arXiv subjects

M. Rahman

Publications and source records attributed to M. Rahman.

32 records · Page 2Linked to original sources

arXiv2020

A repeating fast radio burst source localised to a nearby spiral galaxy

Fast radio bursts (FRBs) are brief, bright, extragalactic radio flashes. Their physical origin remains unknown, but dozens of possible models have been postulated. Some FRB sources exhibit repeat bursts. Though over a hundred FRB sources have been discovered to date, only four have been localised and associated with a host galaxy, with just one of the four known to repeat. The properties of the host galaxies, and the local environments of FRBs, provide important clues about their physical origins. However, the first known repeating FRB has been localised to a low-metallicity, irregular dwarf galaxy, and the apparently non-repeating sources to higher-metallicity, massive elliptical or star-forming galaxies, suggesting that perhaps the repeating and apparently non-repeating sources could have distinct physical origins. Here we report the precise localisation of a second repeating FRB source, FRB 180916.J0158+65, to a star-forming region in a nearby (redshift $z = 0.0337 \pm 0.0002$) massive spiral galaxy, whose properties and proximity distinguish it from all known hosts. The lack of both a comparably luminous persistent radio counterpart and a high Faraday rotation measure further distinguish the local environment of FRB 180916.J0158+65 from that of the one previously localised repeating FRB source, FRB 121102. This demonstrates that repeating FRBs have a wide range of luminosities, and originate from diverse host galaxies and local environments.

astro-ph.HE↗

arXiv2019

CHIME/FRB Detection of Eight New Repeating Fast Radio Burst Sources

We report on the discovery of eight repeating fast radio burst (FRB) sources found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. These sources span a dispersion measure (DM) range of 103.5 to 1281 pc cm$^{-3}$. They display varying degrees of activity: six sources were detected twice, another three times, and one ten times. These eight repeating FRBs likely represent the bright and/or high-rate end of a distribution of infrequently repeating sources. For all sources, we determine sky coordinates with uncertainties of $\sim$10$^\prime$. FRB 180916.J0158+65 has a burst-averaged DM = $349.2 \pm 0.3$ pc cm$^{-3}$ and a low DM excess over the modelled Galactic maximum (as low as $\sim$20 pc cm$^{-3}$); this source also has a Faraday rotation measure (RM) of $-114.6 \pm 0.6$ rad m$^{-2}$, much lower than the RM measured for FRB 121102. FRB 181030.J1054+73 has the lowest DM for a repeater, $103.5 \pm 0.3$ pc cm$^{-3}$, with a DM excess of $\sim$ 70 pc cm$^{-3}$. Both sources are interesting targets for multi-wavelength follow-up due to their apparent proximity. The DM distribution of our repeater sample is statistically indistinguishable from that of the first 12 CHIME/FRB sources that have not repeated. We find, with 4$σ$ significance, that repeater bursts are generally wider than those of CHIME/FRB bursts that have not repeated, suggesting different emission mechanisms. Our repeater events show complex morphologies that are reminiscent of the first two discovered repeating FRBs. The repetitive behavior of these sources will enable interferometric localizations and subsequent host galaxy identifications.

astro-ph.HE↗

arXiv2019

CHIME/FRB Detection of the Original Repeating Fast Radio Burst Source FRB 121102

We report the detection of a single burst from the first-discovered repeating Fast Radio Burst source, FRB 121102, with CHIME/FRB, which operates in the frequency band 400-800 MHz. The detected burst occurred on 2018 November 19 and its emission extends down to at least 600 MHz, the lowest frequency detection of this source yet. The burst, detected with a significance of 23.7$σ$, has fluence 12$\pm$3 Jy ms and shows complex time and frequency morphology. The 34 ms width of the burst is the largest seen for this object at any frequency. We find evidence of sub-burst structure that drifts downward in frequency at a rate of -3.9$\pm$0.2 MHz ms$^{-1}$. Our best fit tentatively suggests a dispersion measure of 563.6$\pm$0.5 pc cm$^{-3}$, which is ${\approx}$1% higher than previously measured values. We set an upper limit on the scattering time at 500 MHz of 9.6 ms, which is consistent with expectations from the extrapolation from higher frequency data. We have exposure to the position of FRB 121102 for a total of 11.3 hrs within the FWHM of the synthesized beams at 600 MHz from 2018 July 25 to 2019 February 25. We estimate on the basis of this single event an average burst rate for FRB 121102 of 0.1-10 per day in the 400-800 MHz band for a median fluence threshold of 7 Jy ms in the stated time interval.

astro-ph.HE↗

arXiv2019

A Second Source of Repeating Fast Radio Bursts

The discovery of a repeating Fast Radio Burst (FRB) source, FRB 121102, eliminated models involving cataclysmic events for this source. No other repeating FRB has yet been detected in spite of many recent FRB discoveries and follow-ups, suggesting repeaters may be rare in the FRB population. Here we report the detection of six repeat bursts from FRB 180814.J0422+73, one of the 13 FRBs detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project during its pre-commissioning phase in July and August 2018. These repeat bursts are consistent with originating from a single position on the sky, with the same dispersion measure (DM), ~189 pc cm-3. This DM is approximately twice the expected Milky Way column density, and implies an upper limit on the source redshift of 0.1, at least a factor of ~2 closer than FRB 121102. In some of the repeat bursts, we observe sub-pulse frequency structure, drifting, and spectral variation reminiscent of that seen in FRB 121102, suggesting similar emission mechanisms and/or propagation effects. This second repeater, found among the first few CHIME/FRB discoveries, suggests that there exists -- and that CHIME/FRB and other wide-field, sensitive radio telescopes will find -- a substantial population of repeating FRBs.

