Ultraluminous X-ray sources (ULXs) are a population of extragalactic objects whose luminosity exceeds the Eddington limit for a 10 M black hole (BH). Their properties have been widely interpreted in ...terms of accreting stellar-mass or intermediate-mass BHs. However at least three neutron stars (NSs) have been recently identified in ULXs through the discovery of periodic pulsations. Motivated by these findings we studied the spectral properties of a sample of bright ULXs using a simple continuum model which was extensively used to fit the X-ray spectra of accreting magnetic NSs in the Galaxy. We found that such a model, consisting of a power-law with a high-energy exponential cut-off, fits most of the ULX spectra analyzed here very well, at a level comparable to that of models involving an accreting BH. On these grounds alone we suggest that other non-pulsating ULXs may host NSs. We also found that above 2 keV the spectrum of known pulsating ULXs is harder than that of the majority of the other ULXs of the sample, with only IC 342 X-1 and Ho IX X-1 displaying spectra of comparable hardness. We thus suggest that these two ULXs may host an accreting NS and encourage searches for periodic pulsations in the flux.
We report on the discovery of a new member of the magnetar class, SGR J1935+2154, and on its timing and spectral properties measured by an extensive observational campaign carried out between 2014 ...July and 2015 March with Chandra and XMM–Newton (11 pointings). We discovered the spin period of SGR J1935+2154 through the detection of coherent pulsations at a period of about 3.24 s. The magnetar is slowing down at a rate of
$\dot{P} = 1.43(1)\times 10^{-11}$
s s−1 and with a decreasing trend due to a negative
$\ddot{P}$
of −3.5(7) × 10−19 s s−2. This implies a surface dipolar magnetic field strength of ∼2.2 × 1014 G, a characteristic age of about 3.6 kyr and a spin-down luminosity Lsd ∼1.7 × 1034 erg s−1. The source spectrum is well modelled by a blackbody with temperature of about 500 eV plus a power-law component with photon index of about 2. The source showed a moderate long-term variability, with a flux decay of about 25 per cent during the first four months since its discovery, and a re-brightening of the same amount during the second four months. The X-ray data were also used to study the source environment. In particular, we discovered a diffuse emission extending on spatial scales from about 1 arcsec up to at least 1 arcmin around SGR J1935+2154 both in Chandra and XMM–Newton data. This component is constant in flux (at least within uncertainties) and its spectrum is well modelled by a power-law spectrum steeper than that of the pulsar. Though a scattering halo origin seems to be more probable we cannot exclude that part, or all, of the diffuse emission is due to a pulsar wind nebula.
We report the INTernational Gamma-ray Astrophysics Laboratory (INTEGRAL) detection of the short gamma-ray burst GRB 170817A (discovered by Fermi-GBM) with a signal-to-noise ratio of 4.6, and, for the ...first time, its association with the gravitational waves (GWs) from binary neutron star (BNS) merging event GW170817 detected by the LIGO and Virgo observatories. The significance of association between the gamma-ray burst observed by INTEGRAL and GW170817 is 3.2 , while the association between the Fermi-GBM and INTEGRAL detections is 4.2 . GRB 170817A was detected by the SPI-ACS instrument about 2 s after the end of the GW event. We measure a fluence of (1.4 0.4 0.6) × 10−7 erg cm−2 (75-2000 keV), where, respectively, the statistical error is given at the 1 confidence level, and the systematic error corresponds to the uncertainty in the spectral model and instrument response. We also report on the pointed follow-up observations carried out by INTEGRAL, starting 19.5 hr after the event, and lasting for 5.4 days. We provide a stringent upper limit on any electromagnetic signal in a very broad energy range, from 3 keV to 8 MeV, constraining the soft gamma-ray afterglow flux to <7.1 × 10−11 erg cm−2 s−1 (80-300 keV). Exploiting the unique capabilities of INTEGRAL, we constrained the gamma-ray line emission from radioactive decays that are expected to be the principal source of the energy behind a kilonova event following a BNS coalescence. Finally, we put a stringent upper limit on any delayed bursting activity, for example, from a newly formed magnetar.
