A low-level accretion flare during the quiescent state of the neutron-star X-ray transient SAX J1750.8-2900

We report on a series of Swift/XRT observations, performed between February and 22 March 2012, during the quiescent state of the neutron-star X-ray binary SAX J1750.8-2900. In these observations, the source was either just detected or undetected, depending on the exposure length (which ranged from ~0.3 to ~3.8 ks). The upper limits for the non-detections were consistent with the detected luminosities (when fitting a thermal model to the spectrum) of ~1E34 erg/s (0.5-10 keV). This level is consistent with what has been measured previously for this source in quiescence. However, on March 17 the source was found to have an order of magnitude larger count rate. When fitting the flare spectrum with an absorbed power-law model, we obtained a flare luminosity of (3-4) 1E34 erg/s (0.5-10 keV). Follow-up Swift observations showed that this flare lasted <16 days. This event was very likely due to a brief episode of low-level accretion onto the neutron star and provides further evidence that the quiescent state of neutron-star X-ray transients might not be as quiet as is generally assumed. The detection of this low-level accretion flare raises the question whether the quiescent emission of the source (outside the flare) could also be due to residual accretion, albeit continuous instead of episodic. However, we provide arguments which would suggest that the lowest intensity level might instead represent the cooling of the accretion-heated neutron star.Comment: Accepted for publication in MNRAS Main Journal on June 18th, 2013. Minor changes to the original submission to incorporate the comments of the refere

Meta-stable low-level accretion rate states or neutron star crust cooling in the Be/X-ray transients V0332+53 and 4U 0115+63

The Be/X-ray transients V0332+53 and 4U 0115+63 exhibited giant, type-II outbursts in 2015. Here we present Swift/XRT follow-up observations at the end of those outbursts. Surprisingly, the sources did not decay back to their known quiescent levels but stalled at a (slowly decaying) meta-stable state with luminosities ~10 times that observed in quiescence. The spectra in these states are considerably softer than the outburst spectra and appear to soften in time when the luminosity decreases. The physical mechanism behind these meta-stable states is unclear and they could be due to low-level accretion (either directly onto the neutron stars or onto their magnetospheres) or due to cooling of the accretion-heated neutron star crusts. Based on the spectra, the slowly decreasing luminosities, and the spectral softening, we favour the crust cooling hypothesis but we cannot exclude the accretion scenarios. On top of this meta-stable state, weak accretion events were observed that occurred at periastron passage and may thus be related to regular type-I outbursts.Comment: Accepted for publication in MNRAS Letter

Cooling of Accretion-Heated Neutron Stars

We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.Comment: Has appeared in Journal of Astrophysics and Astronomy special issue on 'Physics of Neutron Stars and Related Objects', celebrating the 75th birth-year of G. Srinivasan. In case of missing sources and/or references in the tables, please contact the first author and they will be included in updated versions of this revie

Hard state neutron star and black hole X-ray binaries in the radio:X-ray luminosity plane

Motivated by the large body of literature around the phenomenological properties of accreting black hole (BH) and neutron star (NS) X-ray binaries in the radio:X-ray luminosity plane, we carry out a comparative regression analysis on 36 BHs and 41 NSs in hard X-ray states, with data over 7 dex in X-ray luminosity for both. The BHs follow a radio to X-ray (logarithmic) luminosity relation with slope $\beta=0.59\pm0.02$, consistent with the NSs' slope ($\beta=0.44^{+0.05}_{-0.04}$) within 2.5$\sigma$. The best-fitting intercept for the BHs significantly exceeds that for the NSs, cementing BHs as more radio loud, by a factor $\sim$22. \This discrepancy can not be fully accounted for by the mass or bolometric correction gap, nor by the NS boundary layer contribution to the X-rays, and is likely to reflect physical differences in the accretion flow efficiency, or the jet powering mechanism. Once importance sampling is implemented to account for the different luminosity distributions, the slopes of the non-pulsating and pulsating NS subsamples are formally inconsistent ($>3\sigma$), unless the transitional millisecond pulsars (whose incoherent radio emission mechanism is not firmly established) are excluded from the analysis. We confirm the lack of a robust partitioning of the BH data set into separate luminosity tracks.Comment: Accepted by MNRAS as a Lette

A cooling neutron star crust after recurrent outbursts: Modelling the accretion outburst history of Aql X-1

With our neutron star crust cooling code {\tt NSCool} we track the thermal evolution of the neutron star in Aql X-1 over the full accretion outburst history from 1996 until 2015. For the first time, we model many outbursts (23 outbursts were detected) collectively and in great detail. This allows us to investigate the influence of previous outbursts on the internal temperature evolution and to test different neutron star crust cooling scenarios. Aql X-1 is an ideal test source for this purpose, because it shows frequent, short outbursts and thermally dominated quiescence spectra. The source goes into outburst roughly once a year for a few months. Assuming that the quiescent {\it Swift}/XRT observations of Aql X-1 can be explained within the crust cooling scenario (Waterhouse et al. 2016), we find three main conclusions. Firstly, the data are well reproduced by our model if the envelope composition and shallow heating parameters are allowed to change between outbursts. This is not the case if both shallow heating parameters (strength and depth) are tied throughout all accretion episodes, supporting earlier results that the properties of the shallow heating mechanism are not constant between outbursts. Second, from our models shallow heating could not be connected to one specific spectral state during outburst. Third, and most importantly, we find that the neutron star in Aql X-1 does not have enough time between outbursts to cool down to crust-core equilibrium and that heating during one outburst influences the cooling curves of the next.Comment: 20 pages, 8 figures, 4 tables, accepted for publication in MNRA

The X-ray properties of Be/X-ray pulsars in quiescence

Observations of accreting neutron stars (NS) with strong magnetic fields can be used not only for studying the accretion flow interaction with NS magnetospheres, but also for understanding the physical processes inside NSs and for estimating their fundamental parameters. Of particular interest are (i) the interaction of a rotating neutron star (magnetosphere) with the in-falling matter at different accretion rates, and (ii) the theory of deep crustal heating and the influence of a strong magnetic field on this process. Here, we present results of the first systematic investigation of 16 X-ray pulsars with Be optical companions during their quiescent states, based on data from the Chandra, XMM-Newton and Swift observatories. The whole sample of sources can be roughly divided into two distinct groups: i) relatively bright objects with a luminosity around ~10^34 erg/s and (hard) power-law spectra, and ii) fainter ones showing thermal spectra. X-ray pulsations were detected from five objects in group i) with quite a large pulse fraction of 50-70 per cent. The obtained results are discussed within the framework of the models describing the interaction of the in-falling matter with the neutron star magnetic field and those describing heating and cooling in accreting NSs.Comment: 18 pages, 4 figures, 3 tables, accepted by MNRA

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 (NSs) in LMXBs are heated due to the compression of the existing crust by the accreted matter which gives rise to 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. We discuss if NSs in very-faint X-ray transients (VFXTs; those which have peak X-ray luminosities < 1E36 erg/s) can be used to test this model. Unfortunately we cannot conclusively answer this 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 important because it can be so low that the NSs might not have accreted enough matter to become massive enough that enhanced cooling processes become active. Therefore, they could be relatively warm compared to other systems for which such enhanced cooling processed 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 thus very likely decreasing the amount of heat deposited in the crust. Furthermore, better understanding is needed how a hybrid crust affects other properties such as the thermal conductivity. We also show that some individual NS LMXBs might have hybrid crusts as well as the NSs in HMXBs. This has to be taken into account when studying the cooling properties of those systems when they are in quiescence. We show that the VFXTs are likely not the dominate transients that are associated with the brightest low-luminosity X-ray sources in globular clusters as was hypothesized.Comment: Accepted for publication in MNRAS main journa

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 = 54 +/- 2 eV (assuming D=10 kpc). From this, we infer a core temperature in the range 3.5E7-8.3E7 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.Comment: accepted for publication in ApJL, 4 pages, 1 figure

The Quiescent X-ray Spectrum of Accreting Black Holes

The quiescent state is the dominant accretion mode for black holes on all mass scales. Our knowledge of the X-ray spectrum is limited due to the characteristic low luminosity in this state. Herein, we present an analysis of the sample of dynamically-confirmed stellar-mass black holes observed in quiescence in the \textit{Chandra/XMM-Newton/Suzaku} era resulting in a sample of 8 black holes with $\sim$ 570 ks of observations. In contrast to the majority of AGN where observations are limited by contamination from diffuse gas, the stellar-mass systems allow for a clean study of the X-ray spectrum resulting from the accretion flow alone. The data are characterized using simple models. We find a model consisting of a power-law or thermal bremsstrahlung to both provide excellent descriptions of the data, where we measure $\rm \Gamma = 2.06 \pm 0.03$ and $\rm kT = 5.03^{+0.33}_{-0.31} keV$ respectively in the 0.3 -- 10 keV bandpass, at a median luminosity of $\rm L_x \sim 5.5\times10^{-7} L_{Edd}$. This result in discussed in the context of our understanding of the accretion flow onto stellar and supermassive black holes at low luminosities.Comment: 12 pages, 5 figures, 2 tables, MNRAS accepte

A search for near-infrared counterparts of two faint neutron star X-ray transients : XMMU J174716.1-281048 and SAX J1806.5-2215

We present our near-infrared (NIR) imaging observations of two neutron star low mass X-ray binaries XMMU J174716.1-281048 and SAX J1806.5-2215 obtained using the PANIC instrument on the 6.5-meter Magellan telescope and the WHIRC instrument on the 3.5-meter WIYN telescope respectively. Both sources are members of the class of faint to very-faint X-ray binaries and undergo very long X-ray outburst, hence classified as `quasi persistent X-ray binaries'. While XMMU J174716.1-281048 was active for almost 12 years between 2003 and 2015, SAX J1806.5-2215 has been active for more than 5 years now since 2011. From our observations, we identify two NIR stars consistent with the Chandra X-ray error circle of XMMU J174716.1-281048. The comparison of our observations with the UKIRT Galactic plane observations taken during the same outburst, color-color diagram analysis and spectral energy distribution suggest that both stars are probably a part of the field population and are likely high mass stars. Hence possibly neither of the two stars is a true NIR counterpart. For the faint X-ray binary SAX J1806.5-2215 during its current outburst, we detected a NIR star in our K band WIYN observations consistent with its Chandra error circle. The identified NIR star was not detected during the UKIRT observations taken during its quiescent state. The comparison of two observations suggest that there was an increase in flux by at least one magnitude of the detected star during our observations, hence suggests the detection of the likely counterpart of SAX J1806.5-2215.Comment: 9 pages, 4 figures, accepted for publication in MNRA