arXiv subjects

C. Bassa

Publications and source records attributed to C. Bassa.

At least 19 recordsLinked to original sources

arXiv2025

Combining the second data release of the European Pulsar Timing Array with low-frequency pulsar data

Low-frequency radio data improve the sensitivity of pulsar timing arrays (PTAs) to propagation effects such as dispersion measure (DM) variations, enabling better noise characterization essential for detecting the stochastic gravitational wave background (GWB). We combined LOFAR (100-200 MHz) and NenuFAR (30-90 MHz) observations with the recent European and Indian PTA release (DR2new+) into a new dataset, DR2low, spanning ~11 years for 12 pulsars. DR2low allows updated noise models, increasing PTA sensitivity to the GWB. Using Libstempo and Enterprise, we applied standard noise models including red noise (RN) and time-variable DM (DMv) as power laws, and performed Bayesian model selection over RN, DMv, and an additional chromatic noise term (CN4). Compared to DR2new+, DR2low improves DM constraints and separates DM and RN contributions. We found that the RN is required in the final model for 10 out of 12 pulsars, compared to only 5 in the DR2new+ dataset. The improved sensitivity to plasma effects provided by DR2low also favors the identification of significant CN4 in eight pulsars, while none showed such evidence in DR2new+. The analysis also reveals unmodelled solar wind effects, particularly near solar conjunction, with residual delays absorbed into the DM component, highlighting the importance of accurately modelling the solar wind in PTA datasets.

astro-ph.HE↗

arXiv2022

Testing afterglow models of FRB 200428 with early post-burst observations of SGR 1935+2154

We present LOFAR imaging observations from the April/May 2020 active episode of magnetar SGR 1935+2154. We place the earliest radio limits on persistent emission following the low-luminosity fast radio burst FRB 200428 from the magnetar. We also perform an image-plane search for transient emission and find no radio flares during our observations. We examine post-FRB radio upper limits in the literature and find that all are consistent with the multi-wavelength afterglow predicted by the synchrotron maser shock model interpretation of FRB 200428. However, early optical observations appear to rule out the simple versions of the afterglow model with constant-density circumburst media. We show that these constraints may be mitigated by adapting the model for a wind-like environment, but only for a limited parameter range. In addition, we suggest that late-time non-thermal particle acceleration occurs within the afterglow model when the shock is no longer relativistic, which may prove vital for detecting afterglows from other Galactic FRBs. We also discuss future observing strategies for verifying either magnetospheric or maser shock FRB models via rapid radio observations of Galactic magnetars and nearby FRBs.

astro-ph.HE↗

arXiv2021

A repeating fast radio burst source in a globular cluster

Fast radio bursts (FRBs) are exceptionally luminous flashes of unknown physical origin, reaching us from other galaxies (Petroff et al. 2019). Most FRBs have only ever been seen once, while others flash repeatedly, though sporadically (Spitler et al. 2016, CHIME/FRB Collaboration et al. 2021). Many models invoke magnetically powered neutron stars (magnetars) as the engines producing FRB emission (Margalit & Metzger 2018, CHIME/FRB Collaboration et al. 2020). Recently, CHIME/FRB announced the discovery (Bhardwaj et al. 2021) of the repeating FRB 20200120E, coming from the direction of the nearby grand design spiral galaxy M81. Four potential counterparts at other observing wavelengths were identified (Bhardwaj et al. 2021) but no definitive association with these sources, or M81, could be made. Here we report an extremely precise localisation of FRB 20200120E, which allows us to associate it with a globular cluster (GC) in the M81 galactic system and to place it ~2pc offset from the optical center of light of the GC. This confirms (Bhardwaj et al. 2021) that FRB 20200120E is 40 times closer than any other known extragalactic FRB. Because such GCs host old stellar populations, this association strongly challenges FRB models that invoke young magnetars formed in a core-collapse supernova as powering FRB emission. We propose, instead, that FRB 20200120E is a highly magnetised neutron star formed via either accretion-induced collapse of a white dwarf or via merger of compact stars in a binary system (Margalit et al. 2019). Alternative scenarios involving compact binary systems, efficiently formed inside globular clusters, could also be responsible for the observed bursts.

