arXiv subjects

Poshak Gandhi

Publications and source records attributed to Poshak Gandhi.

At least 73 records · Page 4Linked to original sources

arXiv2020

Kinematic study of the association Cyg OB3 with Gaia DR2

We study the stellar kinematic properties and spatial distribution of the association Cyg OB3 using precise astrometric data from Gaia DR2. All known O- and B-type stars in Cyg OB3 region with positions, parallaxes and proper motions available are included, comprising a total of 41 stars. The majority of stars are found to be concentrated at a heliocentric distance of 2.0 +/- 0.3 kpc. The mean peculiar velocity of the sample after removing Galactic rotation and solar motion is ~22 km/s, dominated by the velocity component towards the Galactic center. The relative position and velocity of the black hole X-ray binary Cyg X-1 with respect to the association suggest that Cyg OB3 is most likely its parent association. The peculiar kinematic properties of some of the stars are revealed and are suggestive of past stellar encounters. The sample includes a previously known runaway star HD 227018, and its high peculiar velocity of ~50 km/s is confirmed with Gaia. We estimated the velocities of stars relative to the association and the star HD 225577 exhibits peculiar velocity smaller than its velocity relative to the association. The star has lower value of proper motion than the rest of the sample. The results suggest a slowly expanding nature of the association, which is supported by the small relative speeds <20 km/s with respect to the association for a majority of the sample stars.

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arXiv2020

NuSTAR observations of the Transient Galactic Black Hole Binary Candidate Swift J1858.6-0814: A New Sibling of V404 Cyg and V4641 Sgr?

Swift J1858.6-0814 was discovered by Swift-BAT on October 25, 2018. Here we report on the first follow-up NuSTAR observation of the source, which shows variability spanning two orders of magnitude in count rate on timescales of ~10-100 s. The power-spectrum of the source does not show any quasi-periodic oscillations or periodicity, but has a large fractional rms amplitude of 147%$\pm3$%, exhibiting a number of large flares throughout the observation. The hardness ratio (defined as $R_{10-79 \rm keV}/R_{3-10 \rm keV}$) of the flares tends to be soft, while the source spans a range of hardness ratios during non-flaring periods. The X-ray spectrum of the source shows strong reflection features, which become more narrow and peaked during the non-flaring intervals. We fit an absorbed relativistic reflection model to the source spectra to place physical constraints on the system. Most notably, we find that the source exhibits a large and varying intrinsic absorbing column density ($N_{\rm H}=1.4-4.2\times10^{23}$ cm$^{-2}$). This large intrinsic absorption is further supported by the energy spectra extracted from two flares observed simultaneously by NuSTAR and NICER. We find that the inner accretion disk of the source has a low inclination, $i<29^{\circ}$ ( 3$σ$ upper-limit), while the iron abundance in the disk is close to solar, $A_{\rm Fe}=1.0\pm0.3$. We set a 90% confidence upper limit on the inner radius of the accretion disk of $r_{\rm in}<8 \ r_{\rm ISCO}$, and, by fixing $r_{\rm in}$ to be at $r_{\rm ISCO}$, a 90% confidence lower-limit on the spin of the black hole of $a^{*}>0.0$. Lastly, we compare the properties of Swift J1858.6$-$0814 to those of V404 Cygni and V4641 Sgr, which both show rapid flaring and a strong and variable absorption.

