inside the unstable region predicted by standard physical models. However, this is at odds with theoretical expectations. We discuss different mechanisms that could be at work in the stellar interior, and we show how they can modify the instability window and make it consistent with the inferred temperatures.

A four-year baseline Swift study of enigmatic X-ray transients located near the Galactic center

We report on continued monitoring observations of the Galactic center carried out by the X-ray telescope aboard the Swift satellite in 2008 and 2009. This campaign revealed activity of the five known X-ray transients AX J1745.6‐2901, CXOGC J174535.5‐290124, GRS 1741‐2853, XMM J174457‐2850.3 and CXOGC J174538.0‐290022. All these sources are known to undergo very faint X-ray outbursts with 2‐10 keV peak luminosities of LX,peak ∼ 10 34−36 erg s −1 , although the two confirmed neutron star low-mass X-ray binaries AX J1745.6‐2901 and GRS 1741‐2853 can also become brighter (LX,peak ∼ 10 36−37 erg s −1 ). We discuss the observed longterm lightcurves and X-ray spectra of these five enigmatic transients. In 2008, AX J1745.6‐2901 returned to quiescence following an unusually long accretion outburst of more than 1.5 years. GRS 1741‐2853 was active in 2009 and displayed the brightest outburst ever recorded for this source, reaching up to a 2‐10 keV luminosity of LX ∼ 1 ×10 37 (D/7.2 kpc) 2 erg s −1 . This system appears to undergo recurrent accretion outbursts approximately every 2 years. Furthermore, we find that the unclassified transient XMM J174457‐2850.3 becomes bright only during short episodes (days) and is often found active inbetween quiescence (LX ∼ 10 32 erg s −1 ) and its maximum outburst luminosity of LX ∼ 10 36 erg s −1 . CXOGC J174535.5‐290124 and CXOGC J174538.0‐290022, as well as three other veryfaint X-ray transients that were detected by Swift monitoring observations in 2006, have very low time-averaged mass-accretion rates of � ˙

Strong X-ray variability in the quiescent state of the neutron star low-mass X-ray binary EXO 1745-248

The transient neutron star low-mass X-ray binary EXO 1745−248, located in the globular cluster Terzan 5, was detected during its quiescent state with Chandra in 2003. The source displayed a 0.5-10 keV quiescent X-ray luminosity of Lq∼ 1033 (D/5.5 kpc)2 erg s−1, which was completely dominated by hard non-thermal emission. This is at odds with other non-pulsating neutron stars that typically show detectable soft thermal emission at such quiescent luminosities. Here we use three additional Chandra observations, performed in 2009 and 2011, to further study the quiescent properties of EXO 1745−248. We find that the power-law intensity varies considerably up to a factor of ∼3 within hours and by about one order of magnitude between the different epochs. We discuss the implications of the observed change in quiescent flux for the interpretation of the hard power-law emission. We constrain the neutron star surface temperature as seen by a distant observer to kT∞≲ 42 eV and the thermal bolometric luminosity to Lthq, bol≲ 7 × 1031 (D/5.5 kpc)2 erg s−1. This confirms that EXO 1745−248 harbours a relatively cold neutron star and suggests that, for example, enhanced cooling mechanisms are operating in the stellar core or that the binary on average resides in quiescence for hundreds of years.

Testing the deep-crustal heating model using quiescent neutron-star very-faint X-ray transients and the possibility of partially accreted crusts in accreting neutron stars

It is assumed that accreting neutron stars in low-mass X-ray binaries are heated due to the compression of the existing crust by the freshly accreted matter which gives rise to a variety of nuclear reactions in the crust. It has been shown that most of the energy is released deep in the crust by pycnonuclear reactions involving low-Z elements (the deep-crustal heating scenario). In this paper we discuss if neutron stars in the so-called very-faint X-ray transients (VFXTs; those transients have outburst peak 2–10 keV X-ray luminosities <1 × 10 36 erg s −1 ) can be used to test this deep-crustal heating model. We demonstrate that such systems would indeed be very interesting objects to test the deep-crustal heating model with, but that the interpretation of the results might be challenging because of the large uncertainties in our estimates of the accretion rate history of those VFXTs, both the short-term (less than a few tens of thousands of years) and the one throughout their lifetime. The latter is particularly important because it can be so low that the neutron stars might not have accreted enough matter to become massive enough that enhanced core cooling processes become active. Therefore, they could be relatively warm compared to other systems for which such enhanced cooling processes have been inferred. However, the amount of matter can also not be too low because then the crust might not have been replaced significantly by accreted matter and thus a hybrid crust of partly accreted and partly original, albeit further compressed matter, might be present. This would inhibit the full range of pycnonuclear reactions to occur and therefore possibly decrease the amount of heat deposited in the crust. More detailed calculations of the heating and cooling properties of such hybrid crusts have to be performed to be conclusive. Furthermore, better understanding is needed about how a hybrid crust affects other properties such as the thermal conductivity. A potentially interesting way to observe the effects of a hybrid crust on the heating and cooling of an accreting neutron star is to observe the crust cooling of such a neutron star after a prolonged (years to decades) accretion episode and compare the results with similar studies performed for neutron stars with a fully accreted crust. We also show that some individual neutron-star low-mass X-ray binaries might have hybrid crusts as well as possibly many of the neutron stars in high-mass X-ray binaries. This has to be taken into account when studying the cooling properties of those systems when they are in quiescence. In addition, we show that the VFXTs are likely not the dominate transients that are associated with the brightest (∼10 33 erg s −1 ) low-luminosity X-ray sources in globular clusters as was

Continued Cooling of the Crust in the Neutron Star Low-mass X-ray Binary KS 1731?260

Some neutron star low-mass X-ray binaries have very long outbursts (lasting several years) which can generate a significant amount of heat in the neutron star crust. After the system has returned to quiescence, the crust then thermally relaxes. This provides a rare opportunity to study the thermal properties of neutron star crusts, putting constraints on the thermal conductivity and hence the structure and composition of the crust. KS 1731-260 is one of only four systems where this crustal cooling has been observed. Here, we present a new Chandra observation of this source approximately eight years after the end of the last outburst and four years since the last observation. We find that the source has continued to cool, with the cooling curve displaying a simple power-law decay. This suggests that the crust has not fully thermally relaxed yet and may continue to cool further. A simple power-law decay is in contrast to theoretical cooling models of the crust, which predict that the crust should now have cooled to the same temperature as the neutron star core.