astro-ph.HE↗

arXiv2020

Measuring Interstellar Delays of PSR J0613-0200 over 7 years, using the Large European Array for Pulsars

Using data from the Large European Array for Pulsars (LEAP), and the Effelsberg telescope, we study the scintillation parameters of the millisecond pulsar J0613-0200 over a 7 year timespan. The "secondary spectrum" -- the 2D power spectrum of scintillation -- presents the scattered power as a function of time delay, and contains the relative velocities of the pulsar, observer, and scattering material. We detect a persistent parabolic scintillation arc, suggesting scattering is dominated by a thin, anisotropic region. The scattering is poorly described by a simple exponential tail, with excess power at high delays; we measure significant, detectable scattered power at times out to $\sim 5 \mu s$, and measure the bulk scattering delay to be between 50 to 200\,ns with particularly strong scattering throughout 2013. These delays are too small to detect a change of the pulse profile shape, yet they would change the times-of-arrival as measured through pulsar timing. The arc curvature varies annually, and is well fit by a one-dimensional scattering screen $\sim 40\%$ of the way towards the pulsar, with a changing orientation during the increased scattering in 2013. Effects of uncorrected scattering will introduce time delays correlated over time in individual pulsars, and may need to be considered in gravitational wave analyses. Pulsar timing programs would benefit from simultaneously recording in a way that scintillation can be resolved, in order to monitor the variable time delays caused by multipath propagation.

astro-ph.HE↗

arXiv2019

The LOFAR Tied-Array All-Sky Survey (LOTAAS): Characterization of 20 pulsar discoveries and their single-pulse behavior

We are using the LOw-Frequency ARray (LOFAR) to perform the LOFAR Tied-Array All-Sky (LOTAAS) survey for pulsars and fast transients. Here we present the astrometric and rotational parameters of 20 pulsars discovered as part of LOTAAS. These pulsars have regularly been observed with LOFAR at 149 MHz and the Lovell telescope at 1532 MHz, supplemented by some observations with the Lovell telescope at 334 MHz and the Nancay Radio Telescope at 1484 MHz. Timing models are calculated for the 20 pulsars, some of which are among the slowest-spinning pulsars known. PSR J1236-0159 rotates with a period P ~ 3.6 s, while 5 additional pulsars show P > 2 s. Also, the spin-down rates Pdot are, on average, low, with PSR J0815+4611 showing Pdot ~ 4E-18. Some of the pulse profiles, generically single-peaked, present complex shapes evolving with frequency. Multi-frequency flux measurements show that these pulsars have generically relatively steep spectra but exceptions are present, with values ranging between ~ -4 and -1. Among the pulsar sample, a large fraction shows large single-pulse variability, with 4 pulsars being undetectable more than 15% of the time and one tentatively classified as a Rotating Radio Transient. Two single-peaked pulsars show drifting sub-pulses.

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arXiv2019

Correlated timing noise and high precision pulsar timing: Measuring frequency second derivatives as an example

