40 research outputs found
Very hard states in neutron star low-mass X-ray binaries
We report on unusually very hard spectral states in three confirmed
neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and
IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting
the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed
power-law model, we found photon indices of \Gamma ~ 1, significantly lower
than the \Gamma = 1.5 - 2.0 typically seen when such systems are in their so
called hard state. For individual sources very hard spectra were already
previously identified but here we show for the first time that likely our
sources were in a distinct spectral state (i.e., different from the hard state)
when they exhibited such very hard spectra. It is unclear how such very hard
spectra can be formed; if the emission mechanism is similar to that operating
in their hard states (i.e., up-scattering of soft photons due to hot electrons)
then the electrons should have higher temperatures or a higher optical depth in
the very hard state compared to those observed in the hard state. By using our
obtained \Gamma as a tracer for the spectral evolution with luminosity, we have
compared our results with those obtained by Wijnands et al. (2015). We confirm
their general results in that also our sample of sources follow the same track
as the other neutron star systems, although we do not find that the accreting
millisecond pulsars are systematically harder than the non-pulsating systems.Comment: Accepted for publication in MNRA
LOFAR discovery of the fastest-spinning millisecond pulsar in the Galactic field
We report the discovery of PSR J09520607, a 707-Hz binary millisecond
pulsar which is now the fastest-spinning neutron star known in the Galactic
field (i.e., outside of a globular cluster). PSR J09520607 was found using
LOFAR at a central observing frequency of 135 MHz, well below the 300 MHz to 3
GHz frequencies typically used in pulsar searches. The discovery is part of an
ongoing LOFAR survey targeting unassociated Fermi Large Area Telescope
-ray sources. PSR J09520607 is in a 6.42-hr orbit around a very
low-mass companion ( M) and we identify a
strongly variable optical source, modulated at the orbital period of the
pulsar, as the binary companion. The light curve of the companion varies by 1.6
mag from at maximum to , indicating that it is
irradiated by the pulsar wind. Swift observations place a 3- upper
limit on the keV X-ray luminosity of erg
s (using the 0.97 kpc distance inferred from the dispersion measure).
Though no eclipses of the radio pulsar are observed, the properties of the
system classify it as a black widow binary. The radio pulsed spectrum of PSR
J09520607, as determined through flux density measurements at 150 and 350
MHz, is extremely steep with (where ).
We discuss the growing evidence that the fastest-spinning radio pulsars have
exceptionally steep radio spectra, as well as the prospects for finding more
sources like PSR J09520607.Comment: 9 pages, 3 figures, 1 table, published in ApJ letter
Radio and X-ray monitoring of the accreting millisecond X-ray pulsar IGR J17591-2342 in outburst
IGR J175912342 is a new accreting millisecond X-ray pulsar (AMXP) that was
recently discovered in outburst in 2018. Early observations revealed that the
source's radio emission is brighter than that of any other known neutron star
low-mass X-ray binary (NS-LMXB) at comparable X-ray luminosity, and assuming
its likely kpc distance. It is comparably radio bright to black
hole LMXBs at similar X-ray luminosities. In this work, we present the results
of our extensive radio and X-ray monitoring campaign of the 2018 outburst of
IGR J175912342. In total we collected 10 quasi-simultaneous radio (VLA,
ATCA) and X-ray (Swift-XRT) observations, which make IGR J175912342 one of
the best-sampled NS-LMXBs. We use these to fit a power-law correlation index
between observed radio and X-ray luminosities (
). However, our monitoring revealed a
large scatter in IGR J175912342's radio luminosity (at a similar X-ray
luminosity, erg s, and spectral state), with
erg s during the first three
reported observations, and up to a factor of 4 lower during
later radio observations. Nonetheless, the average radio luminosity of IGR
J175912342 is still one of the highest among NS-LMXBs, and we discuss
possible reasons for the wide range of radio luminosities observed in such
systems during outburst. We found no evidence for radio pulsations from IGR
J175912342 in our Green Bank Telescope observations performed shortly after
the source returned to quiescence. Nonetheless, we cannot rule out that IGR
J175912342 becomes a radio millisecond pulsar during quiescence.Comment: 12 pages, 3 figures, 2 tables, accepted for publication in MNRA
Quasi-simultaneous Radio/X-Ray Observations of the Candidate Transitional Millisecond Pulsar 3FGL J1544.6-1125 during its Low-luminosity Accretion-disk State
3FGL J1544.6-1125 is a candidate transitional millisecond pulsar (tMSP).
Similar to the well-established tMSPs - PSR J1023+0038, IGR J18245-2452, and
XSS J12270-4859 -- 3FGL J1544.6-1125 shows -ray emission and discrete
X-ray "low" and "high" modes during its low-luminosity accretion-disk state.