astro-ph.HE↗

arXiv2018

Modulational instability, rogue waves, and envelope solitons in opposite polarity dusty plasmas

Dust-acoustic (DA) waves (DAWs) and their modulational instability (MI) have been investigated theoretically in a plasma system consisting of inertial opposite polarity (positively and negatively) warm adiabatic charged dust particles as well as inertialess non-extensive $q$-distributed electrons and non-thermal ions. A nonlinear Schrödinger (NLS) equation is derived by using the reductive perturbation method. It has been observed from the analysis of NLS equaion that the modulationally stable solitary DAWs give rise to the existence of dark envelope solitons, and that the modulationally unstable solitary DAWs give rise to the existence of bright envelope solitons or rogue structures. It is also observed for the fast mode of DAWs that the basic features (viz. stability of the DAWs, MI growth rate, amplitude and width of the DA rogue waves, etc.) are significantly modified by the related plasma parameters (viz. dust masses, dust charge state, non-extensive parameter $q$, and non-thermal parameter $α$). The results of our present investigation might be useful for understanding different nonlinear electrostatic phenomena in both space (viz. ionosphere and mesosphere) and laboratory plasmas (viz. high intensity laser irradiation and hot cathode discharge).

physics.plasm-ph↗

arXiv2013

DREEM-ME: Distributed Regional Energy Efficient Multi-hop Routing Protocol based on Maximum Energy in WSNs

Wireless distributed sensor network consists of randomly deployed sensors having low energy assets. These networks can be used for monitoring a variety of environments. Major problems of these networks are energy constraints and their finite lifetimes. To overcome these problems different routing protocols and clustering techniques are introduced. We propose DREEM-ME which uses a unique technique for clustering to overcome these two problems efficiently. DREEM-ME elects a fix number of cluster heads (CHs) in each round instead of probabilistic selection of CHs. Packet Drop Technique is also implemented in our protocol to make it more comprehensive and practical. In DREEM-ME confidence interval is also shown in each graph which helps in visualising the maximum deviation from original course. Our simulations and results show that DREEM-ME is much better than existing protocols of the same nature.

cs.NI↗

arXiv2012

Milky Way Star Forming Complexes and the Turbulent Motion of the Galaxy's Molecular Gas

We analyze Spitzer GLIMPSE, MSX, and WMAP images of the Milky Way to identify 8 micron and free-free sources in the Galaxy. Seventy-two of the eighty-eight WMAP sources have coverage in the GLIMPSE and MSX surveys suitable for identifying massive star forming complexes (SFC). We measure the ionizing luminosity functions of the SFCs and study their role in the turbulent motion of the Galaxy's molecular gas. We find a total Galactic free-free flux f_ν = 46177.6 Jy; the 72 WMAP sources with full 8 micron coverage account for 34263.5 Jy (~75%), with both measurements made at ν=94GHz (W band). We find a total of 280 SFCs, of which 168 have unique kinematic distances and free-free luminosities. We use a simple model for the radial distribution of star formation to estimate the free-free and ionizing luminosity for the sources lacking distance determinations. The total dust-corrected ionizing luminosity is Q = 2.9 \pm 0.5 x 10^53 photons s^-1, which implies a galactic star formation rate of 1.2 \pm 0.2 M_{\sun} yr^-1. We present the (ionizing) luminosity function of the SFCs, and show that 24 sources emit half the ionizing luminosity of the Galaxy. The SFCs appear as bubbles in GLIMPSE or MSX images; the radial velocities associated with the bubble walls allow us to infer the expansion velocity of the bubbles. We calculate the kinetic luminosity of the bubble expansion and compare it to the turbulent luminosity of the inner molecular disk. SFCs emitting 80% of the total galactic free-free luminosity produce a kinetic luminosity equal to 65% of the turbulent luminosity in the inner molecular disk. This suggests that the expansion of the bubbles is a major driver of the turbulent motion of the inner Milky Way molecular gas.

astro-ph.GA↗

arXiv2011

Information Security Synthesis in Online Universities

Information assurance is at the core of every initiative that an organization executes. For online universities, a common and complex initiative is maintaining user lifecycle and providing seamless access using one identity in a large virtual infrastructure. To achieve information assurance the management of user privileges affected by events in the user's identity lifecycle needs to be the determining factor for access control. While the implementation of identity and access management systems makes this initiative feasible, it is the construction and maintenance of the infrastructure that makes it complex and challenging. The objective of this paper1 is to describe the complexities, propose a practical approach to building a foundation for consistent user experience and realizing security synthesis in online universities.