We investigate the impact of noise processes on high-precision pulsar timing. Our analysis focuses on the measurability of the second spin frequency derivative $\ddotν$. This $\ddotν$ can be induced by several factors including the radial velocity of a pulsar. We use Bayesian methods to model the pulsar times-of-arrival in the presence of red timing noise and dispersion measure variations, modelling the noise processes as power laws. Using simulated times-of-arrival that both include red noise, dispersion measure variations and non-zero $\ddotν$ values, we find that we are able to recover the injected $\ddotν$, even when the noise model used to inject and recover the input parameters are different. Using simulations, we show that the measurement uncertainty on $\ddotν$ decreases with the timing baseline $T$ as $T^γ$, where $γ=-7/2+α/2$ for power law noise models with shallow power law indices $α$ ($0<α<4$). For steep power law indices ($α>8$), the measurement uncertainty reduces with $T^{-1/2}$. We applied this method to times-of-arrival from the European Pulsar Timing Array and the Parkes Pulsar Timing Array and determined $\ddotν$ probability density functions for 49 millisecond pulsars. We find a statistically significant $\ddotν$ value for PSR\,B1937+21 and consider possible options for its origin. Significant (95 per cent C.L.) values for $\ddotν$ are also measured for PSRs\,J0621+1002 and J1022+1001, thus future studies should consider including it in their ephemerides. For binary pulsars with small orbital eccentricities, like PSR\,J1909$-$3744, extended ELL1 models should be used to overcome computational issues. The impacts of our results on the detection of gravitational waves are also discussed.

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arXiv2018

Single-pulse classifier for the LOFAR Tied-Array All-sky Survey

Searches for millisecond-duration, dispersed single pulses have become a standard tool used during radio pulsar surveys in the last decade. They have enabled the discovery of two new classes of sources: rotating radio transients and fast radio bursts. However, we are now in a regime where the sensitivity to single pulses in radio surveys is often limited more by the strong background of radio frequency interference (RFI, which can greatly increase the false-positive rate) than by the sensitivity of the telescope itself. To mitigate this problem, we introduce the Single-pulse Searcher (SpS). This is a new machine-learning classifier designed to identify astrophysical signals in a strong RFI environment, and optimized to process the large data volumes produced by the new generation of aperture array telescopes. It has been specifically developed for the LOFAR Tied-Array All-Sky Survey (LOTAAS), an ongoing survey for pulsars and fast radio transients in the northern hemisphere. During its development, SpS discovered 7 new pulsars and blindly identified ~80 known sources. The modular design of the software offers the possibility to easily adapt it to other studies with different instruments and characteristics. Indeed, SpS has already been used in other projects, e.g. to identify pulses from the fast radio burst source FRB 121102. The software development is complete and SpS is now being used to re-process all LOTAAS data collected to date.

astro-ph.IM↗

arXiv2018

A Long-term study of three rotating radio transients

We present the longest-term timing study so far of three Rotating Radio Transients (RRATs) - J1819-1458, J1840-1419 and J1913+1330 - performed using the Lovell, Parkes and Green Bank telescopes over the past decade. We study long-term and short- term variations of the pulse emission rate from these RRATs and report a marginal indication of a long-term increase in pulse detection rate over time for PSR J1819-1458 and J1913+1330. For PSR J1913+1330, we also observe a two orders of magnitude variation in the observed pulse detection rates across individual epochs, which may constrain the models explaining the origin of RRAT pulses. PSR J1913+1330 is also observed to exhibit a weak persistent emission mode. We investigate the post-glitch timing properties of J1819-1458 (the only RRAT for which glitches are observed) and discuss the implications for possible glitch models. Its post-glitch over-recovery of the frequency derivative is magnetar-like and similar behaviour is only observed for two other pulsars, both of which have relatively high magnetic field strengths. Following the over-recovery we also observe that some fraction of the pre-glitch frequency derivative is gradually recovered.

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arXiv2017

280 one-opposition near-Earth asteroids recovered by the EURONEAR with the Isaac Newton Telescope

