Central Compact Objects: some of them could be spinning up?

Among confirmed central compact objects (CCOs), only three sources have measured period and period derivatives. We have investigated possible evolutionary paths of these three CCOs in the fallback disc model. The model can account for the individual X-ray luminosities and rotational properties of the sources consistently with their estimated supernova ages. For these sources, reasonable model curves can be obtained with dipole field strengths $\sim$ a few $\times 10^9$ G on the surface of the star. The model curves indicate that these CCOs were in the spin-up state in the early phase of evolution. The spin-down starts, while accretion is going on, at a time $t \sim 10^3 - 10^4$ yr depending on the current accretion rate, period and the magnetic dipole moment of the star. This implies that some of the CCOs with relatively long periods, weak dipole fields and high X-ray luminosities could be strong candidates to show spin-up behavior if they indeed evolve with fallback discs.Comment: 5 pages, 3 figures, accepted for publication in MNRA

Long-term evolution of anomalous X-ray pulsars and soft gamma repeaters

We have investigated the long-term evolution of individual anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) with relatively well constrained X-ray luminosity and rotational properties. In the frame of the fallback disc model, we have obtained the ranges of disc mass and dipole field strength that can produce the observed source properties. We have compared our results with those obtained earlier for dim isolated neutron stars (XDINs). Our results show that (1) the X-ray luminosity, period and period derivative of the individual AXP/SGR sources can be produced self-consistently in the fallback disc model with very similar basic disc parameters to those used earlier in the same model to explain the long-term evolution of XDINs, (2) except two sources, AXP/SGRs are evolving in the accretion phase; these two exceptional sources, like XDINs, completed their accretion phase in the past and are now evolving in the final propeller phase and still slowing down with the disc torques, (3) the dipole field strength (at the poles) of XDINs are in the $10^{11}-10^{12}$ G range, while AXP/SGRs have relatively strong dipole fields between $1-6 \times 10^{12}$ G, and (4) the source properties can be obtained with large ranges of disc masses which do not allow a clear test of correlation between disc masses and the magnetic dipole fields for the whole AXP/SGRs and XDIN population.Comment: 10 pages, 6 figures, Accepted for publication in MNRA

Impossibility of Three Pass Protocol using Public Abelian Groups

Key transport protocols are designed to transfer a secret key from an initiating principal to other entities in a network. The three-pass protocol is a key transport protocol developed by Adi Shamir in 1980 where Alice wants to transport a secret message to Bob over an insecure channel, and they do not have any pre-shared secret information. In this paper, we prove the impossibility of secret key transportation from a principal to another entity in a network by using the three pass protocol over public Abelian groups. If it were possible to employ public Abelian groups to implement the three-pass protocol, we could use it in post-quantum cryptography for transporting keys providing information theoretic security without relying on any computationally difficult problem

Security in Wireless Sensor Networks: Issues and Challenges

Wireless Sensor Network (WSN) is an emerging technology that shows great promise for various futuristic applications both for mass public and military. The sensing technology combined with processing power and wireless communication makes it lucrative for being exploited in abundance in future. The inclusion of wireless communication technology also incurs various types of security threats. The intent of this paper is to investigate the security related issues and challenges in wireless sensor networks. We identify the security threats, review proposed security mechanisms for wireless sensor networks. We also discuss the holistic view of security for ensuring layered and robust security in wireless sensor networks.Comment: 6 page

Yoğunluk-Uyarlı Kablosuz Şebekeler (DAWN)

TÜBİTAK EEEAG Proje01.10.2018Yüksek veri iletim ihtiyacı olan uygulamaların sayısı artıkça, mobil haberlesme talebi deartmaktadır. Artan gereksinimi karsılamak için hücrelerin küçültülerek kısıtlı olan kablosuzbant genisliginin verimli kullanılması gerekmektedir. Çok sayıda dinamik küçük hücreninortaya çıkmasıyla gezgin aglar da tasarsız sebekelere benzemektedir ve bir paradigmadegisimi yasanmaktadır. Küçük hücrelerin rastlantısal sekilde sebekeye dahil olması veayrılması, sebeke yogunlugunun önceden kestirilemeyecek sekilde degismesine nedenolacaktır. Yogunluk degisimleri ise servis ve tecrübe kalitesini etkileyecek ve yerel kapsamaalanı kontrolü, sinyal girisimi kontrolü gibi yeni sorunlara neden olacaktır. NP-Hard eniyilemeproblem çözümü gerektiren merkezi ve statik yapılandırmalar, sebekenin çok büyük olmasınedeniyle uygulanabilir olmayacaktır. Dagıtık özdüzenleme teknikleriyle, yogunluk-uyarlı yıgıt(stack) ve mimari gelistirilmesi gerekecektir. Bu projenin amacı büyük-ölçekli küçük-hücreliözdüzenlemeli sebekeleri yogunluk-uyarlı ve yogunluk-farkında yapmaktır. Yeni sebekeyogunlugu kestirim teknikleri önerilmis ve yogunluk-uyarlı kapsama alanı kontrolü tekniklerigelistirilmistir. Gelistirilen teknikler benzetim deneyleri ile dogrulanmıstır.As the number of bandwidth-hungry applications and consequently the demand for mobile communicationsincreases, cells have to become smaller to efficiently use the scarce spectrum.This phenomenon causes a paradigm change in mobile communications with the emergenceof small-cell networks that are large in scale and highly dynamic resembling ad hoc networks.The churn in the presence of small cells, which causes sporadic density changes, impacts thequality of service and experience and introduces many novel challenges including distributedcoverage preservation and interference management through local cooperation. Centralizedmanagement and static configuration, which require solving NP-hard optimization problems, arenot feasible any more due to the scale of the network. Distributed self-organization techniques,density-adaptive protocols and architectures have to be developed. This project partially addressedthis challenge with the goal of making the large-scale small-cell networks density-awareand -adaptive. We proposed novel density estimators and density-aware coverage preservationtechniques using the estimators, and validated the expected results by simulations.Keywords: Cellular Networks, Mobile Communications, Long Term Evolution, LTE, Self OrganizingNetworks, Density Estimation, Density-adaptive Wireless Network