cs.CR↗

arXiv2011

An Overview of the Security Concerns in Enterprise Cloud Computing

Deploying cloud computing in an enterprise infrastructure bring significant security concerns. Successful implementation of cloud computing in an enterprise requires proper planning and understanding of emerging risks, threats, vulnerabilities, and possible countermeasures. We believe enterprise should analyze the company/organization security risks, threats, and available countermeasures before adopting this technology. In this paper, we have discussed security risks and concerns in cloud computing and enlightened steps that an enterprise can take to reduce security risks and protect their resources. We have also explained cloud computing strengths/benefits, weaknesses, and applicable areas in information risk management.

cs.CR↗

arXiv2010

Ethical Dilemma of Governmental Wiretapping

USA Government wiretapping activities is a very controversial issue. Undoubtedly this technology can assist law enforced authority to detect / identify unlawful or hostile activities; however, this task raises severe privacy concerns. In this paper, we have discussed this complex information technology issue of governmental wiretapping and how it effects both public and private liberties. Legislation has had a major impact on the uses and the stigma of wiretapping for the war on terrorism. This paper also analyzes the ethical and legal concerns inherent when discussing the benefits and concerns of wiretapping. The analysis has concluded with the effects of wiretapping laws as they relate to future government actions in their fight against terrorists.

cs.CY↗

arXiv2010

Convergence of Corporate and Information Security

As physical and information security boundaries have become increasingly blurry many organizations are experiencing challenges with how to effectively and efficiently manage security within the corporate. There is no current standard or best practice offered by the security community regarding convergence; however many organizations such as the Alliance for Enterprise Security Risk Management (AESRM) offer some excellent suggestions for integrating a converged security program. This paper reports on how organizations have traditionally managed asset protection, why that is changing and how to establish convergence to optimize security value to the business within an enterprise.

cs.CR↗

arXiv2009

Star Formation in Massive Clusters via the Wilkinson Microwave Anisotropy Probe and the Spitzer Glimpse Survey

We use the WMAP maximum entropy method foreground emission map combined with previously determined distances to giant HII regions to measure the free-free flux at Earth and the free-free luminosity of the galaxy. We find a total flux f_ν=54211 Jy and a flux from 88 sources of f_ν=36043 Jy. The bulk of the sources are at least marginally resolved, with mean radii ~60 pc, electron density n_e ~ 9 cm^{-3}, and filling factor ϕ_{HII}=0.005 (over the Galactic gas disk). The total dust-corrected ionizing photon luminosity is Q=3.2x10^{53} photons/s, in good agreement with previous estimates. We use GLIMPSE and MSX 8 micron images to show that the bulk of the free-free luminosity is associated with bubbles having radii r~5-100 pc, with a mean ~20 pc. These bubbles are leaky, so that ionizing photons from inside the bubble excite free-free emission beyond the bubble walls, producing WMAP sources that are larger than the 8 micron bubbles. We suggest that the WMAP sources are the counterparts of the extended low density HII regions described by Mezger (1978). Half the ionizing luminosity from the sources is emitted by the nine most luminous objects, while the seventeen most luminous emit half the total Galactic ionizing flux. These 17 sources have 4x10^{51} < Q <1.6x10^{52}, corresponding to 6x10^4M_\odot < M_*< 2x10^5M_\odot; half to two thirds of this will be in the central massive star cluster. We convert the measurement of Q to a Galactic star formation rate dM/dt=1.3M_\odot/yr, but point out that this is highly dependent on the exponent Γ~1.35 of the high mass end of the stellar initial mass function.

astro-ph.GA↗

arXiv2001

Coulomb Drag Between Quasiballistic Quantum Wires: an Indication of Non-Fermi-Liquid Behavior

The Coulomb drag between two spatially separated, 2 μm long lithographically defined quantum wires has been studied experimentally in the absence of interwire tunneling. The drag resistance R_D shows peaks when the 1D subband bottoms of the wires are aligned and the Fermi wave vector k_F is small. R_D decreases exponentially with the interwire separation d. In the temperature range 0.2K to 1K the drag signal shows the {\it power-law dependence} R_D ~ T**x with x ranging from -0.61 to -0.77 depending on the magnitude of k_F. We interpret our experimental results in the framework of the Tomonaga-Luttinger liquid theory.

cond-mat.mes-hall↗

arXiv2000

Ballistic electron transport in stubbed quantum waveguides: experiment and theory

We present results of experimental and theoretical investigations of electron transport through stub-shaped waveguides or electron stub tuners (ESTs) in the ballistic regime. Measurements of the conductance G as a function of voltages, applied to different gates V_i (i=bottom, top, and side) of the device, show oscillations in the region of the first quantized plateau which we attribute to reflection resonances. The oscillations are rather regular and almost periodic when the height h of the EST cavity is small compared to its width. When h is increased, the oscillations become less regular and broad depressions in G appear. A theoretical analysis, which accounts for the electrostatic potential formed by the gates in the cavity region, and a numerical computation of the transmission probabilities successfully explains the experimental observations. An important finding for real devices, defined by surface Schottky gates, is that the resonance nima result from size quantization along the transport direction of the EST.

cond-mat.mes-hall↗