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arXiv2019

NuSTAR Uncovers an Extremely Local Compton-thick AGN in NGC 4968

We present the analysis of Chandra and NuSTAR spectra of NGC 4968, a local (D$\sim$44 Mpc) 12$μ$m-selected Seyfert 2 galaxy, enshrouded within Compton-thick layers of obscuring gas. We find no evidence of variability between the Chandra and NuSTAR observations (separated by 2 years), and between the two NuSTAR observations (separated by 10 months). Using self-consistent X-ray models, we rule out the scenario where the obscuring medium is nearly spherical and uniform, contradicting the results implied by the $<$10 keV Chandra spectrum. The line-of-sight column density, from intervening matter between the source and observer that intercepts the intrinsic AGN X-ray emission, is well within the Compton-thick regime, with a minimum column density of $2\times10^{24}$ cm$^{-2}$. The average global column density is high ($> 3\times10^{23}$ cm$^{-2}$), with both Compton-thick and Compton-thin solutions permitted depending on the X-ray spectral model. The spectral models provide a range of intrinsic AGN continuum parameters and implied 2-10 keV luminosities ($L_{\rm 2-10keV,intrinsic}$), where the higher end of $L_{\rm 2-10keV,intrinsic}$ is consistent with expectations from the 12$μ$m luminosity ($L_{\rm 2-10keV,intrinisc} \sim 7\times10^{42}$ erg s$^{-1}$). Compared with Compton-thick AGN previously observed by {\it NuSTAR}, NGC 4968 is among the most intrinsically X-ray luminous. However, despite its close proximity and relatively high intrinsic X-ray luminosity, it is undetected by the 105 month Swift-BAT survey, underscoring the importance of multi-wavelength selection for obtaining the most complete census of the most hidden black holes.

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arXiv2019

Parsec-scale Dusty Winds in Active Galactic Nuclei: Evidence for Radiation Pressure Driving

Infrared interferometry of local AGN has revealed a warm (~300K-400K) polar dust structure that cannot be trivially explained by the putative dust torus of the unified model. This led to the development of the disk+wind scenario which comprises of a hot (~1000K) compact equatorial dust disk and a polar dust wind. This wind is assumed to be driven by radiation pressure and, therefore, we would expect that long term variation in radiation pressure would influence the dust distribution. In this paper we attempt to quantify if and how the dust distribution changes with radiation pressure. We analyse so far unpublished VLTI/MIDI data on 8 AGN and use previous results on 25 more to create a sample of 33 AGN. This sample comprises all AGN successfully observed with VLTI/MIDI. For each AGN, we calculate the Eddington ratio, using the intrinsic 2-10keV X-ray luminosity and black hole mass, and compare this to the resolved dust emission fraction as seen by MIDI. We tentatively conclude that there is more dust in the wind at higher Eddington ratios, at least in type 2 AGN where such an effect is expected to be more easily visible.

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arXiv2019

New active galactic nuclei science cases with interferometry: An incomplete preview

Infrared (IR) interferometry has made widely recognised contributions to the way we look at the dusty environment of supermassive black holes on parsec scales. It finally provided direct evidence for orientation-dependent unification of active galaxies, however it also showed that the classical "torus" picture is oversimplified. New scientific opportunities for AGN have been suggested, and will soon be carried out, focusing on the dynamical aspects of spectrally and spatially resolved interferometry, as well as the potential to employ interferometry for cosmology. This will open interferometry to new scientific communities.

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arXiv2019

Hot, dense He II outflows during the 2017 outburst of the X-ray transient Swift J1357.2-0933

Time-resolved SALT spectra of the short-period, dipping X-ray transient, Swift J1357.2-0933, during its 2017 outburst has revealed broad Balmer and HeII4686 absorption features, blue-shifted by ~600 km/s. Remarkably these features are also variable on the ~500s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (>~30,000K) outflowing wind seen at very high inclination, and draw comparisons with other accretion disc corona sources. We argue against previous distance estimates of 1.5 kpc and favour a value >~6 kpc, implying an X-ray luminosity L_X>~4x10^{36} erg/s$. Hence it is not a very faint X-ray transient. Our preliminary 1D Monte-Carlo radiative transfer and photoionization calculations support this interpretation, as they imply a high intrinsic L_X, a column density N_H>~10^{24} cm^{-2} and a low covering factor for the wind. Our study shows that Swift J1357.2-0933 is truly remarkable amongst the cohort of luminous, galactic X-ray binaries, showing the first example of He II absorption, the first (and only) variable dip period and is possibly the first black hole 'accretion disc corona' candidate.

