arXiv subjects

Denis Leahy

Publications and source records attributed to Denis Leahy.

At least 55 records · Page 3Linked to original sources

arXiv2020

New Optically Identified Supernova Remnants in the Large Magellanic Cloud

We present a new optical sample of three Supernova Remnants and 16 Supernova Remnant (SNR) candidates in the Large Magellanic Cloud(LMC). These objects were originally selected using deep H$α$, [SII] and [OIII] narrow-band imaging. Most of the newly found objects are located in less dense regions, near or around the edges of the LMC's main body. Together with previously suggested MCSNR J0541-6659, we confirm the SNR nature for two additional new objects: MCSNR J0522-6740 and MCSNRJ0542-7104. Spectroscopic follow-up observations for 12 of the LMC objects confirm high [SII]/H$α$ a emission-line ratios ranging from 0.5 to 1.1. We consider the candidate J0509-6402 to be a special example of the remnant of a possible Type Ia Supernova which is situated some 2$^\circ$ ($\sim 1.75$kpc) north from the main body of the LMC. We also find that the SNR candidates in our sample are significantly larger in size than the currently known LMC SNRs by a factor of $\sim 2$. This could potentially imply that we are discovering a previously unknown but predicted, older class of large LMC SNRs that are only visible optically. Finally, we suggest that most of these LMC SNRs are residing in a very rarefied environment towards the end of their evolutionary span where they become less visible to radio and X-ray telescopes.

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arXiv2020

AstroSat UVIT Detections of Chandra X-ray Sources in M31

An ultraviolet survey of M31 has been carried out during 2017-19 with the UVIT instrument onboard the AstroSat Observatory. Here we match the M31 UVIT source catalog (Leahy et al. 2020) with the Chandra source catalog (Vulic et al. 2016). We find 67 UVIT/Chandra sources detected in a varying number of UV and X-ray bands. The UV and X-ray photometry is analyzed using powerlaw and blackbody models. The X-ray types include 15 LMXBs and 5 AGNs. Cross-matches with catalogs of stars, clusters and other source types yield the following. 20 of the UVIT/Chandra sources match with M31 globular clusters and 9 with foreground stars. 3 more globular clusters and 2 more foreground stars are consistent with the UVIT source positions although outside the Chandra match radius of 1 arcsec. The UV emission of the UVIT/Chandra sources associated with globular clusters is consistent with emission from blue horizontal branch stars rather than from the X-ray source. The LMXBs in globular clusters are among the most luminous globular clusters in M31.Comparison with stellar evolutionary tracks shows that the UVIT/Chandra sources with high UV blackbody temperatures are consistent with massive (10 to 30 M$_{\odot}$) stars in M31.

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arXiv2020

Swift/BAT and RXTE/ASM Observations of the 35-day X-ray Cycle of Hercules X-1

Swift/BAT and RXTE/ASM observations have monitored the X-ray binary system Her X-1 for approximately 14.5 years each, and both were monitoring Her X-1 for a period of ~5.5 years. Here we study the 35-day cycle using these observations. Using a cross-correlation method we find the times of peaks of the 35-day cycles for ~150 cycles observed by Swift/BAT and ~150 cycles observed by RXTE/ASM. These cycles include ~60 observed with both instruments. The noise level of the RXTE/ASM measurements is larger than that of Swift/BAT, resulting in larger uncertainty in peak times. The distribution of 35-day cycle lengths can be fit with a Gaussian with mean 34.79 d and $σ$ of 1.1 d. The distribution of orbital phases of 35-day cycle peaks is well fit by a uniform distribution, with 76 percent of the cycles, plus a Gaussian distribution peaked at orbital phase ~0.5, with 24 percent of the cycles. We construct the long-term average 35-day lightcurve in the 15-50 keV band from Swift/BAT, and in the 2-12 keV band from RXTE/ASM. The high energy band shows more variability in the Short High state and the low energy band shows more variability in the Main High state. This is consistent with a precessing accretion disk model as cause of the 35-day cycle.