Abstract
A few years after its discovery as a magnetar, SGR J1935+2154 started a new burst-active phase on 2020 April 27, accompanied by a large enhancement of its X-ray persistent emission. Radio ...single bursts were detected during this activation, strengthening the connection between magnetars and fast radio bursts. We report on the X-ray monitoring of SGR J1935+2154 from ∼3 days prior to ∼3 weeks after its reactivation, using Swift, the Nuclear Spectroscopic Telescope Array (NuSTAR), and the Neutron Star Interior Composition Explorer (NICER). We detected X-ray pulsations in the NICER and NuSTAR observations, and constrained the spin period derivative to
s s
−1
(3
σ
c.l.). The pulse profile showed a variable shape switching between single and double-peaked as a function of time and energy. The pulsed fraction decreased from ∼34% to ∼11% (5–10 keV) over ∼10 days. The X-ray spectrum was well fit by an absorbed blackbody model with temperature decreasing from
kT
BB
∼ 1.6 to 0.45–0.6 keV, plus a nonthermal power-law component (Γ ∼ 1.2) observed up to ∼25 keV with NuSTAR. The 0.3–10 keV X-ray luminosity increased in less than 4 days from
erg s
−1
to about
erg s
−1
and then decreased again to
erg s
−1
over the following 3 weeks of the outburst, where
d
6.6
is the source distance in units of 6.6 kpc. We also detected several X-ray bursts, with properties typical of short magnetar bursts.
Abstract
Isolated black holes and neutron stars can be revealed through the observation of long-duration gravitational microlensing events. A few candidates have been found in surveys of stars in the ...direction of the Galactic bulge. Recently, thanks to the addition of astrometric information at milliarcsecond level, it has been possible to reduce the uncertainties in the masses and distances for some of these “dark” gravitational lenses and select the most promising candidates. These isolated compact objects might emit X-rays powered by accretion from the interstellar medium. Using data of the Chandra, XMM-Newton, and INTEGRAL satellites, we searched for X-ray emission in the isolated black hole candidate OGLE-2011-BLG-0462, and in several other putative collapsed objects found with gravitational microlensing. OGLE-2011-BLG-0462 has been recently interpreted as a 7.1
M
⊙
black hole at a distance of 1.6 kpc, although a different group obtained a mass range (1.6–4.4
M
⊙
) that cannot exclude a massive neutron star. We have derived upper limits on the flux from OGLE-2011-BLG-0462 of 9 × 10
−15
erg cm
−2
s
−1
in the 0.5–7 keV range and ∼2 × 10
−12
erg cm
−2
s
−1
in the 17–60 keV range. The implied X-ray luminosity is consistent with the small radiative efficiency expected for a black hole and disfavors a neutron star interpretation. Limits down to a factor of about five lower are obtained for the soft X-ray flux of other candidates, but their interpretation is affected by larger uncertainties in the masses, distances, and spatial velocities.
Abstract
The repeating fast radio burst FRB 20200120E is located in a globular cluster belonging to the nearby M81 galaxy. Its small distance (3.6 Mpc) and accurate localization make it an ...interesting target to search for bursting activity at high energies. From 2003 November to 2021 September, the INTEGRAL satellite has obtained an exposure time of 18 Ms on the M81 sky region. We used these data to search for hard X-ray bursts from FRB 20200120E using the IBIS/ISGRI instrument, without finding any significant candidate, down to an average fluence limit of ∼10
−8
erg cm
−2
(20–200 keV). The corresponding limit on the isotropic luminosity for a burst of duration Δ
t
is
∼
10
45
10
ms
Δ
t
erg s
−1
, the deepest limit obtained for an extragalactic FRB in the hard X-ray range. This rules out the emission of powerful flares at a rate higher than 0.1 yr
−1
that could be expected in models invoking young hyperactive magnetars.