One-opposition near-Earth asteroids (NEAs) are growing in number, and they must be recovered to prevent loss and mismatch risk, and to improve their orbits, as they are likely to be too faint for detection in shallow surveys at future apparitions. We aimed to recover more than half of the one-opposition NEAs recommended for observations by the Minor Planet Center (MPC) using the Isaac Newton Telescope (INT) in soft-override mode and some fractions of available D-nights. During about 130 hours in total between 2013 and 2016, we targeted 368 NEAs, among which 56 potentially hazardous asteroids (PHAs), observing 437 INT Wide Field Camera (WFC) fields and recovering 280 NEAs (76% of all targets). Engaging a core team of about ten students and amateurs, we used the THELI, Astrometrica, and the Find_Orb software to identify all moving objects using the blink and track-and-stack method for the faintest targets and plotting the positional uncertainty ellipse from NEODyS. Most targets and recovered objects had apparent magnitudes centered around V~22.8 mag, with some becoming as faint as V~24 mag. One hundred and three objects (representing 28% of all targets) were recovered by EURONEAR alone by Aug 2017. Orbital arcs were prolonged typically from a few weeks to a few years; our oldest recoveries reach 16 years. The O-C residuals for our 1,854 NEA astrometric positions show that most measurements cluster closely around the origin. In addition to the recovered NEAs, 22,000 positions of about 3,500 known minor planets and another 10,000 observations of about 1,500 unknown objects (mostly main-belt objects) were promptly reported to the MPC by our team. Four new NEAs were discovered serendipitously in the analyzed fields, increasing the counting to nine NEAs discovered by the EURONEAR in 2014 and 2015.

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arXiv2016

Single-Source Gravitational Wave Limits from the J1713+0747 24-hr Global Campaign

Dense, continuous pulsar timing observations over a 24-hr period provide a method for probing intermediate gravitational wave (GW) frequencies from 10 microhertz to 20 millihertz. The European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Parkes Pulsar Timing Array (PPTA), and the combined International Pulsar Timing Array (IPTA) all use millisecond pulsar observations to detect or constrain GWs typically at nanohertz frequencies. In the case of the IPTA's nine-telescope 24-Hour Global Campaign on millisecond pulsar J1713+0747, GW limits in the intermediate frequency regime can be produced. The negligible change in dispersion measure during the observation minimizes red noise in the timing residuals, constraining any contributions from GWs due to individual sources. At 10$^{-5}$Hz, the 95% upper limit on strain is 10$^{-11}$ for GW sources in the pulsar's direction.

astro-ph.IM↗

arXiv2016

The Chandra Galactic Bulge Survey: optical catalogue and point-source counterparts to X-ray sources

As part of the Chandra Galactic Bulge Survey (GBS), we present a catalogue of optical sources in the GBS footprint. This consists of two regions centered at Galactic latitude b = 1.5 degrees above and below the Galactic Centre, spanning (l x b) = (6x1) degrees. The catalogue consists of 2 or more epochs of observations for each line of sight in r', i' and Hα filters. It is complete down to r' = 20.2 and i' = 19.2 mag; the mean 5σ depth is r' = 22.5 and i' = 21.1 mag. The mean root-mean-square residuals of the astrometric solutions is 0.04 arcsec. We cross-correlate this optical catalogue with the 1640 unique X-ray sources detected in Chandra observations of the GBS area, and find candidate optical counterparts to 1480 X-ray sources. We use a false alarm probability analysis to estimate the contamination by interlopers, and expect ~ 10 per cent of optical counterparts to be chance alignments. To determine the most likely counterpart for each X-ray source, we compute the likelihood ratio for all optical sources within the 4σ X-ray error circle. This analysis yields 1480 potential counterparts (~ 90 per cent of the sample). 584 counterparts have saturated photometry (r'<17, i'<16), indicating these objects are likely foreground sources and the real counterparts. 171 candidate counterparts are detected only in the i'-band. These sources are good qLMXB and CV candidates as they are X-ray bright and likely located in the Bulge.