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arXiv2019

Radio Observations of Supernova Remnant G1.9+0.3

We present 1 to 10GHz radio continuum flux density, spectral index, polarisation and Rotation Measure (RM) images of the youngest known Galactic Supernova Remnant (SNR) G1.9+0.3, using observations from the Australia Telescope Compact Array (ATCA). We have conducted an expansion study spanning 8 epochs between 1984 and 2017, yielding results consistent with previous expansion studies of G1.9+0.3. We find a mean radio continuum expansion rate of ($0.78 \pm 0.09$) per cent year$^{-1}$ (or $\sim8900$ km s$^{-1}$ at an assumed distance of 8.5 kpc), although the expansion rate varies across the SNR perimeter. In the case of the most recent epoch between 2016 and 2017, we observe faster-than-expected expansion of the northern region. We find a global spectral index for G1.9+0.3 of $-0.81\pm0.02$ (76 MHz$-$10 GHz). Towards the northern region, however, the radio spectrum is observed to steepen significantly ($\sim -$1). Towards the two so called (east & west) "ears" of G1.9+0.3, we find very different RM values of 400-600 rad m$^{2}$ and 100-200 rad m$^{2}$ respectively. The fractional polarisation of the radio continuum emission reaches (19 $\pm$ 2)~per~cent, consistent with other, slightly older, SNRs such as Cas~A.

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arXiv2019

A Quark-Nova in the wake of a core-collapse Supernova: a unifying model for long duration Gamma-Ray Bursts and Fast Radio Bursts

[Abridged] By appealing to a Quark-Nova (QN; the explosive transition of a neutron star to a quark star) in the wake of a core-collapse Supernova explosion of a massive star, we develop a unified model for long duration Gamma-ray Bursts (LGRBs) and Fast Radio Bursts (FRBs). The time delay (years to decades) between the SN and the QN and, the fragmented nature (i.e. millions of chunks) of the relativistic QN ejecta are key to yielding a robust LGRB engine. In our model, a LGRB light-curve exhibits the interaction of the fragmented QN ejecta with a turbulent (i.e. filamentary and magnetically saturated) SN ejecta which is shaped by its interaction with an underlying pulsar wind nebula (PWN). The afterglow is due to the interaction of the QN chunks, exiting the SN ejecta, with the surrounding medium. Our model can fit BAT/XRT prompt and afterglow light-curves, simultaneously with their spectra, thus yielding the observed properties of LGRBs (e.g. the Band function and the X-ray flares). We find that the Yonetoku law and the Amati law are not fundamental but phenomenological. FRBs result from coherent synchrotron emission when the QN chunks interact with non-turbulent weakly magnetized PWN-SN ejecta, where conditions are prone to the Weibel instability. Magnetic field amplification induced by the Weibel instability sets the bunching length for electrons and pairs to radiate coherently. The resulting emission frequency, luminosity, duration and dispersion measure in our model are consistent with FRB data. We find a natural unification of high-energy burst phenomena from FRBs to LGRBs including X-ray Flashes and X-ray rich GRBs as well as Super-Luminous SNe. We find a possible connection between Ultra-High Energy Cosmic Rays and FRBs and propose that a QN following a binary neutron star merger can yield a short GRB (SGRB) with fits to BAT/XRT light-curves.

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arXiv2019

Multipole Analysis of Radio Continuum Images of Supernova Remnants: Comparison of Type Ia and Core Collapse

A multipole expansion analysis is applied to 1420 MHz radio continuum images of supernova remnants (SNRs) in order to compare Type Ia and core collapse (CC) SNRs. Because the radio synchrotron emission is produced at the outer shock between the SNR and the ISM, we are investigating whether the ISM interaction of SNRs is different between Type Ia and CC SNRs. This is in contrast to previous investigations, which have shown that Type Ia and CC SNRs have different asymmetries in the X-ray emission from their ejecta. The sample consists of 19 SNRs which have been classified as either Type Ia or CC. The quadrupole and octupole moments normalized to their monopole moments (total emission) are used as a measure of asymmetry of the emission. A broad range (by a factor of ~1000) is found for both quadrupole and octupole normalized moments. The strongest correlation we find is that large quadrupole moments are associated with large octupole moments, indicating that both serve as similar indicators of asymmetry. The other correlation we find is that both moments increase with SNR age or radius. This indicates that interstellar medium structure is a strong contributor to asymmetries in the radio emission from SNRs. This does not seem to apply to molecular clouds, because we find that association of a SNR with a molecular cloud is not correlated with larger quadrupole or octupole moments.