We report on International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of the soft γ-ray repeater SGR 1935+2154 performed between 2020 April 28 and May 3. Several short bursts with ...fluence of erg cm−2 were detected by the Imager on-board INTEGRAL (IBIS) instrument in the 20-200 keV range. The burst with the hardest spectrum, discovered and localized in real time by the INTEGRAL Burst Alert System, was spatially and temporally coincident with a short and very bright radio burst detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and Survey for Transient Astronomical Radio Emission 2 (STARE2) radio telescopes at 400-800 MHz and 1.4 GHz, respectively. Its lightcurve shows three narrow peaks separated by ∼29 ms time intervals, superimposed on a broad pulse lasting ∼0.6 s. The brightest peak had a delay of 6.5 1.0 ms with respect to the 1.4 GHz radio pulse (that coincides with the second and brightest component seen at lower frequencies). The burst spectrum, an exponentially cutoff power law with photon index and peak energy , is harder than those of the bursts usually observed from this and other magnetars. By the analysis of an expanding dust-scattering ring seen in X-rays with the Neil Gehrels Swift Observatory X-ray Telescope (XRT) instrument, we derived a distance of kpc for SGR 1935+2154, independent of its possible association with the supernova remnant G57.2+0.8. At this distance, the burst 20-200 keV fluence of erg cm−2 corresponds to an isotropic emitted energy of erg. This is the first burst with a radio counterpart observed from a soft γ-ray repeater and it strongly supports models based on magnetars that have been proposed for extragalactic fast radio bursts.
Low-Magnetic-Field Soft Gamma Repeater Rea, N; Esposito, P; Turolla, R ...
Science (American Association for the Advancement of Science),
11/2010, Letnik:
330, Številka:
6006
Journal Article
Recenzirano
Odprti dostop
Soft gamma repeaters (SGRs) and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, that is, ...neutron stars powered by extreme magnetic fields, B approximately 10¹⁴ to 10¹⁵ gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5 x 10¹² gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages, and evolutionary stages than observed so far.
Abstract
Calvera (1RXS J141256.0+792204) is an isolated neutron star detected only through its thermal X-ray emission. Its location at high Galactic latitude (
b
= +37°) is unusual if Calvera is a ...relatively young pulsar, as suggested by its spin period (59 ms) and period derivative (3.2 × 10
−15
s s
−1
). Using the Neutron Star Interior Composition Explorer, we obtained a phase-connected timing solution spanning four years, which allowed us to measure the second derivative of the frequency
ν
̈
=
−
2.5
×
10
−
23
Hz s
−2
and to reveal timing noise consistent with that of normal radio pulsars. A magnetized hydrogen atmosphere model, covering the entire star surface, provides a good description of the phase-resolved spectra and energy-dependent pulsed fraction. However, we found that a temperature map more anisotropic than that produced by a dipole field is required, with a hotter zone concentrated toward the poles. By adding two small polar caps, we found that the surface effective temperature and that of the caps are ∼0.1 and ∼0.36 keV, respectively. The inferred distance is ∼3.3 kpc. We confirmed the presence of an absorption line at 0.7 keV associated with the emission from the whole star surface, difficult to interpret as a cyclotron feature and more likely originating from atomic transitions. We searched for pulsed
γ
-ray emission by folding seven years of Fermi-LAT data using the X-ray ephemeris, but no evidence for pulsations was found. Our results favor the hypothesis that Calvera is a normal rotation-powered pulsar, with the only peculiarity of being born at a large height above the Galactic disk.
ABSTRACT Observations of PSR B0943+10 with XMM-Newton and the LOFAR, LWA, and Arecibo radio telescopes in 2014 November confirm the synchronous X-ray/radio switching between a radio-bright (B) mode ...and a radio-quiet (Q) mode, in which the X-ray flux is a factor ∼2.4 higher than in the B-mode. We discovered X-ray pulsations during the B-mode (0.5-2 keV pulsed fraction of (38 5)%) and confirm their presence in the Q-mode, where the pulsed fraction increases with energy from ∼20% to ∼65% at 2 keV. We found marginal evidence for an increase in the X-ray pulsed fraction during the B-mode on a timescale of hours. The X-ray spectrum during the Q-mode requires a fit with either a power law plus blackbody or the sum of two blackbodies, while in the B-mode it is well fit by a single blackbody (a single power law is rejected). In the Q-mode, the pulsed emission has a blackbody spectrum with temperature K and the unpulsed emission is a power law with photon index ∼2.5, while during the B-mode both the pulsed and unpulsed emission can be fit by either a blackbody or a power law with similar values of temperature and index. A Chandra image does not show diffuse X-ray emission. These results support a scenario in which unpulsed non-thermal emission, likely magnetospheric, and pulsed thermal emission from a small polar cap (∼1500 m2) with a non-dipolar field (∼1014 G) are present during both modes and vary in a correlated way. This is broadly consistent with the partially screened gap model and does not necessarily imply global magnetospheric rearrangements to explain the mode switching.
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