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arXiv2019

Broadband X-ray Spectral and Timing Analyses of the Black Hole Binary Candidate Swift J1658.2-4242: Rapid Flux Variation and the Turn-on of a Transient QPO

We report results from joint NuSTAR, Swift and XMM-Newton observations of the newly discovered black hole X-ray binary candidate Swift J1658.2-4242 in the intermediate state. We observe a peculiar event in this source, with its X-ray flux rapidly decreasing by $\sim$45\% in $\sim$40~s, accompanied by only subtle changes in the shape of the broadband X-ray spectrum. In addition, we find a sudden turn-on of a transient QPO with a frequency of $6-7$~Hz around the time of the flux change, and the total fractional rms amplitude of the power spectrum increases from $\sim$2\% to $\sim$10\%. X-ray spectral and timing analyses indicate that the flux decrease is driven by intrinsic changes in the accretion flow around the black hole, rather than intervening material along the line of sight. In addition, we do not significantly detect any relativistic disk reflection component, indicating it is much weaker than previously observed while the source was in the bright hard state. We propose accretion disk instabilities triggered at a large disk radius as the origin of the fast transition in spectral and timing properties, and discuss possible causes of the unusual properties observed in Swift J1658.2-4242. The prompt flux variation detected along with the emergence of a QPO makes the event an interesting case for investigating QPO mechanisms in black hole X-ray binaries.

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arXiv2019

Astro2020 Science White Paper: Compact Stellar Jets

This paper outlines the importance of understanding jets from compact binaries for the problem of understanding the broader phenomenology of jet production. Because X-ray binaries are nearby and bright, have well-measured system parameters, and vary by factors of $\sim 10^6$ on $\sim$ year timescales, they provide a unique opportunity to understand how various aspects of the jet physics change in response to changes in the accretion flow, giving the possibility of looking for trends within individual systems and testing their universality with other systems, rather than trying to interpret large samples of objects on a statistical basis.

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arXiv2019

Cosmic evolution of supermassive black holes: A view into the next two decades

The discoveries made over the past 20 years by Chandra and XMM-Newton surveys in conjunction with multiwavelength imaging and spectroscopic data available in the same fields have significantly changed the view of the supermassive black hole (SMBH) and galaxy connection. These discoveries have opened up several exciting questions that are beyond the capabilities of current X-ray telescopes and will need to be addressed by observatories in the next two decades. As new observatories peer into the early Universe, we will begin to understand the physics and demographics of SMBH infancy (at $z>6$) and investigate the influence of their accretion on the formation of the first galaxies ($§$ 2.1). We will also be able to understand the accretion and evolution over the cosmic history (at $z\sim$1-6) of the full population of black holes in galaxies, including low accretion rate, heavily obscured AGNs at luminosities beyond the reach of current X-ray surveys ($§$2.2 and $§$2.3), enabling us to resolve the connection between SMBH growth and their environment.

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arXiv2019

Probing the Black Hole Engine with Measurements of the Relativistic X-ray Reflection Component

Over the last decades X-ray spectroscopy has proven to be a powerful tool for the estimation of black hole spin and several other key parameters in dozens of AGN and black hole X-ray binaries. In this White Paper, we discuss the observational and theoretical challenges expected in the exploration, discovery, and study of astrophysical black holes in the next decade. We focus on the case of accreting black holes and their electromagnetic signatures, with particular emphasis on the measurement of the relativistic reflection component in their X-ray spectra.

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arXiv2019

STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science.