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arXiv2019

Understanding Galactic Star Formation with Next Generation X-ray Spectroscopy and Imaging

This white paper is motivated by open questions in star formation, which can be uniquely addressed by high resolution X-ray imaging and require an X-ray observatory with large collecting area along good spectral resolution. A complete census of star-forming regions in X-rays, combined with well matched infrared (IR) data, will advance our understanding of disk survival times and dissipation mechanisms. In addition, we will be able to directly observe the effects of X-ray irradiation on circumstellar grain growth to compare with grain evolution models in both high- and low-UV environments. X-rays are native to stars at all phases of star formation and affect planet-forming disks especially through flares. Moreover, X-rays trace magnetic fields which weave through the flares, providing a unique, non-gravitational feedback mechanism between disk and star. Finally, the bright X-ray emission emanating from hot plasma associated with massive stars can have large scale impacts on the topology of star-forming regions and their interface with the interstellar medium (ISM).

astro-ph.SR↗

arXiv2019

Supernova Remnants in High Definition

Supernova remnants (SNRs) offer the means to study SN explosions, dynamics, and shocks at sub-parsec scales. X-ray observations probe the hot metals synthesized in the explosion and the TeV electrons accelerated by the shocks, and thus they are key to test recent, high-fidelity three-dimensional SN simulations. In this white paper, we discuss the major advances possible with X-ray spectro-imaging at arcsecond scales, with a few eV spectral resolution and a large effective area. These capabilities would revolutionize SN science, offering a three-dimensional view of metals synthesized in explosions and enabling population studies of SNRs in Local Group galaxies. Moreover, this future X-ray mission could detect faint, narrow synchrotron filaments and shock precursors that will constrain the diffusive shock acceleration process.

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arXiv2016

The Superluminous (Type I) Supernova ASASSN-15lh : A case for a Quark-Nova inside an Oxygen-type Wolf-Rayet supernova remnant

We show that a Quark-Nova (QN; the explosive transition of a neutron star to a quark star) occurring a few days following the supernova explosion of an Oxygen-type Wolf-Rayet (WO) star can account for the intriguing features of ASASSN-15lh, including its extreme energetics, its double-peaked light-curve and the evolution of its photospheric radius and temperature. A two-component configuration of the homologously expanding WO remnant (an extended envelope and a compact core) is used to harness the kinetic energy (>10^52 ergs) of the QN ejecta. The delay between the WO SN and the QN yields a large (~ 10^4 Rsun) envelope which when energized by the QN ejecta/shock gives the first peak in our model. As the envelope's photosphere recedes into the slowly expanding, hot and insulated, denser core (initially heated by the QN shock) a second hump emerges. The spectrum in our model should reflect the composition of an WO SN remnant re-heated by a QN going off in its wake.

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arXiv2016

HI absorption spectra for Supernova Remnants in the VGPS survey

The set of supernova remnants (SNR) from Green's SNR catalog which are found in the VLA Galactic Plane Survey (VGPS) are the objects considered in this study. For these SNR, we extract and analyse HI absorption spectra in a uniform way and construct a catalogue of absorption spectra and distance determinations.

astro-ph.GA↗

arXiv2016

Supernova Remnant Evolution: from explosion to dissipation

Here is considered the full evolution of a spherical supernova remnant. We start by calculating the early time ejecta dominated stage and continue through the different phases of interaction with the circumstellar medium, and end with the dissipation and merger phase. The physical connection between the phases reveals new results. One is that the blast wave radius during the adiabatic phase is significantly smaller than it would be, if one does notaccount for the blast wave interaction with the ejecta.