astro-ph.HE↗

arXiv2016

A Fast Radio Burst Host Galaxy

In recent years, millisecond duration radio signals originating from distant galaxies appear to have been discovered in the so-called Fast Radio Bursts. These signals are dispersed according to a precise physical law and this dispersion is a key observable quantity which, in tandem with a redshift measurement, can be used for fundamental physical investigations. While every fast radio burst has a dispersion measurement, none before now have had a redshift measurement, due to the difficulty in pinpointing their celestial coordinates. Here we present the discovery of a fast radio burst and the identification of a fading radio transient lasting $\sim 6$ days after the event, which we use to identify the host galaxy; we measure the galaxy's redshift to be $z=0.492\pm0.008$. The dispersion measure and redshift, in combination, provide a direct measurement of the cosmic density of ionised baryons in the intergalactic medium of $Ω_{\mathrm{IGM}}=4.9 \pm 1.3\%$, in agreement with the expectation from WMAP, and including all of the so-called "missing baryons". The $\sim6$-day transient is largely consistent with a short gamma-ray burst radio afterglow, and its existence and timescale do not support progenitor models such as giant pulses from pulsars, and supernovae. This contrasts with the interpretation of another recently discovered fast radio burst, suggesting there are at least two classes of bursts.

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arXiv2015

LOFAR discovery of a quiet emission mode in PSR B0823+26

PSR B0823+26, a 0.53-s radio pulsar, displays a host of emission phenomena over timescales of seconds to (at least) hours, including nulling, subpulse drifting, and mode-changing. Studying pulsars like PSR B0823+26 provides further insight into the relationship between these various emission phenomena and what they might teach us about pulsar magnetospheres. Here we report on the LOFAR discovery that PSR B0823+26 has a weak and sporadically emitting 'quiet' (Q) emission mode that is over 100 times weaker (on average) and has a nulling fraction forty-times greater than that of the more regularly-emitting 'bright' (B) mode. Previously, the pulsar has been undetected in the Q-mode, and was assumed to be nulling continuously. PSR B0823+26 shows a further decrease in average flux just before the transition into the B-mode, and perhaps truly turns off completely at these times. Furthermore, simultaneous observations taken with the LOFAR, Westerbork, Lovell, and Effelsberg telescopes between 110 MHz and 2.7 GHz demonstrate that the transition between the Q-mode and B-mode occurs within one single rotation of the neutron star, and that it is concurrent across the range of frequencies observed.

astro-ph.SR↗

arXiv2014

A 24-Hour Global Campaign To Assess Precision Timing of the Millisecond Pulsar J1713+0747

The radio millisecond pulsar J1713+0747 is regarded as one of the highest-precision clocks in the sky, and is regularly timed for the purpose of detecting gravitational waves. The International Pulsar Timing Array collaboration undertook a 24-hour global observation of PSR J1713+0747 in an effort to better quantify sources of timing noise in this pulsar, particularly on intermediate (1 - 24 hr) timescales. We observed the pulsar continuously over 24 hr with the Arecibo, Effelsberg, GMRT, Green Bank, LOFAR, Lovell, Nancay, Parkes, and WSRT radio telescopes. The combined pulse times-of-arrival presented here provide an estimate of what sources of timing noise, excluding DM variations, would be present as compared to an idealized root-N improvement in timing precision, where N is the number of pulses analyzed. In the case of this particular pulsar, we find that intrinsic pulse phase jitter dominates arrival time precision when the S/N of single pulses exceeds unity, as measured using the eight telescopes that observed at L-band/1.4 GHz. We present first results of specific phenomena probed on the unusually long timescale (for a single continuous observing session) of tens of hours, in particular interstellar scintillation, and discuss the degree to which scintillation and profile evolution affect precision timing. This paper presents the data set as a basis for future, deeper studies.