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arXiv2019

The black hole X-ray transient Swift J1357.2-0933 as seen with Swift and NuSTAR during its 2017 outburst

We report on observations of black hole Swift J1357.2-0933, a member of the modest population of very faint X-ray transients. This source has previously shown intense dips in the optical lightcurve, a phenomena that has been linked to the existence of a "unique toroidal structure" in the inner region of the disc, seen at a high inclination. Our observations, carried out by the Neil Gehrels Swift and NuSTAR X-ray observatories, do not show the presence of intense dips in the optical light curves. We find that the X-ray light curves do not show any features that would straightforwardly support an edge-on configuration or high inclination configuration of the orbit. This is similar to what was seen in the X-ray observations of the source during its 2011 outburst. Moreover, the broadband spectra were well described with an absorbed power-law model without any signatures of the cut-off at energies above 10 keV, or any reflection from the disc or the putative torus. Thus, the X-ray data do not support the unique obscuring torus scenario proposed for J1357. We also performed a multi-wavelength study using the data of X-ray telescope and Ultraviolet/Optical Telescope aboard Swift, taken during the 4.5 months duration of the 2017 outburst. This is consistent with what was previously inferred for this source. We found a correlation between the simultaneous X-ray and ultraviolet/optical data and our study suggests that most of the reprocessed flux must be coming out in the ultraviolet.

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arXiv2019

Gaia DR2 Distances and Peculiar Velocities for Galactic Black Hole Transients

We report on a first census of Galactic black hole X-ray binary (BHXRB) properties with the second data release (DR2) of {\em Gaia}, focusing on dynamically confirmed and strong candidate black hole transients. DR2 provides five-parameter astrometric solutions including position, parallax and proper motion for 11 of a sample of 24 systems. Distance estimates are tested with parallax inversion as well as Bayesian inference. We derive an empirically motivated characteristic scale length of $L$=2.17$\pm$0.12 kpc for this BHXRB population to infer distances based upon an exponentially decreasing space density prior. Geometric DR2 parallaxes provide new, independent distance estimates, but the faintness of this population in quiescence results in relatively large fractional distance uncertainties. Despite this, DR2 estimates generally agree with literature distances. The most discrepant case is BW Cir, for which detailed studies of the donor star have suggested a distant location at >~25 kpc. A large DR2 measured parallax and relatively high proper motion instead prefer significantly smaller distances, suggesting that the source may instead be amongst the nearest of XRBs. However, both distances create problems for interpretation of the source, and follow-up data are required to resolve its true nature. DR2 also provides a first distance estimate to one source, MAXI J1820+070, and novel proper motion estimates for 7 sources. Peculiar velocities relative to Galactic rotation exceed $\sim$ 50 km s$^{-1}$ for the bulk of the sample, with a median system kinetic energy of peculiar motion of $\sim$ 5 $\times$ 10$^{47}$ erg. BW Cir could be a new high-velocity BHXRB if its astrometry is confirmed. A putative anti-correlation between peculiar velocity and black hole mass is found, as expected in mass-dependent BH kick formation channels, but this trend remains weak in the DR2 data.

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arXiv2019

Prospecting Period Measurements with LSST - Low Mass X-ray Binaries as a Test Case

The Large Synoptic Survey Telescope (LSST) will provide for unbiased sampling of variability properties of objects with $r$ mag $<$ 24. This should allow for those objects whose variations reveal their orbital periods ($P_{orb}$), such as low mass X-ray binaries (LMXBs) and related objects, to be examined in much greater detail and with uniform systematic sampling. However, the baseline LSST observing strategy has temporal sampling that is not optimised for such work in the Galaxy. Here we assess four candidate observing strategies for measurement of $P_{orb}$ in the range 10 minutes to 50 days. We simulate multi-filter quiescent LMXB lightcurves including ellipsoidal modulation and stochastic flaring, and then sample these using LSST's operations simulator (OpSim) over the (mag, $P_{orb}$) parameter space, and over five sightlines sampling a range of possible reddening values. The percentage of simulated parameter space with correctly returned periods ranges from $\sim$23 %, for the current baseline strategy, to $\sim$70 % for the two simulated specialist strategies. Convolving these results with a $P_{orb}$ distribution, a modelled Galactic spatial distribution and reddening maps, we conservatively estimate that the most recent version of the LSST baseline strategy will allow $P_{orb}$ determination for $\sim$18 % of the Milky Way's LMXB population, whereas strategies that do not reduce observations of the Galactic Plane can improve this dramatically to $\sim$32 %. This increase would allow characterisation of the full binary population by breaking degeneracies between suggested $P_{orb}$ distributions in the literature. Our results can be used in the ongoing assessment of the effectiveness of various potential cadencing strategies.