astro-ph.GA↗

arXiv2016

Quark-Noave in binaries: Observational signatures and implications to astrophysics

The explosive transition of a massive neutron star to a quark star (the Quark-Nova, QN) releases in excess of ~ 10^52 erg in kinetic energy which can drastically impact the surrounding environment of the QN. A QN is triggered when a neutron star gains enough mass to reach the critical value for quark deconfinement to happen in the core. In binaries, a neutron star has access to mass reservoirs (e.g. accretion from a companion or from a Common Envelope, CE). We explain observed light-curves of hydrogen-poor superluminous Supernovae (SLSNe Ia) in the context of a QN occurring in the second CE phase of a massive binary. In particular this model gives good fits to light-curves of SLSNe with double-humped light-curves. Our model suggests the QN as a mechanism for CE ejection and that they be taken into account during binary evolution. In a short period binary with a white dwarf companion, the neutron star can quickly grow in mass and experience a QN event. Part of the QN ejecta collides with the white dwarf, shocking, compressing, and heating it to driving a thermonuclear runaway producing a SN Ia impostor (a QN-Ia). Unlike "normal" Type Ia supernovae where no compact remnant is formed, a QN-Ia produces a quark star undergoing rapid spin-down providing additional power along with the 56Ni decay energy. Type Ia SNe are used as standard candles and contamination of this data by QNe-Ia can infer an incorrect cosmology.

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arXiv2016

Quark-nova compact remnants: Observational signatures in astronomical data and implications to compact stars

Quark-novae leave behind quark stars with a surrounding metal-rich fall-back (ring-like) material. These compact remnants have high magnetic fields and are misconstrued as magnetars; however, several observational features allow us to distinguish a quark star (left behind by a quark-nova) from a neutron star with high magnetic field. In our model, bursting activity is expected from intermittent accretion events from the surrounding fall-back debris leading to X-ray bursts (in the case of a Keplerian ring) or gamma ray bursts (in the case of a co-rotating shell). The details of the spectra are described by a constant background X-ray luminosity from the expulsion of magnetic flux tubes which will be temporarily buried by bursting events caused by accretion of material onto the quark star surface. These accretion events emit high energy photons and heat up the quark star and surrounding debris leading to hot spots which may be observable as distinct blackbodies. Additionally, we explain observed spectral line features as atomic lines from r-process material and explain an observed anti-glitch in an AXP as the transfer of angular momentum from a surrounding Keplerian disk to the quark star.

astro-ph.HE↗

arXiv2016

Quark-Novae occurring in massive binaries : A universal energy source in superluminous Supernovae with double-peaked light curves

A Quark-Nova (QN, the sudden transition from a neutron star into a quark star) which occurs in the second common envelope (CE) phase of a massive binary (Ouyed et al., 2015a&b), gives excellent fits to super-luminous, hydrogen-poor, Supernovae (SLSNe) with double-peaked light curves including DES13S2cmm, SN 2006oz and LSQ14bdq (http://www.quarknova.ca/LCGallery.html). In our model, the H envelope of the less massive companion is ejected during the first CE phase while the QN occurs deep inside the second, He-rich, CE phase after the CE has expanded in size to a radius of a few tens to a few thousands solar radii, this yields the first peak in our model. The ensuing merging of the quark star with the CO core leads to black hole formation and accretion explaining the second long-lasting peak. We study a sample of 8 SLSNe Ic with double-humped light-curves. Our model provides good fits to all of these with a universal explosive energy of 2x10^52 erg (which is the kinetic energy of the QN ejecta) for the first hump. The late-time emissions seen in iPTF13ehe and LSQ14bdq are fit with a shock interaction between the outgoing He-rich (i.e second) CE and the previously ejected H-rich (i.e first) CE.