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arXiv2013

Identification of twenty-three accreting binaries in the Galactic Bulge Survey

(Abridged:) We present the identification of optical counterparts to 23 Galactic Bulge Survey X-ray sources. We report their accurate coordinates and optical spectra acquired at the VLT and Magellan. All sources are classified as accreting binaries according to their emission line characteristics. To distinguish accreting binaries from chromospherically active objects we develop and explain criteria based on Halpha and HeI 5786,6678 emission line properties available in the literature. The spectroscopic properties and photometric variability of all the objects are discussed and a classification of the source is given where possible. Among the 23 systems, at least 9 of them show an accretion-dominated optical spectrum (CX28, CX63, CX70, CX128, CX142, CX207, CX522, CX794, CX1011) and another 6 show photospheric lines from a late-type donor star in addition to accretion disc emission (CX44, CX93, CX137, CX154, CX377 and CX1004) indicating that they are probably accreting binaries in quiescence or in a low accretion rate state. Two sources are confirmed to be eclipsing: CX207 and CX794. CX207 shows a broad asymmetric Halpha profile blue-shifted by >300 km/s. Such line profile characteristics are consistent with a magnetic (Polar) CV. CX794 is an eclipsing nova-like CV in the period gap. Time-resolved photometry and the large broadening of the Halpha emission lines in CX446 (2100 km/s FWHM) suggest that this is also an eclipsing or high-inclination accreting binary. Finally, the low-accretion rate source CX1004 shows a double-peaked Halpha profile with a FWHM of 2100 km/s. This supports a high inclination or even eclipsing system. Whether the compact object is a white dwarf in an eclipsing CV or a black hole primary in a high-inclination LMXB remains to be established.

astro-ph.HE↗

arXiv2013

A strong magnetic field around the supermassive black hole at the centre of the Galaxy

The centre of our Milky Way harbours the closest candidate for a supermassive black hole. The source is thought to be powered by radiatively inefficient accretion of gas from its environment. This form of accretion is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which it can be fed. However, the magnetization of the gas, a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of the accretion, remove angular momentum from the infalling gas, expel matter through relativistic jets and lead to the observed synchrotron emission. Here we report multi-frequency measurements with several radio telescopes of a newly discovered pulsar close to the Galactic Centre and show that its unusually large Faraday rotation indicates a dynamically relevant magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission from the black hole, from radio to X-rays.

astro-ph.GA↗

arXiv2008

On the nature of the intermittent pulsar PSR B1931+24

PSR B1931+24 is the first intermittent radio pulsar discovered to date, characterized by a 0.8 s pulsation which turns on and off quasi-periodically every ~35 days, with a duty cycle of ~10%. We present here X-ray and optical observations of PSR B1931+24 performed with the Chandra X-ray Observatory and Isaac Newton Telescope, respectively. Simultaneous monitoring from the Jodrell Bank Observatory showed that this intermittent pulsar was in the radio-on phase during our observations. We do not find any X-ray or optical counterpart to PSR B1931+24 translating into an upper limit of ~2x10^{31} erg/s on the X-ray luminosity, and of g' > 22.6 on the optical magnitude. If the pulsar is isolated, these limits cannot constrain the dim X-ray and optical emission expected for a pulsar of that age (~1.6 Myr). We discuss the possibility that the quasi-periodic intermittent behavior of PSR B1931+24 is due to the presence of a low mass companion star or gaseous planet, tight with the pulsar in an eccentric orbit. In order to constrain the parameters of this putative binary system we re-analysed the pulsar radio timing residuals and we found that (if indeed hosted in a binary system), PSR B1931+24 should have a very low mass companion and an orbit of low inclination.

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arXiv2008

An Eccentric Binary Millisecond Pulsar in the Galactic Plane

Binary pulsar systems are superb probes of stellar and binary evolution and the physics of extreme environments. In a survey with the Arecibo telescope, we have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 ms in a highly eccentric (e = 0.44) 95-day orbit around a solar mass companion. Infrared observations identify a possible main-sequence companion star. Conventional binary stellar evolution models predict neither large orbital eccentricities nor main-sequence companions around millisecond pulsars. Alternative formation scenarios involve recycling a neutron star in a globular cluster then ejecting it into the Galactic disk or membership in a hierarchical triple system. A relativistic analysis of timing observations of the pulsar finds its mass to be 1.74+/-0.04 Msun, an unusually high value.

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