Coordinated radio/X-ray observations of PSR J1023+0038 in its current
low-luminosity accretion-disk state showed rapidly variable radio continuum
emission-possibly originating from a compact, self-absorbed jet, the
"propellering" of accretion material, and/or pulsar moding. 3FGL J1544.6-1125
is currently the only other (candidate) tMSP system in this state, and can be
studied to see whether tMSPs are typically radio-loud compared to other neutron
star binaries. In this work, we present a quasi-simultaneous Very Large Array
and Swift radio/X-ray campaign on 3FGL J1544.6-1125. We detect 10 GHz radio
emission varying in flux density from Jy down to 15
Jy (3 upper limit) at four epochs spanning three weeks. At the
brightest epoch, the radio luminosity is erg s for a quasi-simultaneous X-ray luminosity
erg s (for an assumed distance of 3.8
kpc). These luminosities are close to those of PSR J1023+0038, and the results
strengthen the case that 3FGL J1544.6-1125 is a tMSP showing similar
phenomenology to PSR J1023+0038.Comment: Accepted for publication in the Astrophysical Journa
Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9-342058
Quasi-simultaneous Radio/X-Ray Observations of the Candidate Transitional Millisecond Pulsar 3FGL J1544.6-1125 during its Low-luminosity Accretion-disk State
3FGL J1544.6-1125 is a candidate transitional millisecond pulsar (tMSP). Similar to the well-established tMSPs - PSR J1023+0038, IGR J18245-2452, and XSS J12270-4859 - 3FGL J1544.6-1125 shows γ-ray emission and discrete X-ray "low"and "high"modes during its low-luminosity accretion-disk state. Coordinated radio/X-ray observations of PSR J1023+0038 in its current low-luminosity accretion-disk state showed rapidly variable radio continuum emission - possibly originating from a compact, self-absorbed jet, the "propellering"of accretion material, and/or pulsar moding. 3FGL J1544.6-1125 is currently the only other (candidate) tMSP system in this state, and can be studied to see whether tMSPs are typically radio-loud compared to other neutron star binaries. In this work, we present a quasi-simultaneous Very Large Array and Swift radio/X-ray campaign on 3FGL J1544.6-1125. We detect 10 GHz radio emission varying in flux density from 47.7 ± 6.0 μJy down to ≲15 μJy (3σ upper limit) at four epochs spanning three weeks. At the brightest epoch, the radio luminosity is L 5 GHz = (2.17 ± 0.17) × 1027 erg s-1 for a quasi-simultaneous X-ray luminosity L 2-10 keV = (4.32 ± 0.23) × 1033 erg s-1 (for an assumed distance of 3.8 kpc). These luminosities are close to those of PSR J1023+0038, and the results strengthen the case that 3FGL J1544.6-1125 is a tMSP showing similar phenomenology to PSR J1023+0038.A.J. and J.W.T.H. acknowledge funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement nr. 337062 (DRAGNET). A.J. also acknowledges support from the NuSTAR mission. A.P. acknowledges support from an NWO Vidi Fellowship. J.C.A.M.-J. is the recipient of an Australian Research Council Future Fellowship (FT 140101082). S.B. was supported in part by NASA Swift Guest Investigator Cycle 12 program grant NNX16AN79G awarded through Columbia University
Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9-342058
1RXS J180408.9–342058 is a transient neutron star low-mass X-ray binary that exhibited a bright accretion outburst in 2015. We present , , and observations obtained around the peak brightness of this outburst. The source was in a soft X-ray spectral state and displayed an X-ray luminosity of (2–3) × 10(/5.8 kpc) erg s (0.5–10 keV). The data reveal a broad Fe–K emission line that we model as relativistically broadened reflection to constrain the accretion geometry. We found that the accretion disc is viewed at an inclination of 27–35 and extended close to the neutron star, down to 5–7.5 gravitational radii (11–17 km). This inner disc radius suggests that the neutron star magnetic field strength is 2 × 10 G. We find a narrow absorption line in the /HEG data at an energy of 7.64 keV with a significance of 4.8. This feature could correspond to blueshifted Fe and arise from an accretion disc wind, which would imply an outflow velocity of 0.086 (25 800 km s). However, this would be extreme for an X-ray binary and it is unclear if a disc wind should be visible at the low inclination angle that we infer from our reflection analysis. Finally, we discuss how the X-ray and optical properties of 1RXS J180408.9–342058 are consistent with a relatively small ( 3 h) binary orbit.ND is supported by an NWO/Vidi grant and an EU Marie Curie Intra-European fellowship under contract no. FP-PEOPLE-2013-IEF-627148. DA acknowledges support from the Royal Society. JCAMJ is supported by an Australian Research Council (ARC) Future Fellowship (FT140101082) and an ARC Discovery Grant (DP120102393). RW and AP are supported by an NWO/TOP grant, module 1, awarded to RW. COH is supported by an NSERC Discovery Grant. ATD is supported by an NWO/Veni grant. JWTH is supported by NWO/Vidi and ERC/starting (337062) grants
Radio emission from the X-ray pulsar Her X-1: a jet launched by a strong magnetic field neutron star?
Her X-1 is an accreting neutron star (NS) in an intermediate-mass X-ray binary. Like lowmass X-ray binaries (LMXBs), it accretes via Roche lobe overflow, but similar to many
high-mass X-ray binaries containing a NS; Her X-1 has a strong magnetic field and slow spin.
Here, we present the discovery of radio emission from Her X-1 with the Very Large Array.
During the radio observation, the central X-ray source was partially obscured by a warped
disc. We measure a radio flux density of 38.7 ± 4.8 µJy at 9 GHz but cannot constrain the
spectral shape. We discuss possible origins of the radio emission, and conclude that coherent
emission, a stellar wind, shocks and a propeller outflow are all unlikely explanations. A jet, as
seen in LMXBs, is consistent with the observed radio properties. We consider the implications
of the presence of a jet in Her X-1 on jet formation mechanisms and on the launching of jets
by NSs with strong magnetic fields