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arXiv2018

Observatory science with eXTP

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

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arXiv2018

The Plane's The Thing: The Case for Wide-Fast-Deep Coverage of the Galactic Plane and Bulge

We argue that the exclusion of the Galactic Plane and Bulge from the uniform wide-fast-deep (WFD) LSST survey cadence is fundamentally inconsistent with two of the main science drivers of LSST: Mapping the Milky Way and Exploring the Transient Optical Sky. We outline the philosophical basis for this claim and then describe a number of important science goals that can only be addressed by WFD-like coverage of the Plane and Bulge.

astro-ph.GA↗

arXiv2018

WISE view of Narrow-Line Seyfert 1 galaxies: mid-infrared color and variability

We present the color and flux variability analysis at 3.4 μm (W1-band) and 4.6 μm (W 2-band) of 492 narrow-line Seyfert 1 (NLSy1) galaxies using archival data from the Wide-field Infrared Survey Explorer (WISE). In the WISE color-color, (W1 - W2) versus (W2 - W3) diagram, ~58% of the NLSy1 galaxies of our sample lie in the region occupied by the blazar category of active galactic nuclei (AGN). The mean W1 - W2 color of candidate variable NLSy1 galaxies is $0.99 \pm 0.18$ mag. The average amplitude of variability is $0.11 \pm 0.07$ mag in long-term (multi-year) with no difference in variability between W1 and W2-bands. The W1 - W2 color of NLSy1 galaxies is anti-correlated with the relative strength of [O III] to Hβ, strongly correlated with continuum luminosity, black hole mass, and Eddington ratio. The long-term amplitude of variability shows weak anti-correlation with the Fe II strength, continuum luminosity and Eddington ratio. A positive correlation between color as well as the amplitude of variability with the radio power at 1.4 GHz was found for the radio-detected NLSy1 galaxies. This suggests non-thermal synchrotron contribution to the mid-infrared color and flux variability in radio-detected NLSy1 galaxies.

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arXiv2018

An X-ray spectroscopic search for dark matter and unidentified line signatures in the Perseus cluster with Hitomi

We present results of a search for unidentified line emission and absorption signals in the 2-12keV energy band of spectra extracted from Perseus Cluster core region observations obtained with the 5eV energy resolution Hitomi Soft X-ray Spectrometer. No significant unidentified line emission or absorption is found. Line flux upper limits (1 sigma per resolution element) vary with photon energy and assumed intrinsic width, decreasing from 100 photons cm$^{-2}$ s$^{-1}$ sr $^{-1}$ at 2keV to $<10$ cm$^{-2}$ s$^{-1}$ sr $^{-1}$ over most of the 5-10 keV energy range for a Gaussian line with Doppler broadening of 640 km/s. Limits for narrower and broader lines have a similar energy dependence and are systematically smaller and larger, respectively. These line flux limits are used to constrain the decay rate of hypothetical dark matter candidates. For the sterile neutrino decay rate, new constraints over the the mass range of 4-24 keV with mass resolution better than any previous X-ray analysis are obtained. Additionally, the accuracy of relevant thermal spectral models and atomic data are evaluated. The Perseus cluster spectra may be described by a composite of multi-temperature thermal and AGN power-law continua. Superposed on these, a few line emission signals possibly originating from unmodeled atomic processes (including Si XIV and Fe XXV) are marginally detected and tabulated. Comparisons with previous X-ray upper limits and future prospects for dark matter searches using high-energy resolution spectroscopy are discussed.

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