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arXiv2015

The superluminous SN DES13S2cmm as a signature of a Quark-Nova in a He-HMXB system

We show that by appealing to a Quark-Nova (the explosive transition of a neutron star to a quark star) occurring in an He-HMXB system we can account for the lightcurve of the first superluminous SN, DES13S2cmm, discovered by the Dark-Energy Survey. The neutron star's explosive conversion is triggered as a result of accretion during the He-HMXB second Common Envelope phase. The dense, relativistic, Quark-Nova ejecta in turn energizes the extended He-rich Common Envelope in an inside-out shock heating process. We find an excellent fit (reduced chi^2 of 1.09) to the bolometric light-curve of SN DES13S2cmm including the late time emission, which we attribute to Black Hole accretion following the conversion of the Quark Star to a Black hole.

astro-ph.HE↗

arXiv2015

Quark-Novae in massive binaries : a model for double-humped, hydrogen-poor, superluminous Supernovae

LSQ14bdq and SN 2006oz are super-luminous, hydrogen-poor, SNe with double-humped light curves. We show that a Quark-Nova (QN; explosive transition of the neutron star to a quark star) occurring in a massive binary, experiencing two Common Envelope (CE) phases, can quantitatively explain the light curves of LSQ14bdq and SN 2006oz. The more massive component (A) explodes first as a normal SN, yielding a Neutron Star which ejects the hydrogen envelope of the companion when the system enters its first CE phase. During the second CE phase, the NS spirals into and inflates the second He-rich CE. In the process it gains mass and triggers a Quark-Nova, outside of the CO core, leaving behind a Quark Star. The first hump in our model is the QN shock re-energizing the expanded He-rich CE. The QN occurs when the He-rich envelope is near maximum size (~ 1000R_sun) and imparts enough energy to unbind and eject the envelope. Subsequent merging of the Quark Star with the CO core of component B, driven by gravitational radiation, turns the Quark star to a Black Hole. The ensuing Black Hole accretion provides sufficient power for the second brighter and long lasting hump. Our model suggests a possible connection between SLSNe-I and type Ic-BL SNe which occur when the Quark Nova is triggered inside the CO core. We estimate the rate of QNe in massive binaries during the second CE phase to be ~ 5x10^(-5) of that of core-collapse SNe.

astro-ph.HE↗

arXiv2015

The puzzling early detection of low velocity 56Ni decay lines in SN 2014J: Hints of a compact remnant

We show that the low-velocity 56Ni decay lines detected earlier than expected in the type Ia SN 2014J find an explanation in the Quark-Nova Ia model which involves the thermonuclear explosion of a tidally disrupted sub-Chandrasekhar White Dwarf in a tight Neutron-Star-White-Dwarf binary system. The explosion is triggered by impact from the Quark-Nova ejecta on the WD material; the Quark-Nova is the explosive transition of the Neutron star to a Quark star triggered by accretion from a CO torus (the circularized WD material). The presence of a compact remnant (the Quark Star) provides: (i) an additional energy source (spin-down power) which allows us to fit the observed light-curve including the steep early rise; (ii) a central gravitational potential which slows down some of the 56Ni produced to velocities of a few 1000 km/s. In our model, the 56Ni decay lines become optically visible at ~20 days from explosion time in agreement with observations. We list predictions that can provide important tests for our model.

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arXiv2014

Hints of a second explosion (a quark nova) in Cassiopeia A Supernova

We show that the explosive transition of the neutron star (NS) to a quark star (QS) (a Quark Nova) in Cassiopeia A (Cas A) a few days following the SN proper can account for several of the puzzling kinematic and nucleosynthetic features observed. The observed decoupling between Fe and 44Ti and the lack of Fe emission within 44Ti regions is expected in the QN model owing to the spallation of the inner SN ejecta by the relativistic QN neutrons. Our model predicts the 44Ti to be more prominent to the NW of the central compact object (CCO) than in the SE and little of it along the NE-SW jets, in agreement with NuStar observations. Other intriguing features of Cas A such as the lack of a pulsar wind nebula (PWN) and the reported a few percent drop of the CCO temperature over a period of 10 years are also addressed.

astro-ph.HE↗