The existence of a 'density cusp'-a localized increase in number-of stellar-mass black holes near a supermassive black hole is a fundamental prediction of galactic stellar dynamics. The best place to ...detect such a cusp is in the Galactic Centre, where the nearest supermassive black hole, Sagittarius A*, resides. As many as 20,000 black holes are predicted to settle into the central parsec of the Galaxy as a result of dynamical friction; however, so far no density cusp of black holes has been detected. Low-mass X-ray binary systems that contain a stellar-mass black hole are natural tracers of isolated black holes. Here we report observations of a dozen quiescent X-ray binaries in a density cusp within one parsec of Sagittarius A*. The lower-energy emission spectra that we observed in these binaries is distinct from the higher-energy spectra associated with the population of accreting white dwarfs that dominates the central eight parsecs of the Galaxy. The properties of these X-ray binaries, in particular their spatial distribution and luminosity function, suggest the existence of hundreds of binary systems in the central parsec of the Galaxy and many more isolated black holes. We cannot rule out a contribution to the observed emission from a population (of up to about one-half the number of X-ray binaries) of rotationally powered, millisecond pulsars. The spatial distribution of the binary systems is a relic of their formation history, either in the stellar disk around Sagittarius A* (ref. 7) or through in-fall from globular clusters, and constrains the number density of sources in the modelling of gravitational waves from massive stellar remnants, such as neutron stars and black holes.
The discovery of a binary neutron star merger (NSM) through both its gravitational wave and electromagnetic emission has revealed these events to be key sites of r-process nucleosynthesis. Here, we ...evaluate the prospects of finding the remnants of Galactic NSMs by detecting the gamma-ray decay lines from their radioactive r-process ejecta. We find that 126Sn, which has several lines in the energy range 415-695 keV and resides close to the second r-process peak, is the most promising isotope, because of its half-life t1/2 = 2.30(14) × 105 yr being comparable to the ages of recent NSMs. Using a Monte Carlo procedure, we predict that multiple remnants are detectable as individual sources by next-generation γ-ray telescopes which achieve sub-MeV line sensitivities of ∼10−8-10−6 γ cm−2 s−1. However, given the unknown locations of the remnants, the most promising search strategy is a systematic survey of the Galactic plane and bulge extending to high Galactic latitudes. Individual known supernova remnants which may be misclassified NSM remnants could also be targeted, especially those located outside the Galactic plane. Detection of a moderate sample of Galactic NSM remnants would provide important clues to unresolved issues such as the production of actinides in NSMs, properties of merging NS binaries, and even help distinguish them from rare supernovae as current Galactic r-process sources. We also investigate the diffuse flux from longer-lived nuclei (e.g., 182Hf) that could in principle trace the Galactic spatial distribution of NSMs over longer timescales, but find that the detection of the diffuse flux appears challenging even with next-generation telescopes.
Abstract
We present an investigation of the quiescent and transient X-ray binaries (XRBs) of the Galactic Center (GC). We extended our Chandra analysis of the non-thermal X-ray sources, located in ...the central parsec, from Hailey et al. (2018), using an additional 4.6 Msec of ACIS-S data obtained in 2012–2018. The individual Chandra spectra of the 12 sources fit to an absorbed power-law model with a mean photon index Γ ≈ 2 and show no Fe emission lines. Long-term variability was detected from nine of them, confirming that a majority are quiescent XRBs. Frequent X-ray monitoring of the GC revealed that the 12 non-thermal X-ray sources, as well as four X-ray transients have shown at most a single outburst over the last two decades. They are distinct from the six known neutron star LMXBs in the GC, which have all undergone multiple outbursts with ≲ 5 year recurrence time on average. Based on the outburst history data of the broader population of X-ray transients, we conclude that the 16 sources represent a population of ∼240–630 tightly bound BH-LMXBs with ∼4−12 hr orbital periods, consistent with the stellar/binary dynamics modeling in the vicinity of Sgr A*. The distribution of the 16 BH-LMXB candidates is disk-like (at 87% CL) and aligned with the nuclear star cluster. Our results have implications for XRB formation and the rate of gravitational wave events in other galactic nuclei.
Abstract
We studied the PeVatron nature of the pulsar wind nebula (PWN) G75.2+0.1 (“Dragonfly”) as part of our NuSTAR observational campaign of energetic PWNe. The Dragonfly is spatially coincident ...with LHAASO J2018+3651, whose maximum photon energy is 0.27 PeV. We detected a compact (radius
1
′
) inner nebula of the Dragonfly without a spectral break in 3–20 keV using NuSTAR. A joint analysis of the inner nebula with archival Chandra and XMM-Newton (XMM) observations yields a power-law spectrum with Γ = 1.49 ± 0.03. Synchrotron burnoff is observed from the shrinkage of the NuSTAR nebula at higher energies, from which we infer the magnetic field in the inner nebula of 24
μ
G at 3.5 kpc. Our analysis of archival XMM data and 13 yr of Fermi-LAT data confirms the detection of an extended (
∼
10
′
) outer nebula in 2–6 keV (Γ = 1.82 ± 0.03) and the nondetection of a GeV nebula, respectively. Using the VLA, XMM, and HAWC data, we modeled a multiwavelength spectral energy distribution of the Dragonfly as a leptonic PeVatron. The maximum injected particle energy of 1.4 PeV from our model suggests that the Dragonfly is likely a PeVatron. Our model prediction of the low magnetic field (2.7
μ
G) in the outer nebula and recent interaction with the host supernova remnant’s reverse shock (4 kyr ago) align with common features of PeVatron PWNe. The origin of its highly asymmetric morphology, pulsar proper motion, PWN–supernova remnant (SNR) interaction, and source distance will require further investigations in the future, including a multiwavelength study using radio, X-ray, and gamma-ray observations.
ABSTRACT Recently, unresolved hard (20-40 keV) X-ray emission has been discovered within the central 10 pc of the Galaxy, possibly indicating a large population of intermediate polars (IPs). Chandra ...and XMM-Newton measurements in the surrounding ∼50 pc imply a much lighter population of IPs with . Here we use broadband NuSTAR observations of two IPs: TV Columbae, which has a fairly typical but widely varying reported mass of - , and IGR J17303-0601, with a heavy reported mass of - . We investigate how varying spectral models and observed energy ranges influences estimated white dwarf mass. Observations of the inner 10 pc can be accounted for by IPs with , consistent with that of the CV population in general and the X-ray observed field IPs in particular. The lower mass derived by Chandra and XMM-Newton appears to be an artifact of narrow energy-band fitting. To explain the (unresolved) central hard X-ray emission (CHXE) by IPs requires an X-ray (2-8 keV) luminosity function (XLF) extending down to at least 5 × 1031 erg s−1. The CHXE XLF, if extended to the surrounding ∼50 pc observed by Chandra and XMM-Newton, requires that at least ∼20%-40% of the ∼9000 point sources are IPs. If the XLF extends just a factor of a few lower in luminosity, then the vast majority of these sources are IPs. This is in contrast to recent observations of the Galactic ridge, where the bulk of the 2-8 keV emission is ascribed to non-magnetic CVs.
Abstract
We report timing and broadband spectral analysis of a Galactic X-ray source, CXOGBS J174517.0−321356 (J1745), with a 614 s periodicity. Chandra discovered the source in the direction of the ...Galactic Bulge. Gong proposed that J1745 was either an intermediate polar (IP) with a mass of ∼1
M
⊙
, or an ultracompact X-ray binary (UCXB). To confirm J1745's nature, we jointly fit XMM-Newton and NuSTAR spectra, ruling out a UCXB origin. We have developed a physically realistic model that considers a finite magnetosphere radius, X-ray absorption from the preshock region, and reflection from the white-dwarf (WD) surface to properly determine the IP properties, especially its WD mass. To assess systematic errors on the WD mass measurement, we consider a broad range of specific accretion rates (
m
̇
=
0.6
–
44
g cm
−2
s
−1
) based on the uncertain source distance (
d
= 3–8 kpc) and fractional accretion area (
f
= 0.001–0.025). Our model properly implements the fitted accretion column height in the X-ray reflection model and accounts for the underestimated mass accretion rate due to the (unobserved) soft X-ray blackbody and cyclotron cooling emissions. We found that the lowest accretion rate of
m
̇
= 0.6 g cm
−2
s
−1
, which corresponds to the nearest source distance and maximum
f
value, yields a WD mass of (0.92 ± 0.08)
M
⊙
. On the other hand, as long as the accretion rate is
m
̇
≳
3
g cm
−2
s
−1
, the WD mass is robustly measured to be (0.81 ± 0.06)
M
⊙
, nearly independent of
m
̇
. The derived WD mass range is consistent with the mean WD mass of nearby IPs. Assuming spin equilibrium between the WD and accretion disk, we constrained the WD magnetic field to
B
≳ 7 MG, indicating that it could be a highly magnetized IP. Our analysis presents the most comprehensive methodology for constraining the WD mass and
B
field of an IP by consolidating the effects of cyclotron cooling, finite magnetospheric radius, and accretion column height.
Aims. For many years it had been claimed that the Galactic ridge X-ray emission at the Galactic Center (GC) is truly diffuse in nature. However, with the advancement of modern X-ray satellites, it ...has been found that most of the diffuse emission actually comprises thousands of previously unresolved X-ray point sources. Furthermore, many studies suggest that a vast majority of these X-ray point sources are magnetic cataclysmic variables (CVs) and active binaries. One unambiguous way to identify these magnetic CVs and other sources is by detecting their X-ray periodicity. Therefore, we systematically searched for periodic X-ray sources in the inner Galactic disk, including the GC region. Methods. We used data from our ongoing XMM-Newton Heritage Survey of the inner Galactic disk (350° ≲ l ≲ +7° and −1° ≲ b ≲ +1°) plus archival XMM-Newton observations of the GC. We computed the Lomb-Scargle periodogram for the soft (0.2–2 keV), hard (2–10 keV), and total (0.2–10 keV) band light curves to search for periodicities. Furthermore, we modeled the power spectrum using a power-law model to simulate 1000 artificial light curves and estimate the detection significance of the periodicity. We fitted the energy spectra of the sources using a simple power-law model plus three Gaussians, at 6.4, 6.7, and 6.9 keV, for the iron K emission complex. Results. We detected periodicity in 26 sources. For 14 of them, this is the first discovery of periodicity. For the other 12 sources, we found periods similar to those already known, indicating no significant period evolution. The intermediate polar (IP) type sources display relatively hard spectra compared to polars. We also searched for the Gaia counterparts of the periodic sources to estimate their distances using the Gaia parallax. We found a likely Gaia counterpart for seven sources. Conclusions. Based on the periodicity, hardness ratio, and the equivalent width of Fe K line emission, we have classified the sources into four categories: IPs, polars, neutron star X-ray binaries, and unknown. Of the 14 sources for which we detect the periodicity for the first time, four are likely IPs, five are likely polars, two are neutron star X-ray binaries, and three are of an unknown nature.
We present NuSTAR spectral and timing studies of the supergiant fast X-ray transient (SFXT) IGR J17544-2619. The spectrum is well described by an ~1 keV blackbody and a hard continuum component, as ...expected from an accreting X-ray pulsar. We detect a cyclotron line at 17 keV, confirming that the compact object in IGR J17544-2619 is indeed a neutron star. This is the first measurement of the magnetic field in an SFXT. The inferred magnetic field strength, B = (1.45 plus or minus 0.03) x 10... G (1 + z) is typical of neutron stars in X-ray binaries, and rules out a magnetar nature for the compact object. We do not find any significant pulsations in the source on time-scales of 1-2000 s. (ProQuest: ... denotes formulae/symbols omitted.)
One of the most unique phenomena in the Galactic center region is the existence of numerous long and narrow filamentary structures within a few hundred parsecs of Sgr A . While more than 100 radio ...filaments have been revealed by MeerKAT, only about two dozen X-ray filaments have been discovered so far. In this article, we report our analysis of deep Chandra and NuSTAR observations of a nonthermal X-ray filament, G0.13-0.11, which is located adjacent to the Radio Arc. Chandra revealed a unique morphology of G0.13-0.11, which is an elongated (0.1 pc in width and 3.2 pc in length) structure slightly bent toward the Radio Arc. A pulsar candidate (Γ ∼ 1.4) is detected in the middle of the filament, with a tail of diffuse nonthermal X-ray emission on one side of the filament. The filament is detected by NuSTAR up to 79 keV, with the hard X-ray centroid consistent with the pulsar candidate. We found that the X-ray intensity decays along the filament farther away from the pulsar candidate, dropping to half of its peak value 2.2 pc away. This system is most likely a pulsar wind nebula (PWN) interacting with the ambient interstellar magnetic field, where the filaments are kinetic jets from the PWN as recently proposed. The nature of this filament adds to the complex origin of X-ray filaments, which serve as powerful tools for probing local and global powerful particle accelerators in the Galactic center.
ABSTRACT The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by the Nuclear Spectroscope Telescope Array in the 3-79 keV energy band for a net exposure time of 50 ks. ...We present the results of this observation which enabled the discovery of a cyclotron resonant scattering feature with a centroid energy of keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5 0.1) × 1012 G. The known pulsation period is now observed at 904.0 0.1 s. Since 2006, the neutron star has undergone a long-term spin-up trend at a rate of s s−1 (−0.6 s per year, or a frequency derivative of Hz s−1). In the power density spectrum, a break appears at the pulse frequency which separates the zero slope at low frequency from the steeper slope at high frequency. This addition of angular momentum to the neutron star could be due to the accretion of a quasi-spherical wind, or it could be caused by the transient appearance of a prograde accretion disk that is nearly in corotation with the neutron star whose magnetospheric radius is around 2 × 